The element id verification only functions with optional chaining, not with type checking

Question

The element id verification only functions with optional chaining, not with type checking

While attempting to apply a gradient to a line chart, I encountered the task of accessing its canvas element. I made sure to implement a typecheck before proceeding with the canvas manipulation. However, I received an error message from Vetur stating that the "Object is possibly 'null'.Vetur(2531)".

  mounted() {
    const canv = document.getElementById("line-chart") as HTMLCanvasElement;
    if (canv !== null && canv !== undefined) {
      const gradient = canv
        .getContext("2d")
        .createLinearGradient(0, 0, 0, canv.height);

After conducting some research, I decided to utilize the optional chaining operator for type checking. Surprisingly, this method proved successful in avoiding any reported errors.

  mounted() {
    const canv = document.getElementById("line-chart") as HTMLCanvasElement;
    const gradient = canv
      ?.getContext("2d")
      ?.createLinearGradient(0, 0, 0, canv.height);

I find it puzzling why the initial approach did not work, considering that canv is declared as a constant and cannot change. In theory, a simple type check should have sufficed.

Why do you think only the optional chaining technique worked in this scenario?

typescript vue.js chart.js optional-chaining

Answer 1

Answer №1

The issue does not lie with canv. The compiler recognizes that canv itself is not null, but the return type for canv.getContext("2d") could potentially be null:

HTMLCanvasElement.getContext(
  contextId: "2d", 
  options?: CanvasRenderingContext2DSettings | undefined
): CanvasRenderingContext2D | null

This is what the "Object is possibly null" error is indicating. You can resolve this by using optional chaining after that call, like so:

if (canv !== null && canv !== undefined) {
    const gradient = canv.getContext("2d")?.createLinearGradient(0, 0, 0, canv.height)
}

Alternatively, you can perform a more explicit type check:

if (canv !== null && canv !== undefined) {
    const context = canv.getContext("2d");
    if (context) {
        const gradient = context.createLinearGradient(0, 0, 0, canv.height)
    }
}

Or any other approach that assures the compiler that canv.getContext("2d") will not be null:

if (canv !== null && canv !== undefined) {
    const gradient = (canv.getContext("2d") || { createLinearGradient: () => undefined })
        .createLinearGradient(0, 0, 0, canv.height);
}

Playground link to code

Answer 2

The issue does not lie with canv. The compiler recognizes that canv itself is not null, but the return type for canv.getContext("2d") could potentially be null:

HTMLCanvasElement.getContext(
  contextId: "2d", 
  options?: CanvasRenderingContext2DSettings | undefined
): CanvasRenderingContext2D | null

This is what the "Object is possibly null" error is indicating. You can resolve this by using optional chaining after that call, like so:

if (canv !== null && canv !== undefined) {
    const gradient = canv.getContext("2d")?.createLinearGradient(0, 0, 0, canv.height)
}

Alternatively, you can perform a more explicit type check:

if (canv !== null && canv !== undefined) {
    const context = canv.getContext("2d");
    if (context) {
        const gradient = context.createLinearGradient(0, 0, 0, canv.height)
    }
}

Or any other approach that assures the compiler that canv.getContext("2d") will not be null:

if (canv !== null && canv !== undefined) {
    const gradient = (canv.getContext("2d") || { createLinearGradient: () => undefined })
        .createLinearGradient(0, 0, 0, canv.height);
}

Playground link to code

The element id verification only functions with optional chaining, not with type checking

Answer №1

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