Property 'flatMap', 'flat' does not exist on type in TS

# Property 'flatMap', 'flat' does not exist on type in TS

To solve the "Property 'flatMap' or 'flat' does not exist on type" error, add the string "es2019" to the lib array in your tsconfig.json file and make sure you are running a recent version of TypeScript.

Open your tsconfig.json file and add es2019 to your lib array.

tsconfig.json

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{
  "compilerOptions": {
    // ... other options
    "lib": [
      // ... other libs
      "es2019"
    ]
  }
}

The code for this article is available on GitHub

This will resolve the error and you will be able to use the flatMap and flat methods.

index.ts

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// 👇️ using Array.flatMap
const arr = ['one two', 'three four'];

const result = arr.flatMap((str) => str.split(' '));

// 👇️ ['one', 'two', 'three', 'four']
console.log(result);

using flatmap in typescript

The code for this article is available on GitHub

Here is an example of using the Array.flat method.

index.ts

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// 👇️ using Array.flat
const arr = [
  ['a', 'b'],
  ['c', 'd'],
];

// 👇️ const flat: string[]
const flat = arr.flat();

// 👇️ ['a', 'b', 'c', 'd']
console.log(flat);

using flat in typescript

# Update your version of `typescript`

If you are still unable to use the flatMap method, make sure you're running a recent version of TypeScript.

shell

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npm install -g typescript@latest

npm install --save-dev typescript@latest

If you use Angular.js, you might have to update your version if the error persists.

# Using `replace` instead of `flatMap`

If you still get an error, here is an example of how I would rewrite the code sample without using flatMap.

index.ts

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const arr = ['one two', 'three four'];

const result = arr.reduce<string[]>(
  (acc, curr) => acc.concat(splitOnSpace(curr)),
  [],
);

console.log(result); // 👉️ ['one', 'two', 'three', 'four']

function splitOnSpace(str: string) {
  return str.split(' ');
}

using replace instead of flatmap

The code for this article is available on GitHub

We used the reduce() method and set its return type and the type of the accumulator variable to be an array of strings ( string[]).

On each iteration, we concatenate the accumulated array to the result of calling the split() method on a string.

I've also written a detailed guide on how to flatten an array of arrays in TS.

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Property 'flatMap', 'flat' does not exist on type in TS