Top Underscore.js Interview Questions and Answers-(2024)

1 .

Underscore is a JavaScript library that provides a whole mess of useful functional programming helpers without extending any built-in objects. Underscore provides over 100 functions that support both your favorite workaday functional helpers: map, filter, invoke — as well as more specialized goodies: function binding, javascript templating, creating quick indexes, deep equality testing, and so on.

2 .

How to Installation Underscore.js?

We can use Underscore.js directly inside the browser and also with node.js. We will discuss both of these methods.

Method 1 : Use directly inside the browser.

Visit the official website (https://underscorejs.org/) and download the latest underscore-min.js file UMD available. After that include the following CDN link inside your code in order to run the underscore.js code inside the browser.

Method 2 : We can install it with npm. Make sure that you have Node.js and npm installed.

npm install underscore

You can also install with yarn

yarn install underscore

<!DOCTYPE html>
<html>
 
<head>
    <script type="text/javascript" src=
"https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js">
    </script>
</head>
 
<body>
    <script type="text/javascript">
        var numbers = [100, 50, 400, 66, 999];
        console.log(_.max(numbers));
    </script>
</body>
 
</html>

Output :

3 .

what is Underscore.js _.max Function?

The _.max() function is used to find the minimum element from the list passed. If an iteratee is given, then it will be applied to each value and generate criteria to rank the values and find the minimum element.

Syntax :

_.max(list, [iteratee], [context])

Parameters : This function accepts three parameters as mentioned above and described below:

* List : This parameter is used to hold the list of items.

* Predicate : This parameter is used to hold the test condition.

* Context : This parameter is used to display the content.

4 .

Passing an array of numbers to the _.max function() in Underscore.js.

The ._max() function takes the element from the list one by one and do compares the elements to find the maximum number in the list. After traversing and comparing all the elements, the _.max() function ends.

Example :

<html>
    <head>
        <script type="text/javascript" src = 
"https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js" >
        </script>
    </head>
    <body>
        <script type="text/javascript">
            var numbers = [100, 50, 400, 66, 999];
            console.log(_.max(numbers));
        </script>
    </body>
</html>

Output :

5 .

What are the Features of Underscore.js?

* Perform common operations of data like arrays, objects, JSON files, etc.

* Compatible with other JS libraries like jQuery to perform DOM operations.

* Contains function for data manipulation.

6 .

What are the iterating collections in Underscore.js?

each : _.each(list, iteratee, [context]) Alias: forEach

Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed. Each invocation of iteratee is called with three arguments: (element, index, list). If list is a JavaScript object, iteratee's arguments will be (value, key, list). Returns the list for chaining.

_.each([1, 2, 3], alert);
=> alerts each number in turn...
_.each({one: 1, two: 2, three: 3}, alert);
=> alerts each number value in turn...

map _.map(list, iteratee, [context]) Alias: collect

Produces a new array of values by mapping each value in list through a transformation function (iteratee). The iteratee is passed three arguments: the value, then the index (or key) of the iteration, and finally a reference to the entire list.

_.map([1, 2, 3], function(num){ return num * 3; });
=> [3, 6, 9]
_.map({one: 1, two: 2, three: 3}, function(num, key){ return num * 3; });
=> [3, 6, 9]
_.map([[1, 2], [3, 4]], _.first);
=> [1, 3]

reduce _.reduce(list, iteratee, [memo], [context]) Aliases : inject, foldl

Also known as inject and foldl, reduce boils down a list of values into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iteratee. The iteratee is passed four arguments: the memo, then the value and index (or key) of the iteration, and finally a reference to the entire list.

If no memo is passed to the initial invocation of reduce, the iteratee is not invoked on the first element of the list. The first element is instead passed as the memo in the invocation of the iteratee on the next element in the list.

var sum = _.reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);
=> 6

reduceRight _.reduceRight(list, iteratee, [memo], [context]) Alias : foldr

The right-associative version of reduce. Foldr is not as useful in JavaScript as it would be in a language with lazy evaluation.

var list = [[0, 1], [2, 3], [4, 5]];
var flat = _.reduceRight(list, function(a, b) { return a.concat(b); }, []);
=> [4, 5, 2, 3, 0, 1]
â€‹

find _.find(list, predicate, [context]) Alias : detect

Looks through each value in the list, returning the first one that passes a truth test (predicate), or undefined if no value passes the test. The function returns as soon as it finds an acceptable element, and doesn't traverse the entire list. predicate is transformed through iteratee to facilitate shorthand syntaxes.

var even = _.find([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> 2

filter _.filter(list, predicate, [context]) Alias : select

Looks through each value in the list, returning an array of all the values that pass a truth test (predicate). predicate is transformed through iteratee to facilitate shorthand syntaxes.

var evens = _.filter([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [2, 4, 6]

findWhere _.findWhere(list, properties)
Looks through the list and returns the first value that matches all of the key-value pairs listed in properties.

If no match is found, or if list is empty, undefined will be returned.

