execution flow analysis disregards narrowing for primitives constants

[zpdDG4gta8XKpMCd]

nightly build

const x = Math.random() > 0.5 ? 'hey' : 1;
if (typeof x === 'number') {
    [].map(() => x + 2); // expected should typecheck, actual no-no-no
}

numbers can't mutate, x holds a number, also it is a constant, constant can't change its value, so ...

[DanielRosenwasser]

My guess is that we stop trying to track the way this was narrowed because you're in an arrow function. Being able to track this would be pretty difficult, but maybe we could say that we still narrow in function expressions/arrow functions.

[RyanCavanaugh]

If we see a const narrowed by typeof, that should be "sticky" even in function bodies.

This isn't true for let, e.g.

let x = Math.random() > 0.5 ? 'hey' : 1;
if (typeof x === 'number') {
  // Unsafe
  window.setImmediate(() => x.toFixed());
  // Safe but not distinguishable from above
  [0].map(() => x.toFixed());
}
x = 'oops';

but we can be sure the typeof a const cannot change.

[mhegazy]

duplicate of #7662.