Summary of ways to traverse using various loops in JavaScript_Basic knowledge

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 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 a 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

// 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('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}
$.each 中的 $(this) 与this有所不同，不过遍历结果却是一样，你可以在测试代码中打印出来看看

$(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 values ​​are 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) {}

Objects like this also have property arguments objects of functions. Of course, strings can also be traversed, but because the enumerable of other properties of the string is set to false, the result of the traversal is the same as the array, that is Don’t worry about this problem anymore.
Small addition

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() {}() Three ways of function self-execution^_^