_.findWhere(publicServicePulitzers, {newsroom: "The New York Times"});
=> {year: 1918, newsroom: "The New York Times",
  reason: "For its public service in publishing in full so many official reports,
  documents and speeches by European statesmen relating to the progress and
  conduct of the war."}

where _.where(list, properties)

Looks through each value in the list, returning an array of all the values that matches the key-value pairs listed in properties.

_.where(listOfPlays, {author: "Shakespeare", year: 1611});
=> [{title: "Cymbeline", author: "Shakespeare", year: 1611},
    {title: "The Tempest", author: "Shakespeare", year: 1611}]

reject _.reject(list, predicate, [context])

Returns the values in list without the elements that the truth test (predicate) passes. The opposite of filter. predicate is transformed through iteratee to facilitate shorthand syntaxes.

var odds = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [1, 3, 5]

every _.every(list, [predicate], [context]) Alias : all

Returns true if all of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a false element is found. predicate is transformed through iteratee to facilitate shorthand syntaxes.

_.every([2, 4, 5], function(num) { return num % 2 == 0; });
=> false

some _.some(list, [predicate], [context]) Alias : any

Returns true if any of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a true element is found. predicate is transformed through iteratee to facilitate shorthand syntaxes.

_.some([null, 0, 'yes', false]);
=> true

7 .

What are the Processing Collections in Underscore.js?

contains _.contains(list, value, [fromIndex]) Aliases : include, includes

Returns true if the value is present in the list. Uses indexOf internally, if list is an Array. Use fromIndex to start your search at a given index.

_.contains([1, 2, 3], 3);
=> true

invoke _.invoke(list, methodName, *arguments)

Calls the method named by methodName on each value in the list. Any extra arguments passed to invoke will be forwarded on to the method invocation.

_.invoke([[5, 1, 7], [3, 2, 1]], 'sort');
=> [[1, 5, 7], [1, 2, 3]]

pluck _.pluck(list, propertyName)

A convenient version of what is perhaps the most common use-case for map: extracting a list of property values.

var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.pluck(stooges, 'name');
=> ["moe", "larry", "curly"]

max _.max(list, [iteratee], [context])

Returns the maximum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked. -Infinity is returned if list is empty, so an isEmpty guard may be required. This function can currently only compare numbers reliably. This function uses operator < (note).

var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.max(stooges, function(stooge){ return stooge.age; });
=> {name: 'curly', age: 60};

min _.min(list, [iteratee], [context])

Returns the minimum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked. Infinity is returned if list is empty, so an isEmpty guard may be required. This function can currently only compare numbers reliably. This function uses operator < (note).

var numbers = [10, 5, 100, 2, 1000];
_.min(numbers);
=> 2

sortBy _.sortBy(list, iteratee, [context])

Returns a (stably) sorted copy of list, ranked in ascending order by the results of running each value through iteratee. iteratee may also be the string name of the property to sort by (eg. length). This function uses operator < (note).

_.sortBy([1, 2, 3, 4, 5, 6], function(num){ return Math.sin(num); });
=> [5, 4, 6, 3, 1, 2]

var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.sortBy(stooges, 'name');
=> [{name: 'curly', age: 60}, {name: 'larry', age: 50}, {name: 'moe', age: 40}];

groupBy _.groupBy(list, iteratee, [context])

Splits a collection into sets, grouped by the result of running each value through iteratee. If iteratee is a string instead of a function, groups by the property named by iteratee on each of the values.

_.groupBy([1.3, 2.1, 2.4], function(num){ return Math.floor(num); });
=> {1: [1.3], 2: [2.1, 2.4]}

_.groupBy(['one', 'two', 'three'], 'length');
=> {3: ["one", "two"], 5: ["three"]}

indexBy _.indexBy(list, iteratee, [context])

Given a list, and an iteratee function that returns a key for each element in the list (or a property name), returns an object with an index of each item. Just like groupBy, but for when you know your keys are unique.

var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.indexBy(stooges, 'age');
=> {
  "40": {name: 'moe', age: 40},
  "50": {name: 'larry', age: 50},
  "60": {name: 'curly', age: 60}
}

countBy _.countBy(list, iteratee, [context])

Sorts a list into groups and returns a count for the number of objects in each group. Similar to groupBy, but instead of returning a list of values, returns a count for the number of values in that group.

_.countBy([1, 2, 3, 4, 5], function(num) {
  return num % 2 == 0 ? 'even': 'odd';
});
=> {odd: 3, even: 2}

shuffle _.shuffle(list)

Returns a shuffled copy of the list, using a version of the Fisher-Yates shuffle.

_.shuffle([1, 2, 3, 4, 5, 6]);
=> [4, 1, 6, 3, 5, 2]

sample _.sample(list, [n])

Produce a random sample from the list. Pass a number to return n random elements from the list. Otherwise a single random item will be returned.

_.sample([1, 2, 3, 4, 5, 6]);
=> 4

_.sample([1, 2, 3, 4, 5, 6], 3);
=> [1, 6, 2]

toArray _.toArray(list)

Creates a real Array from the list (anything that can be iterated over). Useful for transmuting the arguments object.

