Explain "export =" and "export as namespace" syntax in TypeScript

Question

I am trying to make use of the type declarations files in the DefinitelyTyped repository. Many of these files use the following pattern:

export = React; export as namespace React; 

Googling around, I've found references to this being used to make a declaration file usable in both module-based and non-module-based systems. However, I'm struggling to find a clear explanation of (a) what each of these lines exactly does individually and (b) how exactly this combination works to support both types of consumers.

Answer 1

The first form is used for CommonJS and AMD module systems. You have to match the export = React with import React = require('./React')

See the documentation which gives this example:

ZipCodeValidator.ts

let numberRegexp = /^[0-9]+$/; class ZipCodeValidator {     isAcceptable(s: string) {         return s.length === 5 && numberRegexp.test(s);     } } export = ZipCodeValidator; 

Test.ts

import zip = require("./ZipCodeValidator");  // Some samples to try let strings = ["Hello", "98052", "101"];  // Validators to use let validator = new zip();  // Show whether each string passed each validator strings.forEach(s => {   console.log(`"${ s }" - ${ validator.isAcceptable(s) ? "matches" : "does not match" }`); }); 

The export as namespace form creates a global variable so it can be used without importing, but you may still import it with the import { name } from "some-library" form of import. See the documentation which gives this example:

math-lib.d.ts

export const isPrime(x: number): boolean; export as namespace mathLib; 

The library can then be used as an import within modules:

import { isPrime } from "math-lib"; isPrime(2); mathLib.isPrime(2); // ERROR: can't use the global definition from inside a module 

It can also be used as a global variable, but only inside of a script. (A script is a file with no imports or exports.)

mathLib.isPrime(2); 

Here you have a generic type library which doesn't know which module system is being used so it tries to cover all bases.