In JavaScript, arrays aren'tprimitives but are insteadArray objects with the following core characteristics:

• JavaScript arrays are resizable andcan contain a mix of differentdata types. (When those characteristics are undesirable, usetyped arrays instead.)
• JavaScript arrays are not associative arrays and so, array elements cannot be accessed using arbitrary strings as indexes, but must be accessed using nonnegative integers (or their respective string form) as indexes.
• JavaScript arrays arezero-indexed: the first element of an array is at index0, the second is at index1, and so on — and the last element is at the value of the array'slength property minus1.
• JavaScriptarray-copy operations createshallow copies. (All standard built-in copy operations withany JavaScript objects create shallow copies, rather thandeep copies).

Array objects cannot use arbitrary strings as element indexes (as in anassociative array) but must use nonnegative integers (or their respective string form). Setting or accessing via non-integers will not set or retrieve an element from the array list itself, but will set or access a variable associated with that array'sobject property collection. The array's object properties and list of array elements are separate, and the array'straversal and mutation operations cannot be applied to these named properties.

Array elements are object properties in the same way thattoString is a property (to be specific, however,toString() is a method). Nevertheless, trying to access an element of an array as follows throws a syntax error because the property name is not valid:

arr.0; // a syntax error

JavaScript syntax requires properties beginning with a digit to be accessed usingbracket notation instead ofdot notation. It's also possible to quote the array indices (e.g.,years['2'] instead ofyears[2]), although usually not necessary.

The2 inyears[2] is coerced into a string by the JavaScript engine through an implicittoString conversion. As a result,'2' and'02' would refer to two different slots on theyears object, and the following example could betrue:

console.log(years["2"] !== years["02"]);

Onlyyears['2'] is an actual array index.years['02'] is an arbitrary string property that will not be visited in array iteration.

A JavaScript array'slength property and numerical properties are connected.

Several of the built-in array methods (e.g.,join(),slice(),indexOf(), etc.) take into account the value of an array'slength property when they're called.

Other methods (e.g.,push(),splice(), etc.) also result in updates to an array'slength property.

const fruits = [];fruits.push("banana", "apple", "peach");console.log(fruits.length); // 3

When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the engine will update the array'slength property accordingly:

fruits[5] = "mango";console.log(fruits[5]); // 'mango'console.log(Object.keys(fruits)); // ['0', '1', '2', '5']console.log(fruits.length); // 6

Increasing thelength extends the array by adding empty slots without creating any new elements — not evenundefined.

fruits.length = 10;console.log(fruits); // ['banana', 'apple', 'peach', empty x 2, 'mango', empty x 4]console.log(Object.keys(fruits)); // ['0', '1', '2', '5']console.log(fruits.length); // 10console.log(fruits[8]); // undefined

Decreasing thelength property does, however, delete elements.

fruits.length = 2;console.log(Object.keys(fruits)); // ['0', '1']console.log(fruits.length); // 2

This is explained further on thelength page.

Array methods have different behaviors when encountering empty slots insparse arrays. In general, older methods (e.g.,forEach) treat empty slots differently from indices that containundefined.

Methods that have special treatment for empty slots include the following:concat(),copyWithin(),every(),filter(),flat(),flatMap(),forEach(),indexOf(),lastIndexOf(),map(),reduce(),reduceRight(),reverse(),slice(),some(),sort(), andsplice(). Iteration methods such asforEach don't visit empty slots at all. Other methods, such asconcat,copyWithin, etc., preserve empty slots when doing the copying, so in the end the array is still sparse.

const colors = ["red", "yellow", "blue"];colors[5] = "purple";colors.forEach((item, index) => {  console.log(`${index}: ${item}`);});// Output:// 0: red// 1: yellow// 2: blue// 5: purplecolors.reverse(); // ['purple', empty × 2, 'blue', 'yellow', 'red']