(function(){ return _.toArray(arguments).slice(1); })(1, 2, 3, 4);
=> [2, 3, 4]

size _.size(list)

Return the number of values in the list.

_.size([1, 2, 3, 4, 5]);
=> 5

_.size({one: 1, two: 2, three: 3});
=> 3

partition _.partition(list, predicate)

Split list into two arrays: one whose elements all satisfy predicate and one whose elements all do not satisfy predicate. predicate is transformed through iteratee to facilitate shorthand syntaxes.

_.partition([0, 1, 2, 3, 4, 5], isOdd);
=> [[1, 3, 5], [0, 2, 4]]

compact _.compact(list)

Returns a copy of the list with all falsy values removed. In JavaScript, false, null, 0, "", undefined and NaN are all falsy.

_.compact([0, 1, false, 2, '', 3]);
=> [1, 2, 3]

8 .

What are the Iterating Arrays in Underscore.js?

first _.first(array, [n]) Aliases : head, take

Returns the first element of an array. Passing n will return the first n elements of the array.

_.first([5, 4, 3, 2, 1]);
=> 5

initial _.initial(array, [n])

Returns everything but the last entry of the array. Especially useful on the arguments object. Pass n to exclude the last n elements from the result.

_.initial([5, 4, 3, 2, 1]);
=> [5, 4, 3, 2]

last _.last(array, [n])

Returns the last element of an array. Passing n will return the last n elements of the array.

_.last([5, 4, 3, 2, 1]);
=> 1

rest _.rest(array, [index]) Aliases : tail, drop

Returns the rest of the elements in an array. Pass an index to return the values of the array from that index onward.

_.rest([5, 4, 3, 2, 1]);
=> [4, 3, 2, 1]

indexOf _.indexOf(array, value, [isSorted])

Returns the index at which value can be found in the array, or -1 if value is not present in the array. If you're working with a large array, and you know that the array is already sorted, pass true for isSorted to use a faster binary search ... or, pass a number as the third argument in order to look for the first matching value in the array after the given index. If isSorted is true, this function uses operator < (note).

_.indexOf([1, 2, 3], 2);
=> 1

lastIndexOf _.lastIndexOf(array, value, [fromIndex])

Returns the index of the last occurrence of value in the array, or -1 if value is not present. Pass fromIndex to start your search at a given index.

_.lastIndexOf([1, 2, 3, 1, 2, 3], 2);
=> 4

sortedIndex _.sortedIndex(array, value, [iteratee], [context])

Uses a binary search to determine the smallest index at which the value should be inserted into the array in order to maintain the array's sorted order. If an iteratee function is provided, it will be used to compute the sort ranking of each value, including the value you pass. The iteratee may also be the string name of the property to sort by (eg. length). This function uses operator < (note).

_.sortedIndex([10, 20, 30, 40, 50], 35);
=> 3

var stooges = [{name: 'moe', age: 40}, {name: 'curly', age: 60}];
_.sortedIndex(stooges, {name: 'larry', age: 50}, 'age');
=> 1

findIndex _.findIndex(array, predicate, [context])

Similar to _.indexOf, returns the first index where the predicate truth test passes; otherwise returns -1.

_.findIndex([4, 6, 8, 12], isPrime);
=> -1 // not found
_.findIndex([4, 6, 7, 12], isPrime);
=> 2

findLastIndex _.findLastIndex(array, predicate, [context])

Like _.findIndex but iterates the array in reverse, returning the index closest to the end where the predicate truth test passes.

var users = [{'id': 1, 'name': 'Bob', 'last': 'Brown'},
             {'id': 2, 'name': 'Ted', 'last': 'White'},
             {'id': 3, 'name': 'Frank', 'last': 'James'},
             {'id': 4, 'name': 'Ted', 'last': 'Jones'}];
_.findLastIndex(users, {
  name: 'Ted'
});
=> 3

9 .

What are the Processing Arrays in Underscore.js?

flatten _.flatten(array, [depth])

Flattens a nested array. If you pass true or 1 as the depth, the array will only be flattened a single level. Passing a greater number will cause the flattening to descend deeper into the nesting hierarchy. Omitting the depth argument, or passing false or Infinity, flattens the array all the way to the deepest nesting level.

_.flatten([1, [2], [3, [[4]]]]);
=> [1, 2, 3, 4];

_.flatten([1, [2], [3, [[4]]]], true);
=> [1, 2, 3, [[4]]];

_.flatten([1, [2], [3, [[4]]]], 2);
=> [1, 2, 3, [4]];

without _.without(array, *values)

Returns a copy of the array with all instances of the values removed.

_.without([1, 2, 1, 0, 3, 1, 4], 0, 1);
=> [2, 3, 4]

union _.union(*arrays)

Computes the union of the passed-in arrays: the list of unique items, in order, that are present in one or more of the arrays.

_.union([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2, 3, 101, 10]

intersection _.intersection(*arrays)

Computes the list of values that are the intersection of all the arrays. Each value in the result is present in each of the arrays.

