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snippets/js/s/invert-key-values.md

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+---
+title: Invert object
+type: snippet
+language: javascript
+tags: [object]
+cover: pineapple-on-green
+dateModified: 2020-10-20T23:02:01+03:00
+---
+
+Inverts the key-value pairs of an object, without mutating it.
+
+- Use `Object.keys()` and `Array.prototype.reduce()` to invert the key-value pairs of an object and apply the function provided (if any).
+- Omit the second argument, `fn`, to get the inverted keys without applying a function to them.
+- The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.
+
+```js
+const invertKeyValues = (obj, fn) =>
+  Object.keys(obj).reduce((acc, key) => {
+    const val = fn ? fn(obj[key]) : obj[key];
+    acc[val] = acc[val] || [];
+    acc[val].push(key);
+    return acc;
+  }, {});
+```
+
+```js
+invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
+invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value);
+// { group1: [ 'a', 'c' ], group2: [ 'b' ] }
+```