Detailed explanation of various JavaScript traversal methods, there are black technologies you don't know about

In order 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 look at the example directly, it is used too much, it is 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(&#39;demo1Arr[&#39;+ i +&#39;]:&#39; + demo1Arr[i]);
    }
})();

About the for loop, there are a few 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 , use the function self-execution method to isolate it ()();

• Avoid using for(var i=0; i

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

• There are several ways to jump out of the loop

• return function execution is terminated

• break 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, arr represents the element corresponding to the current index value arr[item]

• When traversing the object, item represents the key value, arr Indicates the value obj[item] corresponding to the key value

(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 the for loop Like in a for in loop, 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 if it is code outside the loop. No further execution is required. 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);
})

Specific things to note are as follows

• forEach cannot traverse objects

• forEach cannot be used in IE, firefox and Chrome implements 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. Finally Trapped in an infinite loop. Therefore, you must be careful when using do/while.

It is not recommended to use do/while to traverse the array

// 直接使用while
(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(&#39;demoArr[&#39;+ i +&#39;]:&#39; + demoArr[i]);
        i ++;
    }
    console.log(&#39;------------------------&#39;);
})();

// do while
(function() {
    var i = 0,
        len = demo3Arr.length;
    do {
        if (i == 2) {
            break; // 循环被终止
        };
        console.log(&#39;demo2Arr[&#39;+ i +&#39;]:&#39; + 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 here Things to note

• Use return or return true to skip a loop and continue executing the subsequent loop

• Use return false to terminate the loop execution, but does not terminate the function execution

• Cannot use break and continue to skip the loop

• The this value output 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, traverse it

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

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

Why 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 property 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}

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

　$(selecter).each

Specifically 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 being traversed

• this is the DOM element currently being traversed , jQuery methods cannot be called

• $(this) == $(ele) The jquery object of the currently traversed element can call jquery methods to perform dom operations

Use for in to traverse DOMList

Because domList is not an array, but an object, just because its key values ​​​​are 0, 1, 2... It feels similar to an array, but it is traversed directly The result 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() {}

Therefore, 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) {}

Objects like this also have the attribute arguments object of the function, and of course the string It can also be traversed, but because the enumerable of other attributes of the string is set to false, the result of the traversal is the same as the array, so there is no need to worry about this problem.

Small supplement

If you find that some people write functions like this, don’t panic, and don’t think that they are too high-minded to afford a bird

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

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

 以上就是JavaScript 各种遍历方式详解，有你不知道的黑科技的内容，更多相关内容请关注PHP中文网（www.php.cn）！