TypeScript failing to infer type is not undefined after type guard with logical or

In the code below, in the else block, it is clear that b cannot be undefined (a||b is truthy and a is falsy so b must be truthy).

Why does Typescript produce the error 'b' is possibly 'undefined', and how can I avoid this error?

function test(a: string | undefined, b: string | undefined) {
  if (a || b) {
    if (a) {
      const x = a.length
    } else {
      const y = b.length
    }
  }
}

You can debug it like this:

function test(a: string | undefined, b: string | undefined) {
  type A1 = typeof a; // string | undefined
  type B1 = typeof b; // string | undefined
  if (a || b) {
    type A2 = typeof a; // string | undefined
    type B2 = typeof b; // string | undefined
    if (a) {
      type A3 = typeof a; // string
      type B3 = typeof b; // string | undefined
      const x = a.length
    } else {
      type A4 = typeof a; // string | undefined
      type B4 = typeof b; // string | undefined
      const y = b.length
    }
  }
}

Basically, (a || b) is not able to narrow down the types.

The conditional flow analysis look for !== undefined and !== null checks (this is a known limitation of TS), you should re-write it as follows:

function test(a: string | undefined, b: string | undefined) {
  if (a !== undefined ) {
    const x = a.length; // number
  } else if(b !== undefined){
    const y = b.length; // number
  }
}