_.intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2]

difference _.difference(array, *others)

Similar to without, but returns the values from array that are not present in the other arrays.

_.difference([1, 2, 3, 4, 5], [5, 2, 10]);
=> [1, 3, 4]

uniq _.uniq(array, [isSorted], [iteratee]) Alias : unique

Produces a duplicate-free version of the array, using === to test object equality. In particular only the first occurrence of each value is kept. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass an iteratee function.

_.uniq([1, 2, 1, 4, 1, 3]);
=> [1, 2, 4, 3]

zip _.zip(*arrays)

Merges together the values of each of the arrays with the values at the corresponding position. Useful when you have separate data sources that are coordinated through matching array indexes.

_.zip(['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]);
=> [["moe", 30, true], ["larry", 40, false], ["curly", 50, false]]

unzip _.unzip(array) Alias: transpose

The opposite of zip. Given an array of arrays, returns a series of new arrays, the first of which contains all of the first elements in the input arrays, the second of which contains all of the second elements, and so on. If you're working with a matrix of nested arrays, this can be used to transpose the matrix.

_.unzip([["moe", 30, true], ["larry", 40, false], ["curly", 50, false]]);
=> [['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]]

object _.object(list, [values])

Converts arrays into objects. Pass either a single list of [key, value] pairs, or a list of keys, and a list of values. Passing by pairs is the reverse of pairs. If duplicate keys exist, the last value wins.

_.object(['moe', 'larry', 'curly'], [30, 40, 50]);
=> {moe: 30, larry: 40, curly: 50}

_.object([['moe', 30], ['larry', 40], ['curly', 50]]);
=> {moe: 30, larry: 40, curly: 50}

chunk _.chunk(array, length)

Chunks an array into multiple arrays, each containing length or fewer items.

var partners = _.chunk(_.shuffle(kindergarten), 2);
=> [["Tyrone", "Elie"], ["Aidan", "Sam"], ["Katrina", "Billie"], ["Little Timmy"]]

range _.range([start], stop, [step])

A function to create flexibly-numbered lists of integers, handy for each and map loops. start, if omitted, defaults to 0; step defaults to 1. Returns a list of integers from start (inclusive) to stop (exclusive), incremented (or decremented) by step. Note that ranges that stop before they start are considered to be zero-length instead of negative — if you'd like a negative range, use a negative step.

_.range(10);
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
_.range(1, 11);
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
_.range(0, 30, 5);
=> [0, 5, 10, 15, 20, 25]
_.range(0, -10, -1);
=> [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
_.range(0);
=> []

10 .

What is Object-Oriented Style in Underscore.js?

You can use Underscore in either an object-oriented or a functional style, depending on your preference. The following two lines of code are identical ways to double a list of numbers.

_.map([1, 2, 3], function(n){ return n * 2; });
_([1, 2, 3]).map(function(n){ return n * 2; });

11 .

What is Chaining in Underscore.js?

Calling chain will cause all future method calls to return wrapped objects. When you've finished the computation, call value to retrieve the final value. Here's an example of chaining together a map/flatten/reduce, in order to get the word count of every word in a song.

var lyrics = [
  {line: 1, words: "I'm a lumberjack and I'm okay"},
  {line: 2, words: "I sleep all night and I work all day"},
  {line: 3, words: "He's a lumberjack and he's okay"},
  {line: 4, words: "He sleeps all night and he works all day"}
];

_.chain(lyrics)
  .map(function(line) { return line.words.split(' '); })
  .flatten()
  .reduce(function(counts, word) {
    counts[word] = (counts[word] || 0) + 1;
    return counts;
  }, {})
  .value();

=> {lumberjack: 2, all: 4, night: 2 ... }

In addition, the Array prototype's methods are proxied through the chained Underscore object, so you can slip a reverse or a push into your chain, and continue to modify the array.

chain _.chain(obj)

Returns a wrapped object. Calling methods on this object will continue to return wrapped objects until value is called.

var stooges = [{name: 'curly', age: 25}, {name: 'moe', age: 21}, {name: 'larry', age: 23}];
var youngest = _.chain(stooges)
  .sortBy(function(stooge){ return stooge.age; })
  .map(function(stooge){ return stooge.name + ' is ' + stooge.age; })
  .first()
  .value();
=> "moe is 21"

value _.chain(obj).value()

Extracts the value of a wrapped object.

_.chain([1, 2, 3]).reverse().value();
=> [3, 2, 1]

12 .