Newer methods (e.g.,keys) do not treat empty slots specially and treat them as if they containundefined. Methods that conflate empty slots withundefined elements include the following:entries(),fill(),find(),findIndex(),findLast(),findLastIndex(),includes(),join(),keys(),toLocaleString(),toReversed(),toSorted(),toSpliced(),values(), andwith().

const colors = ["red", "yellow", "blue"];colors[5] = "purple";const iterator = colors.keys();for (const key of iterator) {  console.log(`${key}: ${colors[key]}`);}// Output// 0: red// 1: yellow// 2: blue// 3: undefined// 4: undefined// 5: purpleconst newColors = colors.toReversed(); // ['purple', undefined, undefined, 'blue', 'yellow', 'red']

Some methods do not mutate the existing array that the method was called on, but instead return a new array. They do so by first constructing a new array and then populating it with elements. The copy always happensshallowly — the method never copies anything beyond the initially created array. Elements of the original array(s) are copied into the new array as follows:

• Objects: the object reference is copied into the new array. Both the original and new array refer to the same object. That is, if a referenced object is modified, the changes are visible to both the new and original arrays.
• Primitive types such as strings, numbers and booleans (notString,Number, andBoolean objects): their values are copied into the new array.

Other methods mutate the array that the method was called on, in which case their return value differs depending on the method: sometimes a reference to the same array, sometimes the length of the new array.

The following methods create new arrays by accessingthis.constructor[Symbol.species] to determine the constructor to use:concat(),filter(),flat(),flatMap(),map(),slice(), andsplice() (to construct the array of removed elements that's returned).

The following methods always create new arrays with theArray base constructor:toReversed(),toSorted(),toSpliced(), andwith().

The following table lists the methods that mutate the original array, and the corresponding non-mutating alternative:

An easy way to change a mutating method into a non-mutating alternative is to use thespread syntax orslice() to create a copy first:

arr.copyWithin(0, 1, 2); // mutates arrconst arr2 = arr.slice().copyWithin(0, 1, 2); // does not mutate arrconst arr3 = [...arr].copyWithin(0, 1, 2); // does not mutate arr

Many array methods take a callback function as an argument. The callback function is called sequentially and at most once for each element in the array, and the return value of the callback function is used to determine the return value of the method. They all share the same signature:

method(callbackFn, thisArg)

WherecallbackFn takes three arguments:

element

The current element being processed in the array.

index

The index of the current element being processed in the array.

array

The array that the method was called upon.

WhatcallbackFn is expected to return depends on the array method that was called.

ThethisArg argument (defaults toundefined) will be used as thethis value when callingcallbackFn. Thethis value ultimately observable bycallbackFn is determined according tothe usual rules: ifcallbackFn isnon-strict, primitivethis values are wrapped into objects, andundefined/null is substituted withglobalThis. ThethisArg argument is irrelevant for anycallbackFn defined with anarrow function, as arrow functions don't have their ownthisbinding.

Thearray argument passed tocallbackFn is most useful if you want to read another index during iteration, because you may not always have an existing variable that refers to the current array. You should generally not mutate the array during iteration (seemutating initial array in iterative methods), but you can also use this argument to do so. Thearray argument isnot the array that is being built, in the case of methods likemap(),filter(), andflatMap() — there is no way to access the array being built from the callback function.

All iterative methods arecopying andgeneric, although they behave differently withempty slots.

The following methods are iterative:every(),filter(),find(),findIndex(),findLast(),findLastIndex(),flatMap(),forEach(),map(), andsome().

In particular,every(),find(),findIndex(),findLast(),findLastIndex(), andsome() do not always invokecallbackFn on every element — they stop iteration as soon as the return value is determined.

Thereduce() andreduceRight() methods also take a callback function and run it at most once for each element in the array, but they have slightly different signatures from typical iterative methods (for example, they don't acceptthisArg).

Thesort() method also takes a callback function, but it is not an iterative method. It mutates the array in-place, doesn't acceptthisArg, and may invoke the callback multiple times on an index.

