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Array

Introduction

JavaScript arrays are a specialized form of JavaScript object, and array indexes are really little more than property names that happen to be integers. Arrays inherit properties from Array.prototype, which defines a rich set of array manipulation methods.

Creating Array

There are several ways to create arrays. The subsections that follow explain how to create arrays with:

  • Array literals
  • The spread operator
  • The Array() constructor
  • The Array.of() and Array.from() factory method

Array literals

The simplest way to create an array is with an array literal, which is simply a comma-separated list of array elements within square brackets. For example:

let empty = []; // An array with no elements
let primes = [2, 3, 5, 7, 11]; // An array with 5 numeric elements
let misc = [ 1.1, true, "a", ]; // 3 elements of various types + trailing comma

The values in an array literal need not be constants; they may be arbitrary expressions:

let base = 1024;
let table = [base, base+1, base+2, base+3];

Array literals can contain object literals or other array literals:

let b = [[1, {x: 1, y: 2}], [2, {x: 3, y: 4}]];

If an array literal contains multiple commas in a row, with no value between, the array is sparse. Array elements for which values are omitted do not exist but appear to be undefined if you query them:

let count = [1,,3]; // Elements at indexes 0 and 2. No element at index 1
let undefs = [,,]; // An array with no elements but a length of 2

Array literal syntax allows an optional trailing comma, so [,,] has a length of 2, not 3.

The spread operator

In ES6 and later, you can use the “spread operator,” …, to include the elements of one array within an array literal:

let a = [1, 2, 3];
let b = [0, ...a, 4]; // b == [0, 1, 2, 3, 4]

The three dots “spread” the array a so that its elements become elements within the array literal that is being created. It is as if the …a was replaced by the elements of the array a, listed literally as part of the enclosing array literal. (Note that, although we call these three dots a spread operator, this is not a true operator because it can only be used in array literals and, as we’ll see later in the book, function invocations.) The spread operator is a convenient way to create a (shallow) copy of an array:

let original = [1,2,3];
let copy = [...original];
copy[0] = 0; // Modifying the copy does not change the original
original[0] // => 1

The spread operator works on any iterable object. (Iterable objects are what the for/of loop iterates over; we first saw them in §5.4.4, and we’ll see much more about them in Chapter 12.) Strings are iterable, so you can use a spread operator to turn any string into an array of single-character strings:

let digits = [..."0123456789ABCDEF"];
digits // => ["0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F"]

Set objects are iterable, so an easy way to remove duplicate elements from an array is to convert the array to a set and then immediately convert the set back to an array using the spread operator:

let letters = [..."hello world"];
[...new Set(letters)] // => ["h","e","l","o"," ","w","r","d"]

The Array() constructor

Another way to create an array is with the Array() constructor. You can invoke this constructor in three distinct ways:

Call it with no arguments:

let a = new Array();

This method creates an empty array with no elements and is equivalent to the array literal [].

Call it with a single numeric argument, which specifies a length:

let a = new Array(10);

This technique creates an array with the specified length. This form of the Array() constructor can be used to preallocate an array when you know in advance how many elements will be required. Note that no values are stored in the array, and the array index properties “0”, “1”, and so on are not even defined for the array.

Explicitly specify two or more array elements or a single non-numeric element for the array:

let a = new Array(5, 4, 3, 2, 1, "testing, testing");

In this form, the constructor arguments become the elements of the new array. Using an array literal is almost always simpler than this usage of the Array() constructor.

The Array.of() factory method

When the Array() constructor function is invoked with one numeric argument, it uses that argument as an array length. But when invoked with more than one numeric argument, it treats those arguments as elements for the array to be created. This means that the Array() constructor cannot be used to create an array with a single numeric element.

