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


Borislav Hadzhiev

Sat Mar 05 20222 min read


Photo by Wira Dyatmika

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.

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

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

// 👇️ using Array.flatMap const arr = ['one two', 'three four']; const result = arr.flatMap((str) => str.split(' ')); // 👇️ ['one', 'two', 'three', 'four'] console.log(result); // 👇️ using Array.flat const arr2 = [ ['a', 'b'], ['c', 'd'], ]; // 👇️ const flat: string[] const flat = arr2.flat(); // 👇️ ['a', 'b', 'c', 'd'] console.log(flat);

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

npm install -g typescript@latest npm install --save-dev typescript@latest
If you are using Angular.js, you might have to update your version if the error persist.

If you are still getting an error, here is an example of how I would rewrite the code snippet above without using flatMap.

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(' '); }
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 with the result of calling the split() method on a string, which is another array.

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