instanceof type guards
If you’ve read about typeof type guards and are familiar with the instanceof operator in JavaScript, you probably have some idea of what this section is about.
instanceof type guards are a way of narrowing types using their constructor function. For instance, let’s borrow our industrial string-padder example from earlier:
interface Padder {
getPaddingString(): string
}
class SpaceRepeatingPadder implements Padder {
constructor(private numSpaces: number) { }
getPaddingString() {
return Array(this.numSpaces + 1).join(" ");
}
}
class StringPadder implements Padder {
constructor(private value: string) { }
getPaddingString() {
return this.value;
}
}
function getRandomPadder() {
return Math.random() < 0.5 ?
new SpaceRepeatingPadder(4) :
new StringPadder(" ");
}
// Type is 'SpaceRepeatingPadder | StringPadder'
let padder: Padder = getRandomPadder();
if (padder instanceof SpaceRepeatingPadder) {
padder; // type narrowed to 'SpaceRepeatingPadder'
}
if (padder instanceof StringPadder) {
padder; // type narrowed to 'StringPadder'
}
The right side of the instanceof needs to be a constructor function, and TypeScript will narrow down to:
- the type of the function’s
prototypeproperty if its type is notany - the union of types returned by that type’s construct signatures
in that order.
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