//@ts-check and DOM element properties gives an error

Issue

I’ve got some simple browser-side JS code, and thought I’d try using @ts-check to pick out any fluff. Some valid bugs were found, and I’ve added js-doc parameter type information as well. I don’t want a transpile step so this needs to be vanilla Javascript.

When I access DOM element properties I get errors, because TS doesn’t know the real type of these elements.

s.YWindows = document.getElementById("rows-input").valueAsNumber

gives…

Property 'valueAsNumber' does not exist on type 'HTMLElement'

I thought I could use a JSDoc type hint to resolve that, but it only moves the problem.

     /** @type {HTMLInputElement} */
    let r =  document.getElementById("rows-input")
    s.YWindows = r.valueAsNumber

Type 'HTMLElement' is missing the following properties from type 'HTMLInputElement': accept, align, alt, autocomplete, and 52 more.

Suggestions, or do I just have to disable around this section somehow?

Solution

The TypeScript compiler supports inline type casting when using JSDoc syntax. From the link:

TypeScript borrows cast syntax from Google Closure. This lets you cast types to other types by adding a @type tag before any parenthesized expression.

/**
 * @type {number | string}
 */
var numberOrString = Math.random() < 0.5 ? "hello" : 100;
var typeAssertedNumber = /** @type {number} */ (numberOrString);

You can even cast to const just like TypeScript:

let one = /** @type {const} */(1);

Here’s an example which addresses the details in your question:

TS Playground

/** @type {{ YWindows?: number }} */
const s = {};

// It's not safe to assume that the ID exists in the document,
// so null must be included in the union:
const input = /** @type {HTMLInputElement | null} */(document.getElementById("rows-input"));

if (input) {
  s.YWindows; // number | undefined
  s.YWindows = input.valueAsNumber;
  s.YWindows; // number
}

s.YWindows; // number | undefined

Answered By – jsejcksn

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