5 ways to iterate over arrays in JavaScript

The content of this article is to share with you 5 iteration methods about JavaScript arrays, which have certain reference value. Friends in need can refer to them

ECMAScript5 defines 5 iteration methods for arrays. Each method accepts two parameters: the function to be run on each item and (optionally) the scope object in which the function is run - the value that affects this. The functions passed in to these methods accept three parameters: the value of the array item, the item's position in the array, and the array object itself.

every(): Run the given function on each item of the array. If the function returns true for each item, it returns true.

filter(): Run the given function on each item of the array and return an array of items that the function will return true.

forEach(): Run the given function on each item in the array. This method has no return value.

map (): Run the given function on each item in the array and return an array composed of the results of each function call.

some(): Given a function for each item in the array, if the function returns true for any item, it will return true,

The above methods will Modify the values ​​contained in the array.

    var numbers = [1,2,3,4,5,4,3,2,1];
    var everyResult=numbers.every(function (item,index,array) {
    	return(item>2);
    });
    alert(everyResult);   //false      every()必须没一项都返回true 则返回true

  var someResult=numbers.some(function (item,index,array) {
	return(item>2);
   });
   alert(someResult);   //true         some()任意一项返回true 则返回true

	var filterResult=numbers.filter(function (item,index,array) {
	     return(item>2);
	});
	alert(filterResult);  // 34543  filter()利用指定的函数确定返回在数组中包含某一项。 返回一个所有的数值都要大于2的数组。

       var mapResult=numbers.map(function (item,index,array) {
	     return item * 2;
	});
	alert(mapResult);  //2,4,6,8,10,8,6,4,2  map()也返回一个数组。而这个数组的每一项都是在原始数组中的对应上运行入函数的结果。

    numbers.forEach(function(item,index,array){
    	//执行某些操作
   })     //forEach()它只是对数组的每一项运行传入的函数，没有返回值。本质和for循环迭代数组一样

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