Typescript: Feature Request / Proposal: constructor type guard

Created on 9 Apr 2018 · 6Comments · Source: microsoft/TypeScript

In most cases, constructor comparison is faster than instanceof and typeof, thus using it as type guard can improve performance. The code below demonstrates the desired behavior:

const el:HTMLElement = document.createElement('canvas');

if (el.constructor === HTMLCanvasElement) {
    //inside of this scope, el is parsed as an instance of HTMLCanvasElement
    el.getContext('2d');
}

Moderate Suggestion help wanted

Source

sheley1998

👍8

Most helpful comment

i’d like to work on this :smile:

Kingwl on 22 Apr 2018

👍6

All 6 comments

mhegazy on 10 Apr 2018

Accepting PRs for a new kind of type guard (IOW one that specifically looks for this syntactic pattern). Note that the false arm here can't perform any narrowing of a union due to constructor checks not identifying subclasses.