forEach、map、some、every、filter

1.forEach没有返回值，不能跳出循环，（可通过some、every来实现）
[1, 4, 9, 16].forEach((value, key, array) => {
    console.log(value, key, array)
})

2.map有返回值
[1, 4, 9, 16].map(x => x*2)
// [2, 8, 18, 32]

3.有一个符合返回true
[1, 4, 9, 16].some((value, key, array) => {
    return value > 4
})
// true

4.所有都符合才返回true
[1, 4, 9, 16].every((value, key, array) => {
    return value > 4
})
// false

5.过滤出符合的，返回新值
[1, 4, 9, 16].filter((value, key, array) => {
    return value > 4
})
//[9, 16]

删除数组多项值

问题：删除数组中的某两项值

var arr = [1, 2, 3, 4, 5];
for(var i = 0; i < arr.length; i++){
    console.log('arr.length`````', arr.length);
    console.warn('i``````````````', i);
    console.log('arr[i]`````````', arr[i]);
    if(arr[i] === 2 || arr[i] === 3){
        arr.splice(i, 1);
    }
    console.log(arr);
}
-----------结果-----------
[Log] arr.length````` – 5
[Warning] i`````````````` – 0
[Log] arr[i]````````` – 1
[Log] [1, 2, 3, 4, 5] (5)
[Log] arr.length````` – 5
[Warning] i`````````````` – 1
[Log] arr[i]````````` – 2
[Log] [1, 3, 4, 5] (4)
[Log] arr.length````` – 4
[Warning] i`````````````` – 2
[Log] arr[i]````````` – 4
[Log] [1, 3, 4, 5] (4)
[Log] arr.length````` – 4
[Warning] i`````````````` – 3
[Log] arr[i]````````` – 5
[Log] [1, 3, 4, 5] (4)

结果：数组中为3的那一项并没有删掉

分析：for循环的（i < arr.length）的长度length在删掉为2的那一项后改变为4，下一轮中的i此时已变为2，而arr[2]是4，成功避开为3的那一项，因此并没有被遍历到被删掉。

解决：倒序遍历数组，防止因删掉数组中的某一项后，继续用下标查找漏遍历某一项

var arr = [1, 2, 3, 4, 5];
for(var i = arr.length - 1; i >= 0; i--){
    console.log('arr.length`````', arr.length);
    console.warn('i``````````````', i);
    console.log('arr[i]`````````', arr[i]);
    if(arr[i] === 2 || arr[i] === 3){
        arr.splice(i, 1);
    }
    console.log(arr);
}
-----------结果-----------
[Log] arr.length````` – 5
[Warning] i`````````````` – 4
[Log] arr[i]````````` – 5
[Log] [1, 2, 3, 4, 5] (5)
[Log] arr.length````` – 5
[Warning] i`````````````` – 3
[Log] arr[i]````````` – 4
[Log] [1, 2, 3, 4, 5] (5)
[Log] arr.length````` – 5
[Warning] i`````````````` – 2
[Log] arr[i]````````` – 3
[Log] [1, 2, 4, 5] (4)
[Log] arr.length````` – 4
[Warning] i`````````````` – 1
[Log] arr[i]````````` – 2
[Log] [1, 4, 5] (3)
[Log] arr.length````` – 3
[Warning] i`````````````` – 0
[Log] arr[i]````````` – 1
[Log] [1, 4, 5] (3)