Iterative methods iterate the array like the following (with a lot of technical details omitted):

function method(callbackFn, thisArg) {  const length = this.length;  for (let i = 0; i < length; i++) {    if (i in this) {      const result = callbackFn.call(thisArg, this[i], i, this);      // Do something with result; maybe return early    }  }}

Note the following:

1. Not all methods do thei in this test. Thefind,findIndex,findLast, andfindLastIndex methods do not, but other methods do.
2. Thelength is memorized before the loop starts. This affects how insertions and deletions during iteration are handled (seemutating initial array in iterative methods).
3. The method doesn't memorize the array contents, so if any index is modified during iteration, the new value might be observed.
4. The code above iterates the array in ascending order of index. Some methods iterate in descending order of index (for (let i = length - 1; i >= 0; i--)):reduceRight(),findLast(), andfindLastIndex().
5. reduce andreduceRight have slightly different signatures and do not always start at the first/last element.

Array methods are always generic — they don't access any internal data of the array object. They only access the array elements through thelength property and the indexed elements. This means that they can be called on array-like objects as well.

const arrayLike = {  0: "a",  1: "b",  length: 2,};console.log(Array.prototype.join.call(arrayLike, "+")); // 'a+b'

Normalization of the length property

Thelength property isconverted to an integer and then clamped to the range between 0 and 2⁵³ - 1.NaN becomes0, so even whenlength is not present or isundefined, it behaves as if it has value0.

The language avoids settinglength to anunsafe integer. All built-in methods will throw aTypeError iflength will be set to a number greater than 2⁵³ - 1. However, because thelength property of arrays throws an error if it's set to greater than 2³² - 1, the safe integer threshold is usually not reached unless the method is called on a non-array object.

Array.prototype.flat.call({}); // []

Some array methods set thelength property of the array object. They always set the value after normalization, solength always ends as an integer.

const a = { length: 0.7 };Array.prototype.push.call(a);console.log(a.length); // 0

Array-like objects

The termarray-like object refers to any object that doesn't throw during thelength conversion process described above. In practice, such object is expected to actually have alength property and to have indexed elements in the range0 tolength - 1. (If it doesn't have all indices, it will be functionally equivalent to asparse array.) Any integer index less than zero or greater thanlength - 1 is ignored when an array method operates on an array-like object.

Many DOM objects are array-like — for example,NodeList andHTMLCollection. Thearguments object is also array-like. You can call array methods on them even if they don't have these methods themselves.

function f() {  console.log(Array.prototype.join.call(arguments, "+"));}f("a", "b"); // 'a+b'

Array()

Creates a newArray object.

Array[Symbol.species]

Returns theArray constructor.

Array.from()

Creates a newArray instance from an iterable or array-like object.

Array.fromAsync()

Creates a newArray instance from an async iterable, iterable, or array-like object.

Array.isArray()

Returnstrue if the argument is an array, orfalse otherwise.

Array.of()

Creates a newArray instance with a variable number of arguments, regardless of number or type of the arguments.

These properties are defined onArray.prototype and shared by allArray instances.

Array.prototype.constructor

The constructor function that created the instance object. ForArray instances, the initial value is theArray constructor.

Array.prototype[Symbol.unscopables]

Contains property names that were not included in the ECMAScript standard prior to the ES2015 version and that are ignored forwith statement-binding purposes.

These properties are own properties of eachArray instance.

length

Reflects the number of elements in an array.

Array.prototype.at()

Returns the array item at the given index. Accepts negative integers, which count back from the last item.

Array.prototype.concat()

Returns a new array that is the calling array joined with other array(s) and/or value(s).

Array.prototype.copyWithin()

Copies a sequence of array elements within an array.

Array.prototype.entries()

Returns a newarray iterator object that contains the key/value pairs for each index in an array.

Array.prototype.every()

Returnstrue if every element in the calling array satisfies the testing function.

Array.prototype.fill()

Fills all the elements of an array from a start index to an end index with a static value.

