Typescript throws an "Object is possibly 'null'" error, despite type guard

user_4307181 0
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I have a Typescript behaviour that I don't undestand, and I would like to have your input on it.

The compiler throws me an "Object is possibly 'null'" error even if there is a type check before.

Here is a simplified example:

class Bar {
  public a: string
}

class Foo {
  public bar: Bar | null
}

function functionDoingSomeStuff(callback: any) {
  callback()
}

const foo = new Foo()
if (!foo.bar) {
  return new Error()
}

console.log(foo.bar.a) // no compiler error thanks to the previous if

if (foo.bar) {
  functionDoingSomeStuff(() => {
    console.log(foo.bar.a) // a compiler error
  })
  console.log(foo.bar.a) // no compiler error
}

So why is the compiler warning me if I access to the possibly null property inside a function call, even if the catching if checks it?

Thanks in advance!

typescript null types
1 Answers

This is by design. TypeScript doesn't assume type guards remain active in callbacks as making this assumption is dangerous.

Fix

Capture the variable locally to ensure it doesn't get changed externally and TypeScript can easily understand that. 

if (foo.bar) {
  const bar = foo.bar;
  functionDoingSomeStuff(() => {
    console.log(bar.a) // no compiler error
  })
}
user_3903300
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