js forEach

最新推荐文章于 2024-01-18 11:32:47 发布

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最新推荐文章于 2024-01-18 11:32:47 发布

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forEach这种方法是一个JavaScript扩展到ECMA-262标准;因此它可能不存在在标准的其他实现。

两种常见的形式

1. http://www.jb51.net/article/67441.htm

 
      <html> 
     
      <head> 
     
      <title>JavaScript Array forEach Method</title> 
     
      </head> 
     
      <body> 
     
      <script type= 
      "text/javascript" 
      > 
     
      if 
       (!Array.prototype.forEach) 
     
      { 
     
      Array.prototype.forEach =  
      function 
      (fun  
      /*, thisp*/ 
      ) 
     
      { 
     
      var 
      len =  
      this 
      .length; 
     
      if 
      ( 
      typeof 
       fun !=  
      "function" 
      ) 
     
      throw 
      new 
      TypeError(); 
     
      var 
      thisp = arguments[1]; 
     
      for 
      ( 
      var 
       i = 0; i < len; i++) 
     
      { 
     
      if 
      (i  
      in 
       this 
      ) 
     
      fun.call(thisp,  
      this 
      [i], i,  
      this 
      ); 
     
      } 
     
      }; 
     
      } 
     
      function 
       printBr(element, index, array) { 
     
      document.write( 
      "<br />[" 
      + index +  
      "] is " 
       + element );  
     
      } 
     
      [12, 5, 8, 130, 44].forEach(printBr); 
     
      </script> 
     
      </body> 
     
      </html>

这将产生以下结果：

2. mozilla的实现：

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.io/#x15.4.4.18
if (!Array.prototype.forEach) {

  Array.prototype.forEach = function(callback, thisArg) {

    var T, k;

    if (this == null) {
      throw new TypeError(' this is null or not defined');
    }

    // 1. Let O be the result of calling ToObject passing the |this| value as the argument.
    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get internal method of O with the argument "length".
    // 3. Let len be ToUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If IsCallable(callback) is false, throw a TypeError exception.
    // See: http://es5.github.com/#x9.11
    if (typeof callback !== "function") {
      throw new TypeError(callback + ' is not a function');
    }

    // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
    if (arguments.length > 1) {
      T = thisArg;
    }

    // 6. Let k be 0
    k = 0;

    // 7. Repeat, while k < len
    while (k < len) {

      var kValue;

      // a. Let Pk be ToString(k).
      //   This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the HasProperty internal method of O with argument Pk.
      //   This step can be combined with c
      // c. If kPresent is true, then
      if (k in O) {

        // i. Let kValue be the result of calling the Get internal method of O with argument Pk.
        kValue = O[k];

        // ii. Call the Call internal method of callback with T as the this value and
        // argument list containing kValue, k, and O.
        callback.call(T, kValue, k, O);
      }
      // d. Increase k by 1.
      k++;
    }
    // 8. return undefined
  };
}