Array.prototype.filter()

Returns a new array containing all elements of the calling array for which the provided filtering function returnstrue.

Array.prototype.find()

Returns the value of the first element in the array that satisfies the provided testing function, orundefined if no appropriate element is found.

Array.prototype.findIndex()

Returns the index of the first element in the array that satisfies the provided testing function, or-1 if no appropriate element was found.

Array.prototype.findLast()

Returns the value of the last element in the array that satisfies the provided testing function, orundefined if no appropriate element is found.

Array.prototype.findLastIndex()

Returns the index of the last element in the array that satisfies the provided testing function, or-1 if no appropriate element was found.

Array.prototype.flat()

Returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.

Array.prototype.flatMap()

Returns a new array formed by applying a given callback function to each element of the calling array, and then flattening the result by one level.

Array.prototype.forEach()

Calls a function for each element in the calling array.

Array.prototype.includes()

Determines whether the calling array contains a value, returningtrue orfalse as appropriate.

Array.prototype.indexOf()

Returns the first (least) index at which a given element can be found in the calling array.

Array.prototype.join()

Joins all elements of an array into a string.

Array.prototype.keys()

Returns a newarray iterator that contains the keys for each index in the calling array.

Array.prototype.lastIndexOf()

Returns the last (greatest) index at which a given element can be found in the calling array, or-1 if none is found.

Array.prototype.map()

Returns a new array containing the results of invoking a function on every element in the calling array.

Array.prototype.pop()

Removes the last element from an array and returns that element.

Array.prototype.push()

Adds one or more elements to the end of an array, and returns the newlength of the array.

Array.prototype.reduce()

Executes a user-supplied "reducer" callback function on each element of the array (from left to right), to reduce it to a single value.

Array.prototype.reduceRight()

Executes a user-supplied "reducer" callback function on each element of the array (from right to left), to reduce it to a single value.

Array.prototype.reverse()

Reverses the order of the elements of an arrayin place. (First becomes the last, last becomes first.)

Array.prototype.shift()

Removes the first element from an array and returns that element.

Array.prototype.slice()

Extracts a section of the calling array and returns a new array.

Array.prototype.some()

Returnstrue if at least one element in the calling array satisfies the provided testing function.

Array.prototype.sort()

Sorts the elements of an array in place and returns the array.

Array.prototype.splice()

Adds and/or removes elements from an array.

Array.prototype.toLocaleString()

Returns a localized string representing the calling array and its elements. Overrides theObject.prototype.toLocaleString() method.

Array.prototype.toReversed()

Returns a new array with the elements in reversed order, without modifying the original array.

Array.prototype.toSorted()

Returns a new array with the elements sorted in ascending order, without modifying the original array.

Array.prototype.toSpliced()

Returns a new array with some elements removed and/or replaced at a given index, without modifying the original array.

Array.prototype.toString()

Returns a string representing the calling array and its elements. Overrides theObject.prototype.toString() method.

Array.prototype.unshift()

Adds one or more elements to the front of an array, and returns the newlength of the array.

Array.prototype.values()

Returns a newarray iterator object that contains the values for each index in the array.

Array.prototype.with()

Returns a new array with the element at the given index replaced with the given value, without modifying the original array.

Array.prototype[Symbol.iterator]()

An alias for thevalues() method by default.

This section provides some examples of common array operations in JavaScript.

This example shows three ways to create new array: first usingarray literal notation, then using theArray() constructor, and finally usingString.prototype.split() to build the array from a string.

