JS with Types Start

JSDoc tag

@extends and @augments

  • Checked by TypeScript
  • Tested with TypeScript 5.9
  • Alias @augments

In JavaScript you cannot write class Tally extends Map<string, number>. @extends {Map<string, number>} above the class says the same thing, and TypeScript checks every inherited method with those types. @augments is another name for the same tag.

The tag only supplies type arguments. The class still needs its own extends Map clause; the tag never creates inheritance.

At a glance

Use JSDoc in a .js file TypeScript in a .ts file
A generic base class /** @extends {Map<string, number>} */ class Tally extends Map {} class Tally extends Map<string, number> {}

Examples

tally.js tally.ts TypeScript 5.9.3 TypeScript 5.9.3, run 21 October 2026: No errors. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. No errors. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. No errors. checked 21 October 2026 
/** @extends {Map<string, number>} */
export class Tally extends Map {
  /** @param {string} key */
  bump(key) {
    this.set(key, (this.get(key) ?? 0) + 1);
    return this;
  }
}

const views = new Tally();
views.bump("home").bump("cart");
views.get("home")?.toFixed(0);
export class Tally extends Map<string, number> {
  bump(key: string) {
    this.set(key, (this.get(key) ?? 0) + 1);
    return this;
  }
}

const views = new Tally();
views.bump("home").bump("cart");
views.get("home")?.toFixed(0);
Giving a generic base class its type argument with @extends.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}
tags.js Error confirmed 5.9.3 TypeScript 5.9.3, run 21 October 2026: tags.js(11,10): error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. tags.js(11,10): error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. tags.js(11,10): error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'. checked 21 October 2026 
/** @augments {Set<string>} */
export class TagSet extends Set {
  /** @returns {string} */
  toLabel() {
    return [...this].sort().join(", ");
  }
}

const tags = new TagSet(["cotton", "summer"]);
tags.add(42);error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.
The type argument from @augments (the other name for @extends) is checked on every call.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}
tally.js TypeScript 5.9.3 TypeScript 5.9.3, run 21 October 2026: No errors. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. No errors. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. No errors. checked 21 October 2026 
export class Tally extends Map {
  /** @param {string} key */
  bump(key) {
    this.set(key, (this.get(key) ?? 0) + 1);
    return this;
  }
}

// No error: the base class is Map<any, any>, so a number key and a string value pass.
new Tally().set(1, "one");
Without @extends, a generic base class gets any for its types.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}

When you need it

Only when the base class is generic. Extending Map, Set, Array or your own generic class leaves the type parameters as any unless you supply them, as the third example shows: new Tally().set(1, "one") passes because the base is Map<any, any>.

A class that extends a non-generic class needs no tag at all; extends Base is enough.

Rules TypeScript applies

The type in the tag must name the same class as the extends clause; @extends {Set<string>} on extends Map is TS8023 ("JSDoc '@extends Set' does not match the 'extends Map' clause"). The tag works only on classes; the handbook notes that a constructor function cannot extend a class this way.

A class with @extends and no extends clause gets no error and no base class either: TypeScript 5.9 ignores the tag in that case, so make sure the clause is there.

Gotchas

  • Give the type arguments in the order the base class declares its type parameters: Map<K, V> takes the key type first.
  • To type a subclass's own generic parameter, combine the tags: @template T and @extends {Set<T>}.

Questions

What is the difference between @extends and @augments?

None. TypeScript and the JSDoc documentation tool treat them as the same tag.

  • JSDoc tag

    @implements

    The JSDoc @implements tag makes TypeScript check that a JavaScript class has every member of a type, as implements does in a .ts file. Syntax and examples.

  • JSDoc tag

    @template

    The JSDoc @template tag declares type parameters for generic functions, classes and typedefs, with constraints and defaults. Syntax and checked examples.

  • JSDoc tag

    @class and @constructor

    @class and @constructor mark a JavaScript function as a constructor, so TypeScript requires new and types this. When you need them, and when a class is simpler.

  • JSDoc tag

    @override

    The JSDoc @override tag marks a method that replaces one in the base class. TypeScript checks the base has it, and noImplicitOverride makes the tag required.

  • Type expression

    Generic classes

    A generic class in JSDoc: @template T above the class, T in the members' tags, and the type argument on the variable, since new Queue<T>() is not JavaScript.

  • TypeScript error

    TS8023: An @extends tag that does not match the extends clause

    TS8023 means a class's @extends (or @augments) tag names a different class from its extends clause. The tag should repeat the class, adding type arguments.

Sources

Last reviewed 20 October 2026.