Add dom definition for canvas about flow

[avikchaudhuri]

We'll update the dom definitions to be sensitive to string arguments.

[rajsite]

When performing a createElement('canvas') the type can be known from the string, but how would you annotate getElementById('mycanvas') to type as HTMLCanvasElement instead of HTMLElement?

[lbolla]

What I would like, would be to be able to do something like:

var canvas : HTMLCanvasElement = document.createElement('canvas');
var mycanvas : HTMLCanvasElement = document.getElementById('mycanvas');

In other words, createElement and getElementById would be polimorphic. I don't know if Flow supports such feature.

[agentcooper]

+1

Thank you for reporting this issue and appreciate your patience. We've notified the core team for an update on this issue. We're looking for a response within the next 30 days or the issue may be closed.

[samwgoldman]

createElement("canvas") does currently return a HTMLCanvasElement, so this issue is resolved.

@lbolla Regarding your comment about getElementById returning a specific HTMLElement subtype—Flow does not do that, and it is unsafe to do. Flow doesn't know what HTML the code will execute against, and therefore doesn't know anything more specific.

This is a great use case for dynamic type tests.

var canvas = document.getElementById("mycanvas");
if (canvas instanceof HTMLCanvasElement) {
  // good to go -- type of canvas is refined this block
} else {
  // error condition -- provide some useful debug info
}

I see this as a good thing. Flow is helping you out and directing you to add error handling to make your code safer.