Various issues with Type Guards

[bcherny]

type A = number
type B = string
type C = number|string

const cc: C[] = [1, 2, "a", "b"]

function isNumber(a): a is A {
    return typeof a === 'number'
}

// OK
const dd = []
for (let n = 0; n < cc.length; n++) {
  const d = cc[n]
  if (typeof d === 'number') {
    dd.push(d.toFixed())
  }
}

// OK
const ee = []
for (let n = 0; n < cc.length; n++) {
  const d = cc[n]
  if (isNumber(d)) {
    ee.push(d.toFixed())
  }
}

// NOT OK - property 'toFixed' does not exist on type 'number' | 'string'
// (guard fails when using "cc[n]" instead of first assigning to "d")
const ff = []
for (let n = 0; n < cc.length; n++) {
  if (typeof cc[n] === 'number') {
    ff.push(cc[n].toFixed())
  }
}

// NOT OK - property 'toFixed' does not exist on type 'number' | 'string'
// (guard fails when guarding the inverse)
const gg = []
for (let n = 0; n < cc.length; n++) {
  const d = cc[n]
  if (typeof d !== 'number') { continue }
  gg.push(d.toFixed())
}

// NOT OK - property 'toFixed' does not exist on type 'number' | 'string'
// (guard fails when used in a filter)
// @see https://github.com/Microsoft/TypeScript/issues/7657
const hh = cc
  .filter(isNumber)
  .map(c => c.toFixed())

[Arnavion]

(1) is probably part of #3812

(2) is part of #2388 (continue is ignored for the purpose of narrowing in the rest of the loop body.)

(3) is #7657 as you already mentioned.

[RyanCavanaugh]

Type guards are basically being completely rewritten, so we'll have to check these examples again later.

[bcherny]

Sounds good. Will close this as a duplicate.

Getting this support in would be great, otherwise when this topic comes up around my scala friends I need to change the subject