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# Assert Never [![npm version][npm-image]][npm-url]
Helper function for [exhaustive checks][exhaustive-checks] of discriminated
unions in TypeScript.
## Installation
```
npm install --save assert-never
```
## Usage
```ts
import {assertNever} from "assert-never";
type A = {type: 'a'};
type B = {type: 'b'};
type Union = A | B;
function doSomething(arg: Union) {
if (arg.type === 'a') {
return something;
}
if (arg.type === 'b') {
return somethingElse;
}
// TS will error if there are other types in the union
// Will throw an Error when called at runtime. Use `assertNever(arg, true)`
// instead to fail silently.
return assertNever(arg);
}
```
[npm-image]: https://badge.fury.io/js/assert-never.svg
[npm-url]: https://badge.fury.io/js/assert-never
[exhaustive-checks]: https://basarat.gitbooks.io/typescript/docs/types/discriminated-unions.html#exhaustive-checks