// 'fruits' array created using array literal notation.const fruits = ["Apple", "Banana"];console.log(fruits.length);// 2// 'fruits2' array created using the Array() constructor.const fruits2 = new Array("Apple", "Banana");console.log(fruits2.length);// 2// 'fruits3' array created using String.prototype.split().const fruits3 = "Apple, Banana".split(", ");console.log(fruits3.length);// 2

This example uses thejoin() method to create a string from thefruits array.

const fruits = ["Apple", "Banana"];const fruitsString = fruits.join(", ");console.log(fruitsString);// "Apple, Banana"

This example shows how to access items in thefruits array by specifying the index number of their position in the array.

const fruits = ["Apple", "Banana"];// The index of an array's first element is always 0.fruits[0]; // Apple// The index of an array's second element is always 1.fruits[1]; // Banana// The index of an array's last element is always one// less than the length of the array.fruits[fruits.length - 1]; // Banana// Using an index number larger than the array's length// returns 'undefined'.fruits[99]; // undefined

This example uses theindexOf() method to find the position (index) of the string"Banana" in thefruits array.

const fruits = ["Apple", "Banana"];console.log(fruits.indexOf("Banana"));// 1

This example shows two ways to check if thefruits array contains"Banana" and"Cherry": first with theincludes() method, and then with theindexOf() method to test for an index value that's not-1.

const fruits = ["Apple", "Banana"];fruits.includes("Banana"); // truefruits.includes("Cherry"); // false// If indexOf() doesn't return -1, the array contains the given item.fruits.indexOf("Banana") !== -1; // truefruits.indexOf("Cherry") !== -1; // false

This example uses thepush() method to append a new string to thefruits array.

const fruits = ["Apple", "Banana"];const newLength = fruits.push("Orange");console.log(fruits);// ["Apple", "Banana", "Orange"]console.log(newLength);// 3

This example uses thepop() method to remove the last item from thefruits array.

const fruits = ["Apple", "Banana", "Orange"];const removedItem = fruits.pop();console.log(fruits);// ["Apple", "Banana"]console.log(removedItem);// Orange

This example uses thesplice() method to remove the last 3 items from thefruits array.

const fruits = ["Apple", "Banana", "Strawberry", "Mango", "Cherry"];const start = -3;const removedItems = fruits.splice(start);console.log(fruits);// ["Apple", "Banana"]console.log(removedItems);// ["Strawberry", "Mango", "Cherry"]

This example uses thesplice() method to truncate thefruits array down to just its first 2 items.

const fruits = ["Apple", "Banana", "Strawberry", "Mango", "Cherry"];const start = 2;const removedItems = fruits.splice(start);console.log(fruits);// ["Apple", "Banana"]console.log(removedItems);// ["Strawberry", "Mango", "Cherry"]

This example uses theshift() method to remove the first item from thefruits array.

const fruits = ["Apple", "Banana"];const removedItem = fruits.shift();console.log(fruits);// ["Banana"]console.log(removedItem);// Apple

This example uses thesplice() method to remove the first 3 items from thefruits array.

const fruits = ["Apple", "Strawberry", "Cherry", "Banana", "Mango"];const start = 0;const deleteCount = 3;const removedItems = fruits.splice(start, deleteCount);console.log(fruits);// ["Banana", "Mango"]console.log(removedItems);// ["Apple", "Strawberry", "Cherry"]

This example uses theunshift() method to add, at index0, a new item to thefruits array — making it the new first item in the array.

const fruits = ["Banana", "Mango"];const newLength = fruits.unshift("Strawberry");console.log(fruits);// ["Strawberry", "Banana", "Mango"]console.log(newLength);// 3

This example uses thesplice() method to remove the string"Banana" from thefruits array — by specifying the index position of"Banana".

const fruits = ["Strawberry", "Banana", "Mango"];const start = fruits.indexOf("Banana");const deleteCount = 1;const removedItems = fruits.splice(start, deleteCount);console.log(fruits);// ["Strawberry", "Mango"]console.log(removedItems);// ["Banana"]

This example uses thesplice() method to remove the strings"Banana" and"Strawberry" from thefruits array — by specifying the index position of"Banana", along with a count of the number of total items to remove.

const fruits = ["Apple", "Banana", "Strawberry", "Mango"];const start = 1;const deleteCount = 2;const removedItems = fruits.splice(start, deleteCount);console.log(fruits);// ["Apple", "Mango"]console.log(removedItems);// ["Banana", "Strawberry"]

