Typescript flatMap, flat, flatten doesn't exist on type any[]

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[]'.

By the way I'm using Angular 6, which use 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.

You should add es2019 or es2019.array to your --lib setting for TypeScript to recognize array.flat() and flatMap().

Example:

{
  "compilerOptions": {
    "target": "es5",
    "lib": [
      "es2019"
    ]
  }
}

Previously this was available as part of esnext or esnext.array, but it's now officially part of ES2019.

If you're are in a lower version than es2019, you can implement a shiv to provide similar functionality to .flat() and .flatMap() provided by later libraries.

To flat single level array

arr.reduce((acc, val) => acc.concat(val), []);

To flat multi level array

function flatDeep(arr, d = 1) {
   return d > 0 ? arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatDeep(val, d - 1) : val), []) : arr.slice();
};

to know deeply you can also check below link

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat

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"],}