How to Handle a Possibly Null Variable … That I Know is Not Null?

If I am using (strict null checks) TypeScript, and I have some code with a possibly null variable:

let x = null;
if (someCondition) {
  x = { a: 1 };
}

doSomethingWith(x.a);

I get an error, as expected:

‘x’ is possibly ‘null’.

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I can fix that error with a null check:

if (!x) return;
doSomethingWith(x.a);

but then I get:

Property ‘a’ does not exist on type ‘never’.

And really, instead of an if/return, what I’d like to write is an assert call:

const assert = value => { if (!value) throw Error(); }

assert(x);
doSomethingWith(x.a);

… but that doesn’t even get rid of the first error.

I figured out that I could fix the second error with:

(rootGroup as {id: number}).id

but I’m not sure if all that is the simplest solution, and it still doesn’t solve the problem with using an assert function.

My question is, how can I …?

1. create a "maybe null" variable
2. run a function that proves it’s not null (by throwing an error if it is)
3. add (the least amount possible of) TypeScript code, and then
4. use that variable, without any TS errors

>Solution :

You can use assertion signatures like this:

let x: { a: 1 } | null = null

if (someCondition) {
  x = { a: 1 }
}

// @ts-expect-error x maybe null
console.log(x.a)

assert(x) // asserting that x is truthy

console.log(x.a) // no error here

function assert(value: unknown): asserts value {
  if (!value) {
    throw new Error('foo')
  }
}

If you just want to throw on nullish values, you can do something like this:

function assertNotNull<T>(value: T): asserts value is NonNullable<T> {
  if (value == null) {
    throw new Error('bar')
  }
}

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