This example uses thesplice() method to replace the last 2 items in thefruits array with new items.

const fruits = ["Apple", "Banana", "Strawberry"];const start = -2;const deleteCount = 2;const removedItems = fruits.splice(start, deleteCount, "Mango", "Cherry");console.log(fruits);// ["Apple", "Mango", "Cherry"]console.log(removedItems);// ["Banana", "Strawberry"]

This example uses afor...of loop to iterate over thefruits array, logging each item to the console.

const fruits = ["Apple", "Mango", "Cherry"];for (const fruit of fruits) {  console.log(fruit);}// Apple// Mango// Cherry

Butfor...of is just one of many ways to iterate over any array; for more ways, seeLoops and iteration, and see the documentation for theevery(),filter(),flatMap(),map(),reduce(), andreduceRight() methods — and see the next example, which uses theforEach() method.

This example uses theforEach() method to call a function on each element in thefruits array; the function causes each item to be logged to the console, along with the item's index number.

const fruits = ["Apple", "Mango", "Cherry"];fruits.forEach((item, index, array) => {  console.log(item, index);});// Apple 0// Mango 1// Cherry 2

This example uses theconcat() method to merge thefruits array with amoreFruits array, to produce a newcombinedFruits array. Notice thatfruits andmoreFruits remain unchanged.

const fruits = ["Apple", "Banana", "Strawberry"];const moreFruits = ["Mango", "Cherry"];const combinedFruits = fruits.concat(moreFruits);console.log(combinedFruits);// ["Apple", "Banana", "Strawberry", "Mango", "Cherry"]// The 'fruits' array remains unchanged.console.log(fruits);// ["Apple", "Banana", "Strawberry"]// The 'moreFruits' array also remains unchanged.console.log(moreFruits);// ["Mango", "Cherry"]

This example shows three ways to create a new array from the existingfruits array: first by usingspread syntax, then by using thefrom() method, and then by using theslice() method.

const fruits = ["Strawberry", "Mango"];// Create a copy using spread syntax.const fruitsCopy = [...fruits];// ["Strawberry", "Mango"]// Create a copy using the from() method.const fruitsCopy2 = Array.from(fruits);// ["Strawberry", "Mango"]// Create a copy using the slice() method.const fruitsCopy3 = fruits.slice();// ["Strawberry", "Mango"]

All built-in array-copy operations (spread syntax,Array.from(),Array.prototype.slice(), andArray.prototype.concat()) createshallow copies. If you instead want adeep copy of an array, you can useJSON.stringify() to convert the array to a JSON string, and thenJSON.parse() to convert the string back into a new array that's completely independent from the original array.

const fruitsDeepCopy = JSON.parse(JSON.stringify(fruits));

You can also create deep copies using thestructuredClone() method, which has the advantage of allowingtransferable objects in the source to betransferred to the new copy, rather than just cloned.

Finally, it's important to understand that assigning an existing array to a new variable doesn't create a copy of either the array or its elements. Instead the new variable is just a reference, or alias, to the original array; that is, the original array's name and the new variable name are just two names for the exact same object (and so will always evaluate asstrictly equivalent). Therefore, if you make any changes at all either to the value of the original array or to the value of the new variable, the other will change, too:

const fruits = ["Strawberry", "Mango"];const fruitsAlias = fruits;// 'fruits' and 'fruitsAlias' are the same object, strictly equivalent.fruits === fruitsAlias; // true// Any changes to the 'fruits' array change 'fruitsAlias' too.fruits.unshift("Apple", "Banana");console.log(fruits);// ['Apple', 'Banana', 'Strawberry', 'Mango']console.log(fruitsAlias);// ['Apple', 'Banana', 'Strawberry', 'Mango']

The following creates a chessboard as a two-dimensional array of strings. The first move is made by copying the'p' inboard[6][4] toboard[4][4]. The old position at[6][4] is made blank.