In ES6, the Array.of() function addresses this problem: it is a factory method that creates and returns a new array, using its argument values (regardless of how many of them there are) as the array elements:

Array.of()// => []; returns empty array with no arguments
Array.of(10) // => [10]; can create arrays with a single numeric argument
Array.of(1,2,3) // => [1, 2, 3]

The Array.from() factory method

Array.from is another array factory method introduced in ES6. It expects an iterable or array-like object as its first argument and returns a new array that contains the elements of that object. With an iterable argument, Array.from(iterable) works like the spread operator […iterable] does. It is also a simple way to make a copy of an array:

let copy = Array.from(original);

Array.from() is also important because it defines a way to make a true-array copy of an array-like object. Array-like objects are non-array objects that have a numeric length property and have values stored with properties whose names happen to be integers. When working with client-side JavaScript, the return values of some web browser methods are array-like, and it can be easier to work with them if you first convert them to true arrays:

Reading and Writing Array Element

ou access an element of an array using the [] operator. A reference to the array should appear to the left of the brackets. An arbitrary expression that has a nonnegative integer value should be inside the brackets. You can use this syntax to both read and write the value of an element of an array. Thus, the following are all legal JavaScript statements:

let a = ["world"]; // Start with a one-element array
let value = a[0]; // Read element 0
a[1] = 3.14; // Write element 1
let i = 2;
a[i] = 3; // Write element 2
a[i + 1] = "hello"; // Write element 3
a[a[i]] = a[0]; // Read elements 0 and 2, write element 3

What is special about arrays is that when you use property names that are nonnegative integers less than 232–1, the array automatically maintains the value of the length property for you. In the preceding, for example, we created an array a with a single element. We then assigned values at indexes 1, 2, and 3. The length property of the array changed as we did, so:

a.length // => 4

Remember that arrays are a specialized kind of object. The square brackets used to access array elements work just like the square brackets used to access object properties. JavaScript converts the numeric array index you specify to a string—the index 1 becomes the string “1”—then uses that string as a property name. There is nothing special about the conversion of the index from a number to a string: you can do that with regular objects, too:

let o = {}; // Create a plain object
o[1] = "one"; // Index it with an integer
o["1"] // => "one"; numeric and string property names are the same

It is helpful to clearly distinguish an array index from an object property name. All indexes are property names, but only property names that are integers between 0 and 232–2 are indexes. All arrays are objects, and you can create properties of any name on them. If you use properties that are array indexes, however, arrays have the special behavior of updating their length property as needed.
Note that you can index an array using numbers that are negative or that are not integers. When you do this, the number is converted to a string, and that string is used as the property name. Since the name is not a non-negative integer, it is treated as a regular object property, not an array index. Also, if you index an array with a string that happens to be a non-negative integer, it behaves as an array index, not an object property. The same is true if you use a floating-point number that is the same as an integer:

a[-1.23] = true; // This creates a property named "-1.23"
a["1000"] = 0;// This the 1001st element of the array
a[1.000] = 1; // Array index 1. Same as a[1] = 1;

The fact that array indexes are simply a special type of object property name means that JavaScript arrays have no notion of an “out of bounds” error. When you try to query a nonexistent property of any object, you don’t get an error; you simply get undefined. This is just as true for arrays as it is for objects:

let a = [true, false]; // This array has elements at indexes 0 and 1
a[2] // => undefined; no element at this index.
a[-1] // => undefined; no property with this name.

Array Methods

forEach()

The forEach() method iterates through an array, invoking a function you specify for each element. As we’ve described, you pass the function as the first argument to forEach(). forEach() then invokes your function with three arguments: the value of the array element, the index of the array element, and the array itself. If you only care about the value of the array element, you can write a function with only one parameter the additional arguments will be ignored:

let data = [1,2,3,4,5], sum = 0;

// Compute the sum of the elements of the array
data.forEach(value => { 
  sum += value; 
}); // sum == 15

// Now increment each array element
data.forEach(function(v, i, a) { 
  a[i] = v + 1; 
}); // data == [2,3,4,5,6]

map()

The map() method passes each element of the array on which it is invoked to the function you specify and returns an array containing the values returned by your function. For example:

let a = [1, 2, 3];
a.map(x => x*x) // => [1, 4, 9]: the function takes input x and returns x*x

