How to convince typescript that a record has a value?

If I have this declaration:

type MyAwesomeLookup = Record<string, number>

How can I then write a hasNumber type guard that makes this compile without errors:

const lookup = { [Math.random().toString()]: 42 }
const key = '0.5'

if (hasNumber(lookup, key)) {
  const x: number = lookup[key]
  console.log({ x })
}

I am using TypeScript 5.x.

You could use an intersection to ensure the lookup contains a key:

Playground

type MyAwesomeLookup = Record<string, number>

const lookup = { [Math.random().toString()]: 42 }
const key = '0.5'

const hasNumber = <K extends string>(lookup: MyAwesomeLookup, key: K): lookup is MyAwesomeLookup & {[P in K]: number } => key in lookup;

if (hasNumber(lookup, key)) {
  const x = lookup[key]
  console.log({ x })
}

Though I would suggest to narrow directly:

Playground

type MyAwesomeLookup = Record<string, number>

const lookup = { [Math.random().toString()]: 42 }
const key = '0.5'

if (key in lookup) {
  const x = lookup[key]
  console.log({ x })
}