for...in
is used to iterate over all enumerable properties of an object, including inherited enumerable properties. This iteration statement can be used with arrays strings or plain objects, but not with Map
or Set
objects.
代码实现
for (let prop in ['a', 'b', 'c'])
console.log(prop); // 0, 1, 2 (array indexes)
for (let prop in 'str')
console.log(prop); // 0, 1, 2 (string indexes)
for (let prop in {a: 1, b: 2, c: 3})
console.log(prop); // a, b, c (object property names)
for (let prop in new Set(['a', 'b', 'a', 'd']))
console.log(prop); // undefined (no enumerable properties)
for...of
is used to iterate over iterable objects, iterating over their values instead of their properties. This iteration statement can be used with arrays, strings, Map
or Set
objects, but not with plain objects.
使用样例
for (let val of ['a', 'b', 'c'])
console.log(val); // a, b, c (array values)
for (let val of 'str')
console.log(val); // s, t, r (string characters)
for (let val of {a: 1, b: 2, c: 3})
console.log(prop); // TypeError (not iterable)
for (let val of new Set(['a', 'b', 'a', 'd']))
console.log(val); // a, b, d (Set values)
Finally, forEach()
is a method of the Array
prototype, which allows you to iterate over the elements of an array. While forEach()
only iterates over arrays, it can access both the value and the index of each element while iterating.
['a', 'b', 'c'].forEach(
val => console.log(val) // a, b, c (array values)
);
['a', 'b', 'c'].forEach(
(val, i) => console.log(i) // 0, 1, 2 (array indexes)
);
翻译自:https://www.30secondsofcode.org/js/s/for-in-for-of-foreach