Low readability when type casting by JSDoc

Search Terms

jsdoc, type cast

Suggestion

Currently type casting by JSDoc is not pretty 1) because the comment must be after variable declaration 2) it requires parentheses (#18212).

// @ts-check
const img = /** @type {HTMLImageElement} */ (document.getElementById('#cat'))
img.src = 'cat.gif'

How about the JSDoc type declaration automatically casts the assigned type, only when the variable declaration has the value assigning part?

Use Cases

The code will be prettier: https://github.com/w3c/respec/pull/1949#discussion_r240971984

Examples

Current:

// @ts-check

/** @type {HTMLImageElement} */
const img = document.getElementById('#cat') // Error: Type 'Element' is not assignable to type 'HTMLImageElement'
img.src = 'cat.gif'

Suggestion:

// @ts-check

/** @type {HTMLImageElement} */
const img = document.getElementById('#cat') // Element being casted as HTMLImageElement
img.src = 'cat.gif'

/** @type {HTMLImageElement} */
let img2;
img2 = document.getElementById('#cat') // Still an error

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Commencez par lire les exemples de castings JSDoc proposés et la discussion respec référencée. Comparez le comportement demandé pour les déclarations const avec le comportement explicitement inchangé pour les affectations ultérieures. Le travail est terminé lorsque la proposition dispose d’une implémentation approuvée et d’un comportement de vérification des types vérifié, sans modifier le JavaScript émis.