Detailed explanation of javascript traversal method_javascript skills

To facilitate the example explanation, the existing arrays and json objects are as follows

var demoArr = ['Javascript', 'Gulp', 'CSS3', 'Grunt', 'jQuery', 'angular'];
var demoObj = {
 aaa: 'Javascript',
 bbb: 'Gulp',
 ccc: 'CSS3',
 ddd: 'Grunt',
 eee: 'jQuery',
 fff: 'angular'
};

for

You can just look at the example, it’s used too much, it’s very simple

(function() {
 for(var i=0, len=demoArr.length; i<len; i++) {
 if (i == 2) {
 // return; // 函数执行被终止
 // break; // 循环被终止
 continue; // 循环被跳过
 };
 console.log('demo1Arr['+ i +']:' + demo1Arr[i]);
 }
})();
关于for循环，有一下几

About the for loop, there are the following points to note

• i in the for loop still exists in the scope after the loop ends. In order to avoid affecting other variables in the scope, it is isolated using self-execution of the function ()();
• Avoid using for(var i=0; i

var i = 0, len = demo1Arr.length;
for(; i<len; i++) {};

• There are several ways to break out of the loop

return function execution is terminated
break The loop is terminated
continue loop is skipped
Complete example

for in

for(var item in arr|obj){} can be used to traverse arrays and objects

• When traversing the array, item represents the index value, and arr represents the element corresponding to the current index value arr[item]
• When traversing the object, item represents the key value, and arr represents the value corresponding to the key value obj[item]

(function() {
 for(var i in demoArr) {
 if (i == 2) {
 return; // 函数执行被终止
 // break; // 循环被终止
 // continue; // 循环被跳过
 };
 console.log('demoArr['+ i +']:' + demoArr[i]);
 }
 console.log('-------------');
})();

Regarding for in, there are the following points to note:

• In for loops and for in loops, the i value will be retained after the loop ends. Therefore, use self-executing functions to avoid this.
• Using return, break, and continue to jump out of the loop is consistent with the for loop. However, you need to pay attention to return. In the function body, return indicates the termination of function execution. Even the code outside the loop will no longer continue to execute. Break only terminates the loop, and the subsequent code will continue to execute.

function res() {
 var demoArr = ['Javascript', 'Gulp', 'CSS3', 'Grunt', 'jQuery', 'angular'];

 for(var item in demoArr) {
 if (item == 2) {
 return;
 };
 console.log(item, demoArr[item]);
 }
 console.log('desc', 'function res'); //不会执行
}

forEach

demoArr.forEach(function(arg) {})
The parameter arg represents the element of each item in the array. The example is as follows

demoArr.forEach(function(e) {
 if (e == 'CSS3') {
 return; // 循环被跳过
 // break; // 报错
 // continue;// 报错
 };
 console.log(e);
})

The following are the specific points that need attention

forEach cannot traverse the object
forEach cannot be used in IE, firefox and chrome implement this method
forEach cannot use break and continue to jump out of the loop. When using return, the effect is the same as using continue in a for loop
do/while

The specific implementation of the function is as follows, but one thing worth noting is that when using continue, if you put i at the end, the value of i will never change, and eventually it will fall into an infinite loop. Therefore, you must be careful when using do/while.

It is not recommended to use do/while to traverse arrays
// Use while
directly

(function() {
 var i = 0,
 len = demoArr.length;
 while(i < len) {
 if (i == 2) {
 // return; // 函数执行被终止
 // break; // 循环被终止
 // continue; // 循环将被跳过，因为后边的代码无法执行，i的值没有改变，因此循环会一直卡在这里，慎用！！
 };
 console.log('demoArr['+ i +']:' + demoArr[i]);
 i ++;
 }
 console.log('------------------------');
})();

// do while
(function() {
 var i = 0,
 len = demo3Arr.length;
 do {
 if (i == 2) {
 break; // 循环被终止
 };
 console.log('demo2Arr['+ i +']:' + demo3Arr[i]);
 i++;
 } while(i<len);
})();

$.each

$.each(demoArr|demoObj, function(e, ele))
Can be used to traverse arrays and objects, where e represents the index value or key value, and ele represents the value value

$.each(demoArr, function(e, ele) {
 console.log(e, ele);
})

The output is

0 "Javascript"
1 "Gulp"
2 "CSS3"
3 "Grunt"
4 "jQuery"
5 "angular"
There are many things to pay attention to here

• Use return or return true to skip a loop and continue executing the subsequent loop
• Use return false to terminate the execution of the loop, but not terminate the function execution
• Cannot use break and continue to skip loops
• The output of this value in the loop is similar to the following

console.log(this);
//String {0: "C", 1: "S", 2: "S", 3: "3", length: 4, [[PrimitiveValue]]: "CSS3"}

console.log(this == ele);
// true

Regarding the this value above, let’s traverse it

$.each(this, function(e, ele) {
 console.log(e, ele);
})

// 0 c
// 1 s
// 2 s
// 4 3

Why are length and [[PrimitiveValue]] not traversed? Suddenly, I had an idea and found the answer in "Javascript Advanced Programming". It probably means that in the internal properties of JavaScript, set the Enumerable in the object data attribute to false

// 查看length的内部属性
console.log(Object.getOwnPropertyDescriptor(this, 'length'));
// Object {value: 4, writable: false, enumerable: false, configurable: false}

$(this) in $.each is different from this, but the traversal result is the same. You can print it out in the test code to see
$(selecter).each

Specially used to traverse DOMList

$('.list li').each(function(i, ele) {
 console.log(i, ele);
 // console.log(this == ele); // true
 $(this).html(i);
 if ($(this).attr('data-item') == 'do') {
 $(this).html('data-item: do');
 };
})

• i: sequence value ele: only the DOM element currently traversed
• This is the currently traversed DOM element, jQuery methods cannot be called
• $(this) == $(ele) The jquery object of the currently traversed element, you can call the jquery method to perform dom operations

Use for in to traverse DOMList

Because domList is not an array, but an object. Just because its key value is 0, 1, 2... it feels similar to an array, but the result of direct traversal is as follows

var domList = document.getElementsByClassName('its');
for(var item in domList) {
 console.log(item, ':' + domList[item]);
}
// 0: <li></li>
// 1: <li></li>
// ...
// length: 5
// item: function item() {}
// namedItem: function namedItem() {}

So when we use for in to traverse the domList, we need to convert the domList into an array

var res = [].slice.call(domList);
for(var item in res) {}

类似这样的对象还有函数的属性arguments对象，当然字符串也是可以遍历的，但是因为字符串其他属性的enumerable被设置成了false，因此遍历出来的结果跟数组是一样的，也就不用担心这个问题了.
小补充

如果你发现有些人写函数这样搞，不要惊慌，也不要觉得他高大上。

+function(ROOT, Struct, undefined) {
 ... 
}(window, function() {
 function Person() {}
})

()(), !function() {}() +function() {}() 三种函数自执行的方式。

以上就是本文的全部内容，希望对大家的学习有所帮助。