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This article mainly explains "how to use for loop sentence in JavaScript". The content of the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "how to use for loop sentence in JavaScript".

For

This is probably the most widely used loop statement, simple and practical, and most of the time the performance is still online, the only disadvantage is probably too common, no features, resulting in many people do not want to use it now.

Const array = [4,7,9,2,6]; for (let index = 0; index < array.length; index++) {const element = array [index]; console.log (element);} / / 4,7,9,2, 6for...in

The for...in statement can traverse the enumerable properties of an object except Symbol in any order.

Const temp = {name: "temp"}; function Apple () {this.color = 'red';} Apple.prototype = temp;const obj = new Apple (); for (const prop in obj) {console.log (`obj.$ {prop} = ${obj [prop]} `);} / / obj.color = red// obj.name = temp

If you only consider the properties of the object itself, rather than its prototype, use getOwnPropertyNames () or execute hasOwnProperty () to determine whether a property is a property of the object itself.

Const temp = {name: "temp"}; function Apple () {this.color = 'red';} Apple.prototype = temp;const obj = new Apple (); for (const prop in obj) {if (obj.hasOwnProperty (prop)) {console.log (`obj.$ {prop} = ${obj [prop]} `);}} / / obj.color = red

Of course, it can also be used to traverse the array.

Const arr = [1Jing 2Jing 3JI 4,5]; for (const key in arr) {console.log (key)} / / 0Jing 1Jing 2Jing 3Jing 4

Using for...in, you can traverse an array, but there are the following problems:

The index index is a string number (note, non-numeric) and cannot perform geometric operations directly.

The traversal order may not be in the internal order of the actual array (perhaps in random order).

So it is generally not recommended to use for...in to traverse the array.

For...of

The for...of statement creates an iteration loop over iterable objects (including Array,Map,Set,String,TypedArray,arguments objects, and so on), invokes custom iteration hooks, and executes the statement for the values of each different attribute.

Const array = ['a', 'baked,' c']; for (const element of array) {console.log (element);} / / a _ amp / b _ amp / c

The difference between for...of and for...in:

The for...in statement iterates over the enumerable properties of an object in any order.

The for...of statement iterates through iterable objects that define the data to be iterated over.

Object.prototype.objCustom = function () {}; Array.prototype.arrCustom = function () {}; let iterable = [3,5,7]; iterable.foo = 'hello';for (const key in iterable) {console.log (key); / / logs 0,1,2, "foo", "arrCustom", "objCustom"} / / 0,1,2, "foo", "arrCustom", "objCustom" for (const key of iterable) {console.log (key) } / / 3, 5, 7

Use for...of to traverse the Map structure:

Let nodes = new Map (); nodes.set ("node1", "T1") .set ("node2", "T2") .set ("node3", "T3"); for (const [node, content] of nodes) {console.log (node, content);} / / node1 T1 take / node2 T2 hand / node3 T3

As you can see, it is very convenient to use for...of to traverse the Map structure, and it is recommended!

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