Js uses the for loop to traverse the array, js records

Js uses the for loop to traverse the array, js records

Today I wrote something boring! For Loop usage! The following code defines array a and array B as a pseudo array!

var a = [1,2,3,0,5,4];var b = document.getElementsByTagName('li'); //[<li>1</li>,<li>2</li>,<li>3</li>,<li>4</li>,<li>5</li>]

First: incorrect type!

For (var I = 0; I <. length; I ++) {// This writing method is the most basic, but the error is that the array length should not be obtained every time, So we usually use the second Writing Method !}

Type 2: General type!

For (var I = 0, l =. length; I <l; I ++) {// This writing method is the most common, better understood, and also common. For the two types of a and B (pseudo) all arrays are supported .}

Type 3: optimized

For (var I = a. length-1; I> = 0; I --) {// This write method is clever, traversing in reverse order, thus saving a temporary variable! For both a and B (pseudo) arrays. // Google compiler compression will optimize the for loop !}

Fourth: Special settings

For (var I = 0; B [I]; I ++) {// This writing method is not generic, and it won't work for Array! But it is very practical for the collection of dom elements such as array B !}

Fifth: Skill

For (var I = 0, elem; (elem = B [I])! = Null; I ++) {// This method is also used in specific situations. When the element of the array is not equal to a certain value, here, the loop is stopped when null or undefined is encountered, so array a can also be used! // The trick is to assign values while declaring conditions! This reduces the code for assigning temporary variables to a row !}

In JS, The forin statement is used to perform cyclic operations on the attributes of an array or object?

Yes

For (variable in object)
If the object is null or undefined, javascript will skip the code after loop execution. In ECMAScript3, a type error exception may be thrown.
The object is the original value and is converted to the corresponding packaging object.
If it is an object type, javascript will enumerate the attributes of the object in order to execute the loop. Before each loop, javascript will calculate the value of the variable expression and assign the attribute name (a string) to it.

Example

Var obj = {type: 'Human ', history: '300000years', use: 'example '};
Var arr2 = [], I = 0;
For (arr2 [I ++] in obj)/* empty */;
Console. log (arr2); // ["type", "history", "use"]

Why use for loop to traverse arrays?

First, the courseInfo method of the School class is used to display information.
S1 s2 s3 is an array already written
String [] s1 = {"java "};
String [] s2 = {"c #"};
String [] s3 = {". net "};

If (courseCode = 1 ){
School. courseInfo (courseCode, s1);} is to judge if the input is 1, will it be clear to traverse s1 and other such pushes ??