What are the Functions in Underscore.js?

bind _.bind(function, object, *arguments)

Bind a function to an object, meaning that whenever the function is called, the value of this will be the object. Optionally, pass arguments to the function to pre-fill them, also known as partial application. For partial application without context binding, use partial.

var func = function(greeting){ return greeting + ': ' + this.name };
func = _.bind(func, {name: 'moe'}, 'hi');
func();
=> 'hi: moe'

bindAll _.bindAll(object, *methodNames)

Binds a number of methods on the object, specified by methodNames, to be run in the context of that object whenever they are invoked. Very handy for binding functions that are going to be used as event handlers, which would otherwise be invoked with a fairly useless this. methodNames are required.

var buttonView = {
  label  : 'underscore',
  onClick: function(){ alert('clicked: ' + this.label); },
  onHover: function(){ console.log('hovering: ' + this.label); }
};
_.bindAll(buttonView, 'onClick', 'onHover');
// When the button is clicked, this.label will have the correct value.
jQuery('#underscore_button').on('click', buttonView.onClick);

partial _.partial(function, *arguments)

Partially apply a function by filling in any number of its arguments, without changing its dynamic this value. A close cousin of bind. You may pass _ in your list of arguments to specify an argument that should not be pre-filled, but left open to supply at call-time. Note: if you need _ placeholders and a this binding at the same time, use both _.partial and _.bind.

var subtract = function(a, b) { return b - a; };
sub5 = _.partial(subtract, 5);
sub5(20);
=> 15

// Using a placeholder
subFrom20 = _.partial(subtract, _, 20);
subFrom20(5);
=> 15

memoize _.memoize(function, [hashFunction])

Memoizes a given function by caching the computed result. Useful for speeding up slow-running computations. If passed an optional hashFunction, it will be used to compute the hash key for storing the result, based on the arguments to the original function. The default hashFunction just uses the first argument to the memoized function as the key. The cache of memoized values is available as the cache property on the returned function.

var fibonacci = _.memoize(function(n) {
  return n < 2 ? n: fibonacci(n - 1) + fibonacci(n - 2);
});

delay _.delay(function, wait, *arguments)

Much like setTimeout, invokes function after wait milliseconds. If you pass the optional arguments, they will be forwarded on to the function when it is invoked.

var log = _.bind(console.log, console);
_.delay(log, 1000, 'logged later');
=> 'logged later' // Appears after one second.

once _.once(function)

Creates a version of the function that can only be called one time. Repeated calls to the modified function will have no effect, returning the value from the original call. Useful for initialization functions, instead of having to set a boolean flag and then check it later.

var initialize = _.once(createApplication);
initialize();
initialize();
// Application is only created once.

after _.after(count, function)

Creates a wrapper of function that does nothing at first. From the count-th call onwards, it starts actually calling function. Useful for grouping asynchronous responses, where you want to be sure that all the async calls have finished, before proceeding.

var renderNotes = _.after(notes.length, render);
_.each(notes, function(note) {
  note.asyncSave({success: renderNotes});
});
// renderNotes is run once, after all notes have saved.

before _.before(count, function)

Creates a wrapper of function that memoizes its return value. From the count-th call onwards, the memoized result of the last invocation is returned immediately instead of invoking function again. So the wrapper will invoke function at most count - 1 times.

var monthlyMeeting = _.before(3, askForRaise);
monthlyMeeting();
monthlyMeeting();
monthlyMeeting();
// the result of any subsequent calls is the same as the second call

wrap _.wrap(function, wrapper)

Wraps the first function inside of the wrapper function, passing it as the first argument. This allows the wrapper to execute code before and after the function runs, adjust the arguments, and execute it conditionally.

var hello = function(name) { return "hello: " + name; };
hello = _.wrap(hello, function(func) {
  return "before, " + func("moe") + ", after";
});
hello();
=> 'before, hello: moe, after'

negate _.negate(predicate)

Returns a new negated version of the predicate function.

var isFalsy = _.negate(Boolean);
_.find([-2, -1, 0, 1, 2], isFalsy);
=> 0

compose _.compose(*functions)

Returns the composition of a list of functions, where each function consumes the return value of the function that follows. In math terms, composing the functions f(), g(), and h() produces f(g(h())).

var greet    = function(name){ return "hi: " + name; };
var exclaim  = function(statement){ return statement.toUpperCase() + "!"; };
var welcome = _.compose(greet, exclaim);
welcome('moe');
=> 'hi: MOE!'

13 .

What is the restArguments in Underscore.js?

restArguments

Syntax :

_.restArguments(function, [startIndex])â€‹

Returns a version of the function that, when called, receives all arguments from and beyond startIndex collected into a single array. If you don’t pass an explicit startIndex, it will be determined by looking at the number of arguments to the function itself. Similar to ES6’s rest parameters syntax.

var raceResults = _.restArguments(function(gold, silver, bronze, everyoneElse) {
  _.each(everyoneElse, sendConsolations);
});

raceResults("Dopey", "Grumpy", "Happy", "Sneezy", "Bashful", "Sleepy", "Doc");

14 .

What is the Mapping Objects in Underscore.js?

keys _.keys(object)

Retrieve all the names of the object's own enumerable properties.

_.keys({one: 1, two: 2, three: 3});
=> ["one", "two", "three"]

allKeys _.allKeys(object)

Retrieve all the names of object's own and inherited properties.

function Stooge(name) {
  this.name = name;
}
Stooge.prototype.silly = true;
_.allKeys(new Stooge("Moe"));
=> ["name", "silly"]

values _.values(object)

Return all of the values of the object's own properties.

