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snippets/javascript-for-in-for-of-foreach.md

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+---
+title: What is the difference between JavaScript's for...in, for...of and forEach?
+shortTitle: JavaScript iteration methods comparison
+type: question
+tags: [javascript,array,object,iterator]
+author: chalarangelo
+cover: lake-loop
+excerpt: Learn the differences between the three most commonly used iteration methods offered by JavaScript, which often confuse beginners and veterans alike.
+dateModified: 2021-06-12T19:30:41+03:00
+---
+
+`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.
+
+```js
+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.
+
+```js
+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.
+
+```js
+['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)
+);
+```