Why are flatMap, flat, and flatten not recognized on any[] in TypeScript?

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#1

Why is flatMap, flat, and flatten not recognized on any[] in TypeScript?

I’m using Chrome 70, which supports methods like .flatMap, .flatten, and .flat. My code runs as expected in the browser, but TypeScript doesn’t recognize these methods.

For example:

// data.flatMap lint error
export const transformData = (data: any[]) => data.flatMap(abc => [
   parentObj(abc),
   ...generateTasks(abc)
]);

The warning I get is TS2339: Property ‘flatMap’ does not exist on type ‘any[]’.

I am using Angular 6, which includes TypeScript ~2.9.2, and I’ve already added import 'core-js/es7/array'; in polyfills.ts. However, I’m still facing issues.

My assumption is that there’s no typing for these methods. I also tried running npm run -dev @types/array.prototype.flatmap, but that didn’t resolve the issue.

Is there a solution to enable typescript flatmap for these array methods in my TypeScript code?

#2

Hey there! It seems like you’re running into an issue with TypeScript’s version compatibility. Here’s what I’d recommend:

Update Your TypeScript Version
Since you’re using TypeScript ~2.9.2, it’s likely these array methods aren’t fully supported or typed. To fix this, upgrade to TypeScript 3.5 or higher—these versions include proper support for methods like flatMap.

Here’s how you can do it:

npm install typescript@latest --save-dev

This will ensure that TypeScript recognizes flatMap and other modern array methods, allowing you to properly use them in your project.

#3

Great suggestion above! If updating TypeScript doesn’t entirely solve it, here’s another thing to try:

Add a Polyfill
Since Angular 6 uses an older setup, even with core-js/es7/array in your polyfills.ts, the type definitions might still be missing. To bridge this gap, install a specific polyfill for flatMap, such as array.prototype.flatmap.

Run this command:

npm install array.prototype.flatmap --save

This polyfill ensures not only the runtime behavior of flatMap but also provides TypeScript with the correct type definitions, resolving the typescript flatmap issue.

#4

These are solid solutions! But if you’re still having trouble or want a quick workaround while addressing typescript flatmap, try this:

Type the Array Explicitly
TypeScript might struggle with any[] because it can’t infer enough details about the array. By explicitly typing your array as Array<any>, you’re telling TypeScript that it’s dealing with an array that supports flatMap and other modern methods.

Here’s an example:

export const transformData = (data: Array<any>) => data.flatMap(abc => [
   parentObj(abc),
   ...generateTasks(abc)
]);

Explicit typing makes your intentions clearer to TypeScript, reducing the chances of type errors while keeping your code consistent.