for...in VS for...of in an array

JavaScriptJunior~1 min read

The difference between for...in and for...of in JavaScript is one of those things you need to understand clearly, especially when working with arrays.

In brief:

Loop	What it iterates	Used for
`for...in`	Keys / indexes (property names of an object)	objects
`for...of`	Values of elements (from iterable objects)	arrays, strings, Set, Map, etc.

Example 1 - array

javascript

const arr = ["a", "b", "c"];

`for...in` iterates indexes (keys):

javascript

for (const index in arr) {
  console.log(index); // "0", "1", "2"
}

Returns string indexes (not numbers!).
Suited for objects, not arrays.
May pick up extra properties, if added manually.

`for...of` iterates values:

javascript

for (const value of arr) {
  console.log(value); // "a", "b", "c"
}

Works with iterable objects (Array, String, Set, Map...).
Returns element values.
Ideal for arrays.

Example 2 - the difference in practice:

javascript

const arr = ["x", "y", "z"];
arr.custom = "hello"; // add a property manually

for (const key in arr) {
  console.log(key); // "0", "1", "2", "custom"
}

for (const value of arr) {
  console.log(value); // "x", "y", "z"
}

for...in walks through all enumerable properties of the object, including manually added ones. for...of visits only the array's elements.

Example 3 - working with a string (an iterable object)

javascript

for (const char of "JS") {
  console.log(char); // "J", "S"
}

for...of works great with strings. for...in on a string returns indexes "0", "1".

In short:

Feature	`for...in`	`for...of`
Iterates	keys (indexes, properties)	values
Works with	objects, arrays	iterable objects (array, string, Set, Map)
Returns	strings, property names	actual values
Order	can be unpredictable	preserves order
Suited for arrays	No	Yes
Suited for objects	Yes	No

Remember it simply:

for...in → iterates keys (for objects) for...of → iterates values (for arrays and iterable objects)

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