ts-assert-exists

Assert that a value exists – and remove | null | undefined from its type

Install

npm install ts-assert-exists

Examples

import assertExists from 'ts-assert-exists';

const twitterToken: string = assertExists(process.env.TWITTER_TOKEN);
// Type of `process.env.TWITTER_TOKEN` is `string | undefined`.
// `assertExists` will cast `process.env.TWITTER_TOKEN` to `string`
// and throw if `process.env.TWITTER_TOKEN` would be null or undefined.

or

import assertExists from 'ts-assert-exists';

const twitterToken: string = assertExists(
    process.env.TWITTER_TOKEN,
    'Twitter token does not exist',
);
// Type of `process.env.TWITTER_TOKEN` is `string | undefined`.
// `assertExists` will cast `process.env.TWITTER_TOKEN` to `string`
// and throw an error with “Twitter token does not exist”
// if `process.env.TWITTER_TOKEN` would be null or undefined.

Why

Imagine you have a variable that might be undefined:

process.env.APP_NAME; // Has a type of `string | undefined`

You want to pass this variable into a function that accepts strings, but TypeScripts gives you an error:

function repeatString(str: string, count: number) {
    // ...
}

repeatString(process.env.APP_NAME, 5);
// TypeScript: type 'string | undefined' is not assignable to type 'string'

To work around this, you could cast the variable with as:

function repeatString(str: string, count: number) {
    // ...
}

// Don’t do this!
repeatString(process.env.APP_NAME as string, 5);

But what if you forget to specify this environment variable, and it turns out to be undefined? You’ll receive a cryptic runtime error – or, ever worse, a wrong value.

To avoid such issue, use this package:

function repeatString(str: string, count: number) {
    // ...
}

const definedVariable = assertExists(process.env.APP_NAME);
repeatString(definedVariable, 5);

API

assertExists<Type>(value: Type | null | undefined, messageToThrow?: string): Type;

License

MIT © Ivan Akulov

Based upon the code snippet shared by @Pauan.