How to use jquery's each(callback)? What effect will it achieve?-JS Tutorial-php.cn

How to use jquery's each(callback)? What effect will it achieve?

伊谢尔伦

Release： 2017-06-16 15:58:48

Original

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Overview

Execute a function with each matching element as the context.

means that each time the function passed in is executed, the this keyword in the function points to a different DOM element (a different matching element each time). Moreover, every time the function is executed, a numeric value representing the position of the element as the execution environment in the matching element set is passed to the function as a parameter (an integer based on zero). Returning 'false' will stop the loop (just like using 'break' in a normal loop). Returns 'true' to jump to the next loop (just like using 'continue' in a normal loop).

Examples are as follows:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
 <title> 遍历元素</title>
  <script src="js/jQuery.js" type="text/JavaScript"></script>
    <!-- 
    <script src="http://code.jquery.com/jquery-latest.js"></script> 
     -->
 <style type="text/css">
  body{font-size:13px}
  img{border:solid 1px #ccc;padding:3px;margin:5px;width:143px;height:101px}
 </style>
 <script type="text/javascript">
  $(function() {
   $("img").each(function(index){
    // 根据形参index 设置元素的title 属性
    this.title = " 第" + index +" 幅风景图片，alt 内容是" + this.alt;
    if(i==1)
     return false;
   })
  })
 </script>
  </head>
  <body>
 <p>
  <img title="picture0" src="images/img05.jpg" alt=" 第0 幅风景画" />
  <img title="picture1" src="images/img06.jpg" alt=" 第1 幅风景画" />
  <img title="picture2" src="images/img07.jpg" alt=" 第2 幅风景画" />
 </p>
  </body>
</html>

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General traversal method, which can be used to traverse objects and arrays.

Unlike the $().each() method that iterates over jQuery objects, this method can be used to iterate over any object. Callback function has two parameters: the first is the member of the object or the index of the array, and the second is the corresponding variable or content. If you need to exit the each loop, you can make the callback function return false, and other return values will be ignored.

var dic={"tom" : 20,"jerry" :30, "jim":40};
$.each(dic,function(key,value){ alert(key+"的年龄是"+value); });

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Result: Tom’s age is 20
Jerry’s age is 30

Jim’s age is 40

var arr=[1,2,3];
$.each(arr,function(key,value){ key++;alert(key+"="+value);});

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Result: 1= 1 2=2 3=3

var arr=[1,2,3];
$.each(arr,function(item){ alert(item);});

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Result: 0 1 2

var dic={"tom" : 20,"jerry" :30, "jim":40};
$.each(dic,function(){ alert(this);});

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Result: 20 30 40

$.each() and $(selector ).each() is different. The latter is dedicated to traversing jquery objects. The former can be used to traverse any collection (whether it is an array or an object). If it is an array, the callback function passes in the index of the array and the corresponding value (value It can also be obtained through the this keyword, but JavaScript will always wrap the this value as an object—even if it is a string or a number), and the method will return the first parameter of the traversed object.

Theeach() method can make the DOM loop structure concise and less error-prone. The each() function encapsulates a very powerful traversal function and is very convenient to use. It can traverse one-dimensional arrays, multi-dimensional arrays, DOM, JSON, etc.
Developed in javaScript Using $each during the process can greatly reduce our workload.

each processes a one-dimensional array

var arr1 = [ "aaa", "bbb", "ccc" ];
$.each(arr1, function(i,val){
alert(i);
alert(val);
});

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alert(i) will output 0, 1, 2
alert(val) will output aaa, bbb, ccc

each Processingtwo-dimensional array

var arr2 = [[&#39;a&#39;, &#39;aa&#39;, &#39;aaa&#39;], [&#39;b&#39;, &#39;bb&#39;, &#39;bbb&#39;], [&#39;c&#39;, &#39;cc&#39;, &#39;ccc&#39;]]
　　$.each(arr, function(i, item){
alert(i);
alert(item);
　　});

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arr2 is a two-dimensional array, and item is equivalent to taking each array in this two-dimensional array.
item[0] is relative to taking the first value in each one-dimensional array
alert(i) will output 0, 1, 2, because this two-dimensional array contains 3 array elements
alert(item) will output as ['a', 'aa', 'aaa'], ['b', 'bb', 'bbb'], ['c', 'cc', 'ccc']

After slightly changing the processing of this two-digit array

var arr = [[&#39;a&#39;, &#39;aa&#39;, &#39;aaa&#39;], [&#39;b&#39;, &#39;bb&#39;, &#39;bbb&#39;], [&#39;c&#39;, &#39;cc&#39;, &#39;ccc&#39;]]
　　$.each(arr, function(i, item){
　　$.each(item,function(j,val){
　　　　 alert(j);
　　　　alert(val);
　});
});

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alert(j) will output 0, 1, 2, 0, 1, 2, 0, 1, 2

alert(val) will output as a, aa, aaa, b, bb, bbb, c, cc, ccc

each processes json data, this each is even more powerful, and can loop through each attribute

var obj = { one:1, two:2, three:3};
each(obj, function(key, val) {
alert(key);
alert(val);
});

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Here alert(key) will output one two three
alert(val) will output one, 1, two, 2, three, 3
Why is the key here not a number but an attribute? , because the json format is a set of unordered attributes and values. Since it is unordered, where are the numbers?
And this val is equivalent to obj[key]

ecah processes dom elements. Here, an input form element is used as an example.

If you have a piece of code like this in your dom

<input name="aaa" type="hidden" value="111" />
<input name="bbb" type="hidden" value="222" />
<input name="ccc" type="hidden" value="333" />
<input name="ddd" type="hidden" value="444"/>

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Then you use each as follows

$.each($("input:hidden"), function(i,val){
alert(val);
alert(i);
alert(val.name);
alert(val.value);
});

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Then, alert(val) will output [object HTMLInputElement] because it is A form element.

alert(i) will output 0, 1, 2, 3

alert(val.name); will output aaa, bbb, ccc, ddd, if this.name is used, it will output The same result
alert(val.value); will output 111,222,333,444. If you use this.value, the same result will be output

If you change the above code into the following form

$("input:hidden").each(function(i,val){
alert(i);
alert(val.name);
alert(val.value);
});

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As you can see, the output results are the same. As for the difference between the two writing methods, I don’t know yet. This change will produce the same results when applied to the array operations above.

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