Cannot use generics in JSDoc object signature

[schmidtk]

TypeScript does not appear to support using generics in JSDoc Object types (Object<string, T>). Doing so results in errors like Index signatures are incompatible. Type 'T' is not assignable to type 'string'..

TypeScript Version: 3.1.0-dev.20180919

Search Terms:

• jsdoc object generic
• object generic is not assignable to type
• jsdoc object generic index signature

Code

/**
 * @param {T} val The value.
 * @return {Object<string, T>} The object.
 * @template T
 */
const createGenericObject = function(val) {
 return {
 'key': val
 };
};

/**
 * @const
 * @type {Object<string, string>}
 */
const myStringObject = createGenericObject('hello');

/**
 * @const
 * @type {Object<string, number>}
 */
const myNumberObject = createGenericObject(42);

Expected behavior:
No errors.

Actual behavior:

objectgeneric.js:16:7 - error TS2322: Type '{ [x: string]: T; }' is not assignable to type '{ [x: string]: string; }'.
 Index signatures are incompatible.
 Type 'T' is not assignable to type 'string'.

16 const myStringObject = createGenericObject('hello');
 ~~~~~~~~~~~~~~

objectgeneric.js:22:7 - error TS2322: Type '{ [x: string]: T; }' is not assignable to type '{ [x: string]: number; }'.
 Index signatures are incompatible.
 Type 'T' is not assignable to type 'number'.

22 const myNumberObject = createGenericObject(42);
 ~~~~~~~~~~~~~~

Playground Link: Not applicable (tsc behavior).

Related Issues: None found.

[schmidtk]

Andy (Andrewkraft) (@Andy-MS) / Nathan Shively-Sanders (@sandersn): Is this something that could make the 3.2 milestone? I'm trying to get a rough estimate for planning purposes. Thanks for your time!

[sandersn]

Kevin Schmidt (@schmidtk) It's planned for 3.2, and next on my list after improving the infer-from-usage codefix (#27610). It's not a simple fix — JSDoc generics do not create actual generic types right now — so plan like you would for any software engineering estimate.

[schmidtk]

Nathan Shively-Sanders (@sandersn) Excellent, appreciate the feedback. Thank you!