The function you pass to map() is invoked in the same way as a function passed to forEach(). For the map() method, however, the function you pass should return a value. Note that map() returns a new array. t does not modify the array it is invoked on.

filter()

The filter() method returns an array containing a subset of the elements of the array on which it is invoked. The function you pass to it should be predicate: a function that returns true or false. The predicate is invoked just as for forEach() and map(). If the return value is true, or a value that converts to true, then the element passed to the predicate is a member of the subset and is added to the array that will become the return value. Examples:

let a = [5, 4, 3, 2, 1];
a.filter(x => x < 3) // => [2, 1]; values less than 3
a.filter((x,i) => i%2 === 0) // => [5, 3, 1]; every other value

Note that filter() skips missing elements in sparse arrays and that its return value is always dense. To close the gaps in a sparse array, you can do this:

let dense = sparse.filter(() => true);

And to close gaps and remove undefined and null elements, you can use filter, like this:

a = a.filter(x => x !== undefined && x !== null);

find() and findIndex()

The find() and findIndex() methods are like filter() in that they iterate through your array looking for elements for which your predicate function returns a truthy value. Unlike filter(), however, these two methods stop iterating the first time the predicate finds an element. When that happens, find() returns the matching element, and findIndex() returns the index of the matching element. If no matching element is found, find() returns undefined and findIndex() returns -1:

let a = [1,2,3,4,5];
a.findIndex(x => x === 3) // => 2; the value 3 appears at index 2
a.findIndex(x => x < 0) // => -1; no negative numbers in the array
a.find(x => x % 5 === 0) // => 5: this is a multiple of 5
a.find(x => x % 7 === 0) // => undefined: no multiples of 7 in the array

every() and some()

The every() and some() methods are array predicates: they apply a predicate function you specify to the elements of the array, then return true or false. The every() method is like the mathematical “for all” quantifier ∀: it returns true if and only if your predicate function returns true for all elements in the array:

let a = [1,2,3,4,5];
a.every(x => x < 10) // => true: all values are < 10.
a.every(x => x % 2 === 0) // => false: not all values are even.

The some() method is like the mathematical “there exists” quantifier ∃: it returns true if there exists at least one element in the array for which the predicate returns true and returns false if and only if the predicate returns false for all elements of the array:

let a = [1,2,3,4,5];
a.some(x => x%2===0) // => true; a has some even numbers.
a.some(isNaN) // => false; a has no non-numbers.

Note that both every() and some() stop iterating array elements as soon as they know what value to return. some() returns true the first time your predicate returns <code>true</code> and only iterates through the entire array if your predicate always returns false. every() is the opposite: it returns false the first time your predicate returns false and only iterates all elements if your predicate always returns true. Note also that, by mathematical convention, every() returns true and some returns false when invoked on an empty array.

reduce() and reduceRight()

The reduce() and reduceRight() methods combine the elements of an array, using the function you specify, to produce a single value. This is a common operation in functional programming and also goes by the names “inject” and “fold.” Examples help illustrate how it works:

let a = [1,2,3,4,5];
a.reduce((x,y) => x+y, 0); // => 15; the sum of the values 
a.reduce((x,y) => x*y, 1) // => 120; the product of the values
a.reduce((x,y) => (x > y) ? x : y) // => 5; the largest of the values reduce() takes two arguments. 

The first is the function that performs the reduction operation. The task of this reduction function is to somehow combine or reduce two values into a single value and to return that reduced value. In the examples we’ve shown here, the functions combine two values by adding them, multiplying them,
and choosing the largest. The second (optional) argument is an initial value to pass to the function. Functions used with reduce() are different than the functions used with forEach() and map().

flat() and flatMap()

The flat() method creates and returns a new array that contains the same elements as the array it is called on, except that any elements that are themselves arrays are “flattened” into the returned array. For example:

[1, [2, 3]].flat() // => [1, 2, 3]
[1, [2, [3]]].flat() // => [1, 2, [3]]

