'esnext.array' compiler option doesn't enable access to Array.prototype.flat()

Question

I'm trying to make my Angular 2 app compile when using JavaScript array experimental features like flat() .

I've added esnext.array option to tsconfig.json , so the lib section looks like this:

"lib": [
    "es2017",
    "esnext.array",
    "dom"
],

I've checked lib.esnext.array.d.ts file and it indeed contains definitions for the flat() method.

But despite this, I'm getting a compilation error:

ERROR in src/app/services/service.ts(286,22): error TS2339: Property 'flat' does not exist on type 'ReadonlyMap[]'.

UPDATE 1: I've checked esnext option which results in the same error.

Also, I've checked that Angular CLI picks up the lib options by typing something non-existent like esnext.array.dfgdfg .

UPDATE 2: To exclude a possibility that it's something wrong with the TypeScript version 2.9.2 or Angular toolchain, I've created a simple project with pure TypeScript:

// greeter.ts
function greeter(persons: string[]) {
    return "Hello, " + persons.flat().join();
}

var user: string[] = ["Jane User"];

document.body.innerHTML = greeter(user);

// tsconfig.json
{
  "compilerOptions": {
    "target": "esnext",
    "lib": [
      "es2017",
      "esnext.array",
      "dom"
    ]
  }
}

And tried to compile it with TypsScript 2.9.2, the current latest 3.0.1 and typescript@next , but I always get the same error:

greeter.ts:2:32 - error TS2339: Property 'flat' does not exist on type 'string[]'.

2 return "Hello, " + persons.flat().join(); ~~~~

Could somebody explain what I'm missing here?

Answer 1

There's probably a Node.js v10/v11 compatibility issue here.

A temporary working solution is to use Lodash's flatten :

Instead of array.flat() use _.flatten(array)

Answer 2

In older TypeScript versions, flat() has been called flatten() . So if you're stuck with those TS versions, you might try using flatten() instead.