_.values({one: 1, two: 2, three: 3});
=> [1, 2, 3]

mapObject _.mapObject(object, iteratee, [context])

Like map, but for objects. Transform the value of each property in turn.

_.mapObject({start: 5, end: 12}, function(val, key) {
  return val + 5;
});
=> {start: 10, end: 17}

pairs _.pairs(object)

Convert an object into a list of [key, value] pairs. The opposite of object.

_.pairs({one: 1, two: 2, three: 3});
=> [["one", 1], ["two", 2], ["three", 3]]

invert _.invert(object)

Returns a copy of the object where the keys have become the values and the values the keys. For this to work, all of your object's values should be unique and string serializable.

_.invert({Moe: "Moses", Larry: "Louis", Curly: "Jerome"});
=> {Moses: "Moe", Louis: "Larry", Jerome: "Curly"};

create _.create(prototype, props)

Creates a new object with the given prototype, optionally attaching props as own properties. Basically, Object.create, but without all of the property descriptor jazz.

var moe = _.create(Stooge.prototype, {name: "Moe"});

functions _.functions(object) Alias : methods

Returns a sorted list of the names of every method in an object — that is to say, the name of every function property of the object.

_.functions(_);
=> ["all", "any", "bind", "bindAll", "clone", "compact", "compose" ...

findKey _.findKey(object, predicate, [context])

Similar to _.findIndex but for keys in objects. Returns the key where the predicate truth test passes or undefined. predicate is transformed through iteratee to facilitate shorthand syntaxes.

15 .

What is the Updating Objects in Underscore.js?

extend _.extend(destination, *sources)

Shallowly copy all of the properties in the source objects over to the destination object, and return the destination object. Any nested objects or arrays will be copied by reference, not duplicated. It's in-order, so the last source will override properties of the same name in previous arguments.

_.extend({name: 'moe'}, {age: 50});
=> {name: 'moe', age: 50}

extendOwn _.extendOwn(destination, *sources) Alias : assign

Like extend, but only copies own properties over to the destination object.

pick _.pick(object, *keys)

Return a copy of the object, filtered to only have values for the allowed keys (or array of valid keys). Alternatively accepts a predicate indicating which keys to pick.

_.pick({name: 'moe', age: 50, userid: 'moe1'}, 'name', 'age');
=> {name: 'moe', age: 50}
_.pick({name: 'moe', age: 50, userid: 'moe1'}, function(value, key, object) {
  return _.isNumber(value);
});
=> {age: 50}

omit _.omit(object, *keys)

Return a copy of the object, filtered to omit the disallowed keys (or array of keys). Alternatively accepts a predicate indicating which keys to omit.

_.omit({name: 'moe', age: 50, userid: 'moe1'}, 'userid');
=> {name: 'moe', age: 50}
_.omit({name: 'moe', age: 50, userid: 'moe1'}, function(value, key, object) {
  return _.isNumber(value);
});
=> {name: 'moe', userid: 'moe1'}

defaults _.defaults(object, *defaults)

Returns object after filling in its undefined properties with the first value present in the following list of defaults objects.

var iceCream = {flavor: "chocolate"};
_.defaults(iceCream, {flavor: "vanilla", sprinkles: "lots"});
=> {flavor: "chocolate", sprinkles: "lots"}

clone _.clone(object)

Create a shallow-copied clone of the provided plain object. Any nested objects or arrays will be copied by reference, not duplicated.

_.clone({name: 'moe'});
=> {name: 'moe'};

tap _.tap(object, interceptor)

Invokes interceptor with the object, and then returns object. The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.

_.chain([1,2,3,200])
  .filter(function(num) { return num % 2 == 0; })
  .tap(alert)
  .map(function(num) { return num * num })
  .value();
=> // [2, 200] (alerted)
=> [4, 40000]

toPath _.toPath(path)

Ensures that path is an array. If path is a string, it is wrapped in a single-element array; if it is an array already, it is returned unmodified.

_.toPath('key');
=> ['key']
_.toPath(['a', 0, 'b']);
=> ['a', 0, 'b'] // (same array)

_.toPath is used internally in has, get, invoke, property, propertyOf and result, as well as in iteratee and all functions that depend on it, in order to normalize deep property paths. You can override _.toPath if you want to customize this behavior, for example to enable Lodash-like string path shorthands. Be advised that altering _.toPath will unavoidably cause some keys to become unreachable; override at your own risk.

// Support dotted path shorthands.
var originalToPath = _.toPath;
_.mixin({
  toPath: function(path) {
    return _.isString(path) ? path.split('.') : originalToPath(path);
  }
});
_.get({a: [{b: 5}]}, 'a.0.b');
=> 5

get _.get(object, path, [default])

Returns the specified property of object. path may be specified as a simple key, or as an array of object keys or array indexes, for deep property fetching. If the property does not exist or is undefined, the optional default is returned.

_.get({a: 10}, 'a');
=> 10
_.get({a: [{b: 2}]}, ['a', 0, 'b']);
=> 2
_.get({a: 10}, 'b', 100);
=> 100

has _.has(object, key)

Does the object contain the given key? Identical to object.hasOwnProperty(key), but uses a safe reference to the hasOwnProperty function, in case it's been overridden accidentally.