When called with no arguments, flat() flattens one level of nesting. Elements of the original array that are themselves arrays are flattened, but array elements of those arrays are not flattened. If you want to flatten more levels, pass a number to flat():

let a = [1, [2, [3, [4]]]];
a.flat(1) // => [1, 2, [3, [4]]]
a.flat(2) // => [1, 2, 3, [4]]
a.flat(3) // => [1, 2, 3, 4]
a.flat(4) // => [1, 2, 3, 4]

The flatMap() method works just like the map() method except that the returned array is automatically flattened as if passed to flat(). That is, calling a.flatMap(f) is the same as (but more efficient than) a.map(f).flat():

let phrases = ["hello world", "the definitive guide"];
let words = phrases.flatMap(phrase => phrase.split(" "));
words // => ["hello", "world", "the", "definitive", "guide"];

You can think of flatMap() as a generalization of map() that allows each element of the input array to map to any number of elements of the output array. In particular, flatMap() allows you to map input elements to an empty array, which flattens to nothing in the output array:

// Map non-negative numbers to their square roots
[-2, -1, 1, 2].flatMap(x => x < 0 ? [] : Math.sqrt(x)) // => [1, 2**0.5]

concat()

The concat() method creates and returns a new array that contains the elements of the original array on which concat() was invoked, followed by each of the arguments to concat(). If any of these arguments is itself an array, then it is the array elements that are concatenated, not the array itself. Note, however, that concat() does not recursively flatten arrays of arrays. concat() does not modify the array on which it is invoked:

let a = [1,2,3];
a.concat(4, 5)// => [1,2,3,4,5]
a.concat([4,5],[6,7]) // => [1,2,3,4,5,6,7]; arrays are flattened
a.concat(4, [5,[6,7]]) // => [1,2,3,4,5,[6,7]]; but not nested arrays
a // => [1,2,3]; the original array is unmodified

Note that concat() makes a new copy of the array it is called on. In many cases, this is the right thing to do, but it is an expensive operation. If you find yourself writing code like a = a.concat(x), then you should think about modifying your array in place with push() or splice() instead of creating a new one.

push() and pop()

The push() and pop() methods allow you to work with arrays as if they were stacks. The push() method appends one or more new elements to the end of an array and returns the new length of the array. Unlike concat(), push() does not flatten array arguments. The pop() method does the reverse: it deletes the last element of an array, decrements the array length, and returns the value that it removed.

let stack = []; // stack == []
stack.push(1,4); // stack == [1,4];
stack.pop(); // stack == [1]; returns 4
stack.push(9); // stack == [1,9]
stack.pop(); // stack == [1]; returns 9
stack.push([4,5]); // stack == [1,[4,5]]
stack.pop() // stack == [1]; returns [4,5]
stack.pop(); // stack == []; returns 1

shift() and unshift()

The unshift() and shift() methods behave much like push() and pop(), except that they insert and remove elements from the beginning of an array rather than from the end. unshift() adds an element or elements to the beginning of the array, shifts the existing array elements up to higher indexes to make room, and returns the new length of the array. shift() removes and returns the first element of the array, shifting all subsequent elements down one place to occupy the newly vacant space at the start of the array. You could use unshift() and shift() to implement a stack, but it would be less efficient than using push() and pop() because the array elements need to be shifted up or down every time an element is added or removed at the start of the array. Instead, though, you can implement a queue data structure by using push() to add elements at the end of an array and shift() to remove them from the start of the array.

let q = []; // q == []
q.push(1,2); // q == [1,2]
q.shift(); // q == [2]; returns 1
q.push(3) // q == [2, 3]
q.shift() // q == [3]; returns 2
q.shift() // q == []; returns 3

When passing multiple arguments to unshift(), they are inserted all at once, which means that they end up in the array in a different order than they would be if you inserted them one at a time:

let a = []; // a == []
a.unshift(1) // a == [1]
a.unshift(2) // a == [2, 1]
a = []; // a == []
a.unshift(1,2) // a == [1, 2]

slice()

