microsoft / microsoft/TypeScript
'Canvas#getContext' almost never returns 'null'
Open
Nobody has claimed this yet.
Awaiting More Feedback
Domain: lib.d.ts
Suggestion
- Dominant language
- Go
- Stars
- 111k
- Forks
- 14.3k
- Avg merge
- 2d 4h
- Merged PRs (30d)
- 132
Description
TypeScript Version: 2.5.3
Code:
let canvas: HTMLCanvasElement = document.createElement("canvas");
let ctx: CanvasRenderingContext2D = canvas.getContext("2d");
ctx.fillRect(0, 0, canvas.width, canvas.height);
Expected behavior:
No errors should occur because the type of canvas is HTMLCanvasElement.
Actual behavior:
test.ts(2,5): error TS2322: Type 'CanvasRenderingContext2D | null' is not assignable to type 'CanvasRenderingContext2D'.
Type 'null' is not assignable to type 'CanvasRenderingContext2D'.
If I change the type of ctx to CanvasRenderingContext2D | null, then I get
test.ts(3,1): error TS2531: Object is possibly 'null'.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the reported TypeScript 2.5.3 snippet using HTMLCanvasElement.getContext("2d") and compare the declared return type with the Canvas API behavior described in the issue. Determine the intended nullability for CanvasRenderingContext2D, then verify that the example type-checks and that calling fillRect produces no nullability error.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- frontend, web-dev
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100