数组的循环和迭代

循环（1） for 循环for 循环是遍历数组最简单的一种，也是使用频率最高的一种。普通版 for 循环var = ["a", "b", "c", "d"];for ( let = 0; < . ; ++ ) {console.log( [ ] ); // a b c d}优化版 for 循环var = ["a", "b", "c", "d"];for ( let = 0, = . ; < ; ++ ) {console.log( [ ] ); // a b c d}（2）for-in 循环var = ["a", "b", "c", "d"];for ( let in ) {console.log( [ ] ); // a b c d}(3)for-of 循环var = ["a", "b", "c", "d"];for ( let of ) {console.log( ); // a b c d}for-in 是遍历集合对象的键名， for-of 是直接遍历集合对象的键值。for-of 的遍历方式性能要高于 for-in ，但仍比不不上普通的 for 循环。for-of 不仅支持数组的遍历，还支持大多数类数组对象。同时也支持 ES6 里新增的 Map 和 Set对象。数组迭代(1)forEach()var = [10, 11, 12, 13, 14];arr.forEach(function( , ) {console.log( ); // 10 11 12 13 14console.log( ); // 0 1 2 3 4})(2)map()var = [10, 11, 12, 13, 14];var = .map(function( , ) {return * 2;})console.log( );(3)filter()var = [10, 11, 12, 13, 14];var = .filter(function( , ) {return % 2 == 0;})console.log( );(4)reduce()var = [10, 11, 12, 13, 14];var = .reduce(function( , ) {return + ;})console.log( );(5)every()var = [10, 11, 12, 13, 14];var = .every(function( , ) {return % 2 == 0;})console.log( );(6)some()var = [10, 11, 12, 13, 14];var = .some(function( , ) {return % 2 == 0;})console.log( );