Understand Node's assert() as a type guard

[jinwoo]

TypeScript Version: 2.5.3

Code

For code like below:

function getFoo(): Foo {
  const a: Foo|null = null;
  // ...
  // some code that conditionally sets a to a Foo instance.
  // ...
  if (!a) throw new Error('a is not a Foo!');
  return a;
}

, typescript understand that a is non-null when returned from getFoo().

But Node.js has its own assert module, and it is not understood by typescript and we need a type-cast like below:

import * as assert from 'assert';

function getFoo(): Foo {
  const a: Foo|null = null;
  // ...
  // some code that conditionally sets a to a Foo instance.
  // ...
  assert(a);
  return a as Foo;
}

It'd be really great if typescript understands that the non-nullness has been already checked by the assert() call and a is non-null in this case.

[mhegazy]

Duplicate of #10470 and #8655

this is a design limitation at the time being. the reachable code analysis happens at an earlier phase, before types are known, and that builds the control flow graph that is used later on. narrowing will use that graph.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.