const board = [  ["R", "N", "B", "Q", "K", "B", "N", "R"],  ["P", "P", "P", "P", "P", "P", "P", "P"],  [" ", " ", " ", " ", " ", " ", " ", " "],  [" ", " ", " ", " ", " ", " ", " ", " "],  [" ", " ", " ", " ", " ", " ", " ", " "],  [" ", " ", " ", " ", " ", " ", " ", " "],  ["p", "p", "p", "p", "p", "p", "p", "p"],  ["r", "n", "b", "q", "k", "b", "n", "r"],];console.log(`${board.join("\n")}\n\n`);// Move King's Pawn forward 2board[4][4] = board[6][4];board[6][4] = " ";console.log(board.join("\n"));

Here is the output:

R,N,B,Q,K,B,N,RP,P,P,P,P,P,P,P , , , , , , , , , , , , , , , , , , , , , , , , , , , ,p,p,p,p,p,p,p,pr,n,b,q,k,b,n,rR,N,B,Q,K,B,N,RP,P,P,P,P,P,P,P , , , , , , , , , , , , , , , , , ,p, , , , , , , , , ,p,p,p,p, ,p,p,pr,n,b,q,k,b,n,r

const values = [];for (let x = 0; x < 10; x++) {  values.push([2 ** x, 2 * x ** 2]);}console.table(values);

Results in

// The first column is the index0  1    01  2    22  4    83  8    184  16   325  32   506  64   727  128  988  256  1289  512  162

The result of a match between aRegExp and a string can create a JavaScript array that has properties and elements which provide information about the match. Such an array is returned byRegExp.prototype.exec() andString.prototype.match().

For example:

// Match one d followed by one or more b's followed by one d// Remember matched b's and the following d// Ignore caseconst myRe = /d(b+)(d)/i;const execResult = myRe.exec("cdbBdbsbz");console.log(execResult.input); // 'cdbBdbsbz'console.log(execResult.index); // 1console.log(execResult); // [ "dbBd", "bB", "d" ]

For more information about the result of a match, see theRegExp.prototype.exec() andString.prototype.match() pages.

Iterative methods do not mutate the array on which it is called, but the function provided ascallbackFn can. The key principle to remember is that only indexes between 0 andarrayLength - 1 are visited, wherearrayLength is the length of the array at the time the array method was first called, but the element passed to the callback is the value at the time the index is visited. Therefore:

• callbackFn will not visit any elements added beyond the array's initial length when the call to the iterative method began.
• Changes to already-visited indexes do not causecallbackFn to be invoked on them again.
• If an existing, yet-unvisited element of the array is changed bycallbackFn, its value passed to thecallbackFn will be the value at the time that element gets visited. Removed elements are not visited.

The following examples use theforEach method as an example, but other methods that visit indexes in ascending order work in the same way. We will first define a helper function:

function testSideEffect(effect) {  const arr = ["e1", "e2", "e3", "e4"];  arr.forEach((elem, index, arr) => {    console.log(`array: [${arr.join(", ")}], index: ${index}, elem: ${elem}`);    effect(arr, index);  });  console.log(`Final array: [${arr.join(", ")}]`);}

Modification to indexes not visited yet will be visible once the index is reached:

testSideEffect((arr, index) => {  if (index + 1 < arr.length) arr[index + 1] += "*";});// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e1, e2*, e3, e4], index: 1, elem: e2*// array: [e1, e2*, e3*, e4], index: 2, elem: e3*// array: [e1, e2*, e3*, e4*], index: 3, elem: e4*// Final array: [e1, e2*, e3*, e4*]

Modification to already visited indexes does not change iteration behavior, although the array will be different afterwards:

testSideEffect((arr, index) => {  if (index > 0) arr[index - 1] += "*";});// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e1, e2, e3, e4], index: 1, elem: e2// array: [e1*, e2, e3, e4], index: 2, elem: e3// array: [e1*, e2*, e3, e4], index: 3, elem: e4// Final array: [e1*, e2*, e3*, e4]

