for…in
iterates over property names, not values, and does so in an unspecified order (yes, even after ES6). You shouldn’t use it to iterate over arrays. For them, there’s ES5’s forEach
method that passes both the value and the index to the function you give it:
var myArray = [123, 15, 187, 32];
myArray.forEach(function (value, i) {
console.log('%d: %s', i, value);
});
// Outputs:
// 0: 123
// 1: 15
// 2: 187
// 3: 32
Or ES6’s Array.prototype.entries
, which now has support across current browser versions:
for (const [i, value] of myArray.entries()) {
console.log('%d: %s', i, value);
}
For iterables in general (where you would use a for…of
loop rather than a for…in
), there’s nothing built-in, however:
function* enumerate(iterable) {
let i = 0;
for (const x of iterable) {
yield [i, x];
i++;
}
}
for (const [i, obj] of enumerate(myArray)) {
console.log(i, obj);
}
If you actually did mean for…in
– enumerating properties – you would need an additional counter. Object.keys(obj).forEach
could work, but it only includes own properties; for…in
includes enumerable properties anywhere on the prototype chain.