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javascript/snippets/index-of-all.md

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+---
+title: Index of all matches
+type: snippet
+tags: [array]
+cover: jars-on-shelf-2
+dateModified: 2020-10-22T20:23:47+03:00
+---
+
+Finds all indexes of `val` in an array.
+If `val` never occurs, returns an empty array.
+
+- Use `Array.prototype.reduce()` to loop over elements and store indexes for matching elements.
+
+```js
+const indexOfAll = (arr, val) =>
+  arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
+```
+
+```js
+indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3]
+indexOfAll([1, 2, 3], 4); // []
+```