Insertingn elements at unvisited indexes that are less than the initial array length will make them be visited. The lastn elements in the original array that now have index greater than the initial array length will not be visited:

testSideEffect((arr, index) => {  if (index === 1) arr.splice(2, 0, "new");});// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e1, e2, e3, e4], index: 1, elem: e2// array: [e1, e2, new, e3, e4], index: 2, elem: new// array: [e1, e2, new, e3, e4], index: 3, elem: e3// Final array: [e1, e2, new, e3, e4]// e4 is not visited because it now has index 4

Insertingn elements with index greater than the initial array length will not make them be visited:

testSideEffect((arr) => arr.push("new"));// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e1, e2, e3, e4, new], index: 1, elem: e2// array: [e1, e2, e3, e4, new, new], index: 2, elem: e3// array: [e1, e2, e3, e4, new, new, new], index: 3, elem: e4// Final array: [e1, e2, e3, e4, new, new, new, new]

Insertingn elements at already visited indexes will not make them be visited, but it shifts remaining elements back byn, so the current index and then - 1 elements before it are visited again:

testSideEffect((arr, index) => arr.splice(index, 0, "new"));// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [new, e1, e2, e3, e4], index: 1, elem: e1// array: [new, new, e1, e2, e3, e4], index: 2, elem: e1// array: [new, new, new, e1, e2, e3, e4], index: 3, elem: e1// Final array: [new, new, new, new, e1, e2, e3, e4]// e1 keeps getting visited because it keeps getting shifted back

Deletingn elements at unvisited indexes will make them not be visited anymore. Because the array has shrunk, the lastn iterations will visit out-of-bounds indexes. If the method ignores non-existent indexes (seearray methods and empty slots), the lastn iterations will be skipped; otherwise, they will receiveundefined:

testSideEffect((arr, index) => {  if (index === 1) arr.splice(2, 1);});// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e1, e2, e3, e4], index: 1, elem: e2// array: [e1, e2, e4], index: 2, elem: e4// Final array: [e1, e2, e4]// Does not visit index 3 because it's out-of-bounds// Compare this with find(), which treats nonexistent indexes as undefined:const arr2 = ["e1", "e2", "e3", "e4"];arr2.find((elem, index, arr) => {  console.log(`array: [${arr.join(", ")}], index: ${index}, elem: ${elem}`);  if (index === 1) arr.splice(2, 1);  return false;});// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e1, e2, e3, e4], index: 1, elem: e2// array: [e1, e2, e4], index: 2, elem: e4// array: [e1, e2, e4], index: 3, elem: undefined

Deletingn elements at already visited indexes does not change the fact that they were visited before they get deleted. Because the array has shrunk, the nextn elements after the current index are skipped. If the method ignores non-existent indexes, the lastn iterations will be skipped; otherwise, they will receiveundefined:

testSideEffect((arr, index) => arr.splice(index, 1));// array: [e1, e2, e3, e4], index: 0, elem: e1// Does not visit e2 because e2 now has index 0, which has already been visited// array: [e2, e3, e4], index: 1, elem: e3// Does not visit e4 because e4 now has index 1, which has already been visited// Final array: [e2, e4]// Index 2 is out-of-bounds, so it's not visited// Compare this with find(), which treats nonexistent indexes as undefined:const arr2 = ["e1", "e2", "e3", "e4"];arr2.find((elem, index, arr) => {  console.log(`array: [${arr.join(", ")}], index: ${index}, elem: ${elem}`);  arr.splice(index, 1);  return false;});// array: [e1, e2, e3, e4], index: 0, elem: e1// array: [e2, e3, e4], index: 1, elem: e3// array: [e2, e4], index: 2, elem: undefined// array: [e2, e4], index: 3, elem: undefined

For methods that iterate in descending order of index, insertion causes elements to be skipped, and deletion causes elements to be visited multiple times. Adjust the code above yourself to see the effects.

• Indexed collections guide
• TypedArray
• ArrayBuffer