The slice() method returns a slice, or subarray, of the specified array. Its two arguments specify the start and end of the slice to be returned. The returned array contains the element specified by the first argument and all subsequent elements up to, but not including, the element specified by the second argument. If only one argument is specified, the returned array contains all elements from the start position to the end of the array. If either argument is negative, it specifies an array element relative to the length of the array. An argument of –1, for example, specifies the last element in the array, and an argument of –2 specifies the element before that one. Note that slice() does not modify the array on which it is invoked.

let a = [1,2,3,4,5];
a.slice(0,3); // Returns [1,2,3]
a.slice(3); // Returns [4,5]
a.slice(1,-1); // Returns [2,3,4]
a.slice(-3,-2); // Returns [3]

splice()

splice() is a general-purpose method for inserting or removing elements from an array. Unlike slice() and concat(), splice() modifies the array on which it is invoked. Note that splice() and slice() have very similar names but perform substantially different operations. splice() can delete elements from an array, insert new elements into an array, or perform both operations at the same time. Elements of the array that come after the insertion or deletion point have their indexes increased or decreased as necessary so that they remain contiguous with the rest of the array. The first argument to splice() specifies the array position at which the insertion and/or deletion is to begin. The second argument specifies the number of elements that should be deleted from (spliced out of) the array. (Note that this is another difference between these two methods. The second argument to slice() is an end position. The second argument to splice() is a length.) If this second argument is omitted, all array elements from the start element to the end of the array are removed. splice() returns an array of the deleted elements, or an empty array if no elements were deleted.

let a = [1,2,3,4,5,6,7,8];
a.splice(4) // => [5,6,7,8]; a is now [1,2,3,4]
a.splice(1,2) // => [2,3]; a is now [1,4]
a.splice(1,1) // => [4]; a is now [1]
let a = [1,2,3,4,5];
a.splice(2,0,"a","b") // => []; a is now [1,2,"a","b",3,4,5]
a.splice(2,2,[1,2],3) // => ["a","b"]; a is now [1,2,[1,2],3,3,4,5]

fill()

The fill() method sets the elements of an array, or a slice of an array, to a specified value. It mutates the array it is called on, and also returns the modified array:

let a = new Array(5); // Start with no elements and length 5
a.fill(0) // => [0,0,0,0,0]; fill the array with zeros
a.fill(9, 1) // => [0,9,9,9,9]; fill with 9 starting at index 1
a.fill(8, 2, -1) // => [0,9,8,8,9]; fill with 8 at indexes 2, 3

The first argument to fill() is the value to set array elements to. The optional second argument specifies the starting index. If omitted, filling starts at index 0. The optional third argument specifies the ending index—array elements up to, but not including, this index will be filled. If this argument is omitted, then the array is filled from the start index to the end. You can specify indexes relative to the end of the array by passing negative numbers, just as you can for slice().

copyWithin()

copyWithin() copies a slice of an array to a new position within the array. It modifies the array in place and returns the modified array, but it will not change the length of the array. The first argument specifies the destination index to which the first element will be copied. The second argument specifies the index of the first element to be copied. If this second argument is omitted, 0 is used. The third argument specifies the end of the slice of elements to be copied. If omitted, the length of the array is used. Elements from the start index up to, but not including, the end index will be copied. You can specify indexes relative to the end of the array by passing negative numbers, just as you can for slice():

let a = [1,2,3,4,5];
a.copyWithin(1) // => [1,1,2,3,4]: copy array elements up one
a.copyWithin(2, 3, 5) // => [1,1,3,4,4]: copy last 2 elements to index 2
a.copyWithin(0, -2) // => [4,4,3,4,4]: negative offsets work, too

indexOf() and lastIndexOf()

indexOf() and lastIndexOf() search an array for an element with a specified value and return the index of the first such element found, or -1 if none is found. indexOf() searches the array from beginning to end, and lastIndexOf() searches from end to beginning.

