ECMAScript 5 定义了9个新的数组方法,分别为:
1.forEach(); 2.map(); 3.filter(); 4.every(); 5.some(); 6.reduce(); 7.reduceRight(); 8.indexOf(); 9.lastIndexOf();
概述:首先,大多数的方法都接受一个函数作为第一个参数,并为数组里的每个元素(或者一些元素)执行这个函数.在稀疏数组中(索引不以0开始,并且元素不连续),不存在的数组元素不调用函数参数.大多数实例中,定义的函数(方法中的参数)包含三个参数:第一个是元素的值,第二个是元素的索引,第三个是数组本身.ECMAScript 5 数组方法不改变调用它们的数组,但是我们自己定义的函数可能会改变数组的值.接下来分别介绍具体方法:
forEach():
为数组中的每个元素执行指定操作.
array1.forEach(callbackfn[, thisArg])
参数:
|
参数 |
定义 |
|---|---|
|
array1 |
必选。 一个数组对象。 |
|
callbackfn |
必选。 最多可以接受三个参数的函数。 对于数组中的每个元素，forEach 都会调用 callbackfn 函数一次。 |
|
thisArg |
可选。 callbackfn 函数中的 this 关键字可引用的对象。 如果省略 thisArg，则 undefined 将用作 this 值。 |
参数
如果callback参数不是函数对象,则将引发TypeError异常.
备注:
对于数组中出现的每个元素，forEach 方法都会调用 callbackfn 函数一次（采用升序索引顺序）。 将不会为数组中缺少的元素调用回调函数。
除了数组对象之外，forEach 方法可由具有 length 属性且具有已按数字编制索引的属性名的任何对象使用。
回调函数语法:
function callbackfn(value, index, array1)
你可使用最多三个参数来声明回调函数。
prototype源码: (来源于官方文档)
// 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
};
}
具体应用如下:
1 //defined the callback function
2 function showResult(val,index,arr){
3 document.write("value:"+val);
4 document.write("index:"+index);
5 document.write("<br />");
6 }
7
8 //defined an array and assignment two values
9 var arrays = ["hello","world"];
10
11 //invoked
12 arrays.forEach(showResult)
13
14 //output
15 value:hello index:0
16 value:world index:1
map():
对数组的每个元素调用定义的回调函数并返回包含结果的数组.
注意:用法,参数和forEach()相似,但是map()要返回一个结果数组.参考上面forEach()的用法.直接上源码:
// Production steps of ECMA-262, Edition 5, 15.4.4.19
// Reference: http://es5.github.io/#x15.4.4.19
if (!Array.prototype.map) { Array.prototype.map = function(callback, thisArg) { var T, A, 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 A be a new array created as if by the expression new Array(len)
// where Array is the standard built-in constructor with that name and
// len is the value of len.
A = new Array(len); // 7. Let k be 0
k = 0; // 8. Repeat, while k < len
while (k < len) { var kValue, mappedValue; // 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. Let mappedValue be the result of calling the Call internal
// method of callback with T as the this value and argument
// list containing kValue, k, and O.
mappedValue = callback.call(T, kValue, k, O); // iii. Call the DefineOwnProperty internal method of A with arguments
// Pk, Property Descriptor
// { Value: mappedValue,
// Writable: true,
// Enumerable: true,
// Configurable: true },
// and false. // In browsers that support Object.defineProperty, use the following:
// Object.defineProperty(A, k, {
// value: mappedValue,
// writable: true,
// enumerable: true,
// configurable: true
// }); // For best browser support, use the following:
A[k] = mappedValue;
}
// d. Increase k by 1.
k++;
} // 9. return A
return A;
};
}
具体实现:
//defined callback function
function AreaOfCircle(radius){
var res = Math.PI(radius*radius);
return res.toFixed(0);
} //defined the array and assignment
var arr = [10,20,30]; //invoked
var arrlist = arr.map(AreaOfCircle); //output
document.write(arrlist); 314,1257,2827
filter():
返回数组中的满足回调函数中指定的条件的元素.
filter()方法的参数也如上,不过区别在于它返回一个包含回调函数为其返回 true 的所有值的新数组。 如果回调函数为 array1 的所有元素返回 false，则新数组的长度为 0。
prototype源码:
if (!Array.prototype.filter) {
Array.prototype.filter = function(fun/*, thisArg*/) {
'use strict';
if (this === void 0 || this === null) {
throw new TypeError();
}
var t = Object(this);
var len = t.length >>> 0;
if (typeof fun !== 'function') {
throw new TypeError();
}
var res = [];
var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
for (var i = 0; i < len; i++) {
if (i in t) {
var val = t[i];
// NOTE: Technically this should Object.defineProperty at
// the next index, as push can be affected by
// properties on Object.prototype and Array.prototype.
// But that method's new, and collisions should be
// rare, so use the more-compatible alternative.
if (fun.call(thisArg, val, i, t)) {
res.push(val);
}
}
}
return res;
};
}
具体实现:
// Define a callback function.
function CheckIfPrime(value, index, ar) {
high = Math.floor(Math.sqrt(value)) + 1; for (var div = 2; div <= high; div++) {
if (value % div == 0) {
return false;
}
}
return true;
} // Create the original array.
var numbers = [31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53]; // Get the prime numbers that are in the original array.
var primes = numbers.filter(CheckIfPrime); document.write(primes);
// Output: 31,37,41,43,47,53
以上资料整理官方文档和javascript权威指南