_.has({a: 1, b: 2, c: 3}, "b");
=> true

property _.property(path)

Returns a function that will return the specified property of any passed-in object. path may be specified as a simple key, or as an array of object keys or array indexes, for deep property fetching.

var stooge = {name: 'moe'};
'moe' === _.property('name')(stooge);
=> true

var stooges = {moe: {fears: {worst: 'Spiders'}}, curly: {fears: {worst: 'Moe'}}};
var curlysWorstFear = _.property(['curly', 'fears', 'worst']);
curlysWorstFear(stooges);
=> 'Moe'

propertyOf _.propertyOf(object)

Inverse of _.property. Takes an object and returns a function which will return the value of a provided property.

var stooge = {name: 'moe'};
_.propertyOf(stooge)('name');
=> 'moe'

16 .

Explain some Comparing Objects in Underscore.js?

matcher _.matcher(attrs) Alias: matches

Returns a predicate function that will tell you if a passed in object contains all of the key/value properties present in attrs.

var ready = _.matcher({selected: true, visible: true});
var readyToGoList = _.filter(list, ready);

isEqual _.isEqual(object, other)

Performs an optimized deep comparison between the two objects, to determine if they should be considered equal.

var stooge = {name: 'moe', luckyNumbers: [13, 27, 34]};
var clone  = {name: 'moe', luckyNumbers: [13, 27, 34]};
stooge == clone;
=> false
_.isEqual(stooge, clone);
=> true

isMatch _.isMatch(object, properties)

Tells you if the keys and values in properties are contained in object.

var stooge = {name: 'moe', age: 32};
_.isMatch(stooge, {age: 32});
=> true

isEmpty _.isEmpty(collection)

Returns true if collection has no elements. For strings and array-like objects _.isEmpty checks if the length property is 0. For other objects, it returns true if the object has no enumerable own-properties. Note that primitive numbers, booleans and symbols are always empty by this definition.

_.isEmpty([1, 2, 3]);
=> false
_.isEmpty({});
=> true

isElement _.isElement(object)

Returns true if object is a DOM element.

_.isElement(jQuery('body')[0]);
=> true
â€‹

isArray _.isArray(object)

Returns true if object is an Array.

(function(){ return _.isArray(arguments); })();
=> false
_.isArray([1,2,3]);
=> true

isObject _.isObject(value)

Returns true if value is an Object. Note that JavaScript arrays and functions are objects, while (normal) strings and numbers are not.

_.isObject({});
=> true
_.isObject(1);
=> false

isArguments _.isArguments(object)

Returns true if object is an Arguments object.

(function(){ return _.isArguments(arguments); })(1, 2, 3);
=> true
_.isArguments([1,2,3]);
=> false

isFunction _.isFunction(object)

Returns true if object is a Function.

_.isFunction(alert);
=> true

isString _.isString(object)

Returns true if object is a String.

_.isString("moe");
=> true

isNumber _.isNumber(object)

Returns true if object is a Number (including NaN).

_.isNumber(8.4 * 5);
=> true

isFinite _.isFinite(object)

Returns true if object is a finite Number.

_.isFinite(-101);
=> true

_.isFinite(-Infinity);
=> false

isBoolean _.isBoolean(object)

Returns true if object is either true or false.

_.isBoolean(null);
=> false

isDate _.isDate(object)

Returns true if object is a Date.

_.isDate(new Date());
=> true

isRegExp _.isRegExp(object)

Returns true if object is a RegExp.

_.isRegExp(/moe/);
=> true

isError _.isError(object)

Returns true if object inherits from an Error.

try {
  throw new TypeError("Example");
} catch (o_O) {
  _.isError(o_O);
}
=> true

isSymbol _.isSymbol(object)

Returns true if object is a Symbol.

_.isSymbol(Symbol());
=> true

isMap _.isMap(object)

Returns true if object is a Map.

_.isMap(new Map());
=> true

isWeakMap _.isWeakMap(object)

Returns true if object is a WeakMap.

_.isWeakMap(new WeakMap());
=> true

isSet _.isSet(object)

Returns true if object is a Set.

_.isSet(new Set());
=> true

isWeakSet _.isWeakSet(object)

Returns true if object is a WeakSet.

_.isWeakSet(WeakSet());
=> true

isArrayBuffer _.isArrayBuffer(object)

Returns true if object is an ArrayBuffer.

_.isArrayBuffer(new ArrayBuffer(8));
=> true

isDataView _.isDataView(object)

Returns true if object is a DataView.

_.isDataView(new DataView(new ArrayBuffer(8)));
=> true

isTypedArray _.isTypedArray(object)

Returns true if object is a TypedArray.

_.isTypedArray(new Int8Array(8));
=> true

isNaN _.isNaN(object)

Returns true if object is NaN.

Note : this is not the same as the native isNaN function, which will also return true for many other not-number values, such as undefined.