let a = [0,1,2,1,0];
a.indexOf(1) // => 1: a[1] is 1
a.lastIndexOf(1) // => 3: a[3] is 1
a.indexOf(3) // => -1: no element has value 3

indexOf() and lastIndexOf() compare their argument to the array elements using the equivalent of the === operator. If your array contains objects instead of primitive values, these methods check to see if two references both refer to exactly the same object. If you want to actually look at the content of an object, try using the find() method with your own custom predicate function instead.

indexOf() and lastIndexOf() take an optional second argument that specifies the array index at which to begin the search. If this argument is omitted, indexOf() starts at the beginning and lastIndexOf() starts at the end. Negative values are allowed for the second argument and are treated as an offset from the end of the array, as they are for the slice() method: a value of –1, for example, specifies the last element of the array.

The following function searches an array for a specified value and returns an array of all matching indexes. This demonstrates how the second argument to indexOf() can be used to find matches beyond the first.

// Find all occurrences of a value x in an array a and return an array
// of matching indexes
function findall(a, x) {
   let results = [], // The array of indexes we'll return
   len = a.length, // The length of the array to be searched
   pos = 0; // The position to search from
   while(pos < len) { // While more elements to search...
      pos = a.indexOf(x, pos); // Search
      if (pos === -1) break; // If nothing found, we're done.
      results.push(pos); // Otherwise, store index in array
      pos = pos + 1; // And start next search at next element
   }
   return results; // Return array of indexes
}

includes()

The includes() method is slightly different than the indexOf() method in one important way. indexOf() tests equality using the same algorithm that the === operator does, and that equality algorithm considers the not-a-number value to be different from every other value, including itself. includes() uses a slightly different version of equality that does consider NaN to be equal to itself. This means that indexOf() will not detect the NaN value in an array, but includes() will:

let a = [1,true,3,NaN];
a.includes(true) // => true
a.includes(2) // => false
a.includes(NaN) // => true
a.indexOf(NaN) // => -1; indexOf can't find NaN

sort()

sort() sorts the elements of an array in place and returns the sorted array. When sort() is called with no arguments, it sorts the array elements in alphabetical order (temporarily converting them to strings to perform the comparison, if necessary). If an array contains undefined elements, they are sorted to the end of the array. To sort an array into some order other than alphabetical, you must pass a comparison function as an argument to sort().

let a = ["banana", "cherry", "apple"];
a.sort(); // a == ["apple", "banana", "cherry"]

This function decides which of its two arguments should appear first in the sorted array. If the first argument should appear before the second, the comparison function should return a number less than zero. If the first argument should appear after the second in the sorted array, the function should return a number greater than zero. And if the two values are equivalent (i.e., if their order is irrelevant), the comparison function should return 0. So, for example, to sort array elements into numerical rather than alphabetical order, you might do this.

let a = [33, 4, 1111, 222];
a.sort(); // a == [1111, 222, 33, 4]; alphabetical order
a.sort(function(a,b) { // Pass a comparator function
   return a-b; // Returns < 0, 0, or > 0, depending on order
}); // a == [4, 33, 222, 1111]; numerical order
a.sort((a,b) => b-a); // a == [1111, 222, 33, 4]; reverse numerical order

You might perform a case-insensitive alphabetical sort on an array of strings by passing a comparison function that converts both of its arguments to lowercase (with the toLowerCase() method) before comparing them.

let a = ["ant", "Bug", "cat", "Dog"];
a.sort(); // a == ["Bug","Dog","ant","cat"]; case-sensitive sort
a.sort(function(s,t) {
   let a = s.toLowerCase();
   let b = t.toLowerCase();
   if (a < b) return -1;
   if (a > b) return 1;
   return 0;
}); // a == ["ant","Bug","cat","Dog"]; case-insensitive sort

reverse()

The reverse() method reverses the order of the elements of an array and returns the reversed array. It does this in place; in other words, it doesn’t create a new array with the elements rearranged but instead rearranges them in the already existing array.

let a = [1,2,3];
a.reverse(); // a == [3,2,1]
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