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Support type parameter defaults in JSDoc template tags #45481

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@rbuckton

Description

@rbuckton

Suggestion

Add support for type parameter defaults in JSDoc @template tags, to match parity with type parameter defaults in TypeScript:

// ts
type Foo<T = number> = T

// js
/**
 * @template [T=number]
 * @typedef {T} Foo
 */

🔍 Search Terms

jsdoc template type parameter default

✅ Viability 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, new syntax sugar for JS, etc.)
  • This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Implement type parameter defaults in @template similar to optional parameters in @param:

/**
 * @param {string} [x=1]  - NOTE: TS parses the expression `1` but ignores it.
 */
function f(x) {}

/**
 * @template {string} [T="hello"]
 * @typedef {T} Foo
 */
  • Brackets around T would be required (so as not to misinterpret existing documentation comments
  • A bracketed template type must have a default (unlike @param which allows [name] on its own).

📃 Motivating Example

Improves JavaScript JSDoc type annotation parity with TypeScript type annotations.

💻 Use Cases

Used for JavaScript development using the TypeScript language service.

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