JavaScript flatMap() error: "flatMap mapper function is not callable"

Question

I am trying to flatten an array with the following code, but when I use .flatMap() I get the error:

Uncaught TypeError: flatMap mapper function is not callable
    at Array.flatMap (<anonymous>)

const data = [
        { key1: 5, key3: 2 },
        { key1: 2, key3: 1 },
        { key1: 3, key3: 2 },
        { key2: 8 },
        { key1: 5, key3: 2 },
      ];
var allKeys = data.map(r => Object.keys(r))
// [
//   [ 'key1', 'key3' ],
//   [ 'key1', 'key3' ],
//   [ 'key1', 'key3' ],
//   [ 'key2' ],
//   [ 'key1', 'key3' ]
// ]
allKeys.flatMap();

because flatMap require function as parameter. Just use `flat()` — demkovych, Jun 25 '20 at 14:43
Have you read `flatMap` [docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap)? — Yury Tarabanko, Jun 25 '20 at 14:43

score 1 · Answer 1 · edited Jun 26 '20 at 04:53

1

In this case I should be using flat() not flatMap(). The below works as expected.

const data = [
    { key1: 5, key3: 2 },
    { key1: 2, key3: 1 },
    { key1: 3, key3: 2 },
    { key2: 8 },
    { key1: 5, key3: 2 },
];
var allKeys = data.map(r => Object.keys(r))
console.log(allKeys.flat());

edited Jun 26 '20 at 04:53

Ziaullhaq Savanur

1,848
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answered Jun 25 '20 at 14:48

Max888

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score 1 · Answer 2 · answered Jun 25 '20 at 14:55

You could use flatMap instead of map to be more concise, which eliminates the need for calling .flat() after, as it is just shorthand for .map(...).flat(). As well, you can directly pass Object.keys as the callback for flatMap, as it ignores all arguments other than the first one that it is given.

const data = [
        { key1: 5, key3: 2 },
        { key1: 2, key3: 1 },
        { key1: 3, key3: 2 },
        { key2: 8 },
        { key1: 5, key3: 2 },
      ];
var allKeys = data.flatMap(Object.keys)
console.log(allKeys);

JavaScript flatMap() error: "flatMap mapper function is not callable"

2 Answers2