_.isNaN(NaN);
=> true
isNaN(undefined);
=> true
_.isNaN(undefined);
=> false

isNull _.isNull(object)

Returns true if the value of object is null.

_.isNull(null);
=> true
_.isNull(undefined);
=> false

isUndefined _.isUndefined(value)

Returns true if value is undefined.

_.isUndefined(window.missingVariable);
=> true

17 .

Explain some Utility Functions in Underscore.js?

noConflict _.noConflict()

Give control of the global _ variable back to its previous owner. Returns a reference to the Underscore object.

var underscore = _.noConflict();

The _.noConflict function is not present if you use the EcmaScript 6, AMD or CommonJS module system to import Underscore.

identity _.identity(value)

Returns the same value that is used as the argument. In math: f(x) = x

This function looks useless, but is used throughout Underscore as a default iteratee.

var stooge = {name: 'moe'};
stooge === _.identity(stooge);
=> true

constant _.constant(value)

Creates a function that returns the same value that is used as the argument of _.constant.

var stooge = {name: 'moe'};
stooge === _.constant(stooge)();
=> true

noop _.noop()

Returns undefined irrespective of the arguments passed to it. Useful as the default for optional callback arguments.

obj.initialize = _.noop;â€‹

times _.times(n, iteratee, [context])

Invokes the given iteratee function n times. Each invocation of iteratee is called with an index argument. Produces an array of the returned values.

_.times(3, function(n){ genie.grantWishNumber(n); });â€‹

random _.random(min, max)

Returns a random integer between min and max, inclusive. If you only pass one argument, it will return a number between 0 and that number.

_.random(0, 100);
=> 42

mixin _.mixin(object)

Allows you to extend Underscore with your own utility functions. Pass a hash of {name: function} definitions to have your functions added to the Underscore object, as well as the OOP wrapper. Returns the Underscore object to facilitate chaining.

_.mixin({
  capitalize: function(string) {
    return string.charAt(0).toUpperCase() + string.substring(1).toLowerCase();
  }
});
_("fabio").capitalize();
=> "Fabio"

uniqueId _.uniqueId([prefix])

Generate a globally-unique id for client-side models or DOM elements that need one. If prefix is passed, the id will be appended to it.

_.uniqueId('contact_');
=> 'contact_104'

escape _.escape(string)

Escapes a string for insertion into HTML, replacing &, <, >, ", `, and ' characters.

_.escape('Curly, Larry & Moe');
=> "Curly, Larry &amp; Moe"

unescape _.unescape(string)

The opposite of escape, replaces &, <, >, ", ` and ' with their unescaped counterparts.

_.unescape('Curly, Larry &amp; Moe');
=> "Curly, Larry & Moe"

result _.result(object, property, [defaultValue])

If the value of the named property is a function then invoke it with the object as context; otherwise, return it. If a default value is provided and the property doesn't exist or is undefined then the default will be returned. If defaultValue is a function its result will be returned.

var object = {cheese: 'crumpets', stuff: function(){ return 'nonsense'; }};
_.result(object, 'cheese');
=> "crumpets"
_.result(object, 'stuff');
=> "nonsense"
_.result(object, 'meat', 'ham');
=> "ham"

now _.now()

Returns an integer timestamp for the current time, using the fastest method available in the runtime. Useful for implementing timing/animation functions.

_.now();
=> 1392066795351

template _.template(templateString, [settings])

Compiles JavaScript templates into functions that can be evaluated for rendering. Useful for rendering complicated bits of HTML from JSON data sources. Template functions can both interpolate values, using <%= … %>, as well as execute arbitrary JavaScript code, with <% … %>. If you wish to interpolate a value, and have it be HTML-escaped, use <%- … %>. When you evaluate a template function, pass in a data object that has properties corresponding to the template's free variables. The settings argument should be a hash containing any _.templateSettings that should be overridden.

var compiled = _.template("hello: <%= name %>");
compiled({name: 'moe'});
=> "hello: moe"

var template = _.template("<b><%- value %></b>");
template({value: '<script>'});
=> "<b>&lt;script&gt;</b>"

You can also use print from within JavaScript code. This is sometimes more convenient than using <%= ... %>.

var compiled = _.template("<% print('Hello ' + epithet); %>");
compiled({epithet: "stooge"});
=> "Hello stooge"

_.templateSettings = {
  interpolate: /\{\{(.+?)\}\}/g
};

var template = _.template("Hello {{ name }}!");
template({name: "Mustache"});
=> "Hello Mustache!"

By default, template places the values from your data in the local scope via the with statement. However, you can specify a single variable name with the variable setting. This can significantly improve the speed at which a template is able to render.

_.template("Using 'with': <%= data.answer %>", {variable: 'data'})({answer: 'no'});
=> "Using 'with': no"

Precompiling your templates can be a big help when debugging errors you can't reproduce. This is because precompiled templates can provide line numbers and a stack trace, something that is not possible when compiling templates on the client. The source property is available on the compiled template function for easy precompilation.

<script>
  JST.project = <%= _.template(jstText).source %>;
</script>