Question: I'm using Chrome 70 and Chrome does add methods `.flatMap`, `.flatten`, `.flat`. So my code does run as expected. Unfortunately, TypeScript doesn't like it.

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

```

The warning I got is `TS2339: Property 'flatMap' does not exist on type 'any[]'.`

I'm using Angular 6, which uses Typescript ~2.9.2, and I already include `import 'core-js/es7/array';` in `polyfills.ts`.

My guess is that there is no typing for these methods, and I did try to `npm run -dev @types/array.prototype.flatmap` but still not solve.

Answer: 
Aaron Beall's answer is excellent. It may be worth knowing that if "lib" is not specified in the tsConfig.JSON file a default list of libraries are injected. The default libraries injected are:
► For --target ES5: DOM,ES5,ScriptHost
► For --target ES6: DOM,ES6,DOM.Iterable,ScriptHost

In other words: We must specify those libs that were previously added automatically. (see: <https://www.typescriptlang.org/docs/handbook/compiler-options.html> for further info) 

```
"compilerOptions": {

   "target": "es6",

   "lib": [ "es2019", "DOM", "ES6" ,"DOM.Iterable", "ScriptHost"],}

```
As of angular 11 and thx to [typescript 3.9](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-9.html) this is now the new config.

```json
"compilerOptions": {
    "target": "es2018",
    "module": "es2020",
    "lib": ["es2020", "dom"],
}

```

### Why `es2020` instead of `esnext`?

> 
> In TypeScript 3.9, the behavior of the TypeScript compiler controlled by module is the same with both "esnext" and "es2020" values. This behavior can change in the future, because the "esnext" option could evolve in a backwards incompatible ways, resulting in build-time or run-time errors during a TypeScript update. As a result, code can become unstable. Using the "es2020" option mitigates this risk.
> 
> 
> 

[for further read](https://angular.io/guide/migration-update-module-and-target-compiler-options)