Typescript accepts null values when type set is non-nullable

Question

I would like to know why the following code accepts null in typescript:

TS playground

// Not sure why null is accepted here when I've specified number as the type
const foo = (): number => 1 || null

// Even when enforcing non-nullable
const foo2 = (): NonNullable<number> => 1 || null

// Here tough, it works: null is not a number
const foo3 = (i: number): number => i || null

Seems to be the same with undefined

For 1 & 2: 1 || null is known by the compiler to be 1 and so the type is number, not number | null. For 3: you need to turn on the --strictNullChecks compiler option on. — jcalz
– jcalz, Commented Mar 17, 2019 at 20:02

Nino Filiu · Accepted Answer · 2019-03-17 19:50:14Z

1

You mistyped your functions. To type foo as () => number and to define it as returning null, you should write that:

const foo
  : () => number // type def
  = () => null;  // function def

With strict null checks enabled, this throws an error, as expected.

answered Mar 17, 2019 at 19:50

Nino Filiu

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