JS with Types Start

Type expression

Primitive types

  • Basics
  • Tested with TypeScript 5.9

JavaScript's primitive values each have a type with the same name in lowercase: string, number, boolean, bigint and symbol, plus null and undefined, which have their own page. Inside JSDoc braces you write them exactly as in a .ts file: @type {string}, @param {number} price.

There is no separate integer or float type: number covers both, as it does at run time. For whole numbers beyond Number.MAX_SAFE_INTEGER, use bigint and the n suffix.

At a glance

Use JSDoc in a .js file TypeScript in a .ts file
Text /** @type {string} */ : string
Numbers /** @type {number} */ : number
True or false /** @type {boolean} */ : boolean
Big integers /** @type {bigint} */ : bigint
Symbols /** @type {symbol} */ : symbol

Examples

product.js product.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 
/** @type {string} */
let name = "Cotton tee";

/** @type {number} */
let price = 19.5;

/** @type {boolean} */
let inStock = true;

/** @type {bigint} */
const views = 12_000_000_000n;

/** @type {symbol} */
const id = Symbol("product");

name = name.toUpperCase();
price = Math.round(price * 100) / 100;
inStock = !inStock;

export { name, price, inStock, views, id };
let name: string = "Cotton tee";

let price: number = 19.5;

let inStock: boolean = true;

const views: bigint = 12_000_000_000n;

const id: symbol = Symbol("product");

name = name.toUpperCase();
price = Math.round(price * 100) / 100;
inStock = !inStock;

export { name, price, inStock, views, id };
The primitive types in JSDoc.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}
price.js Error confirmed 5.9.3 TypeScript 5.9.3, run 21 October 2026: price.js(12,11): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. price.js(12,11): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. price.js(12,11): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'. checked 21 October 2026 
/**
 * @param {number} price
 * @param {number} quantity
 */
export function lineTotal(price, quantity) {
  return price * quantity;
}

const fromForm = "19.5";
lineTotal(Number(fromForm), 2);
lineTotal(fromForm, 2);error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
A string where a number belongs.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}
label.js Error confirmed 5.9.3 TypeScript 5.9.3, run 21 October 2026: label.js(6,7): error TS2322: Type 'String' is not assignable to type 'string'. 'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. label.js(6,7): error TS2322: Type 'String' is not assignable to type 'string'. 'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. label.js(6,7): error TS2322: Type 'String' is not assignable to type 'string'. 'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible. checked 21 October 2026 
/** @type {string} */
const label = "Cotton tee";

/** @type {string} */
const boxed = new String("Cotton tee");error TS2322: Type 'String' is not assignable to type 'string'.
  'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.

export { label, boxed };
new String() makes a wrapper object, not a string.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}

Lowercase, not capitalized

String, Number and Boolean with a capital letter are the wrapper object types, the type of new String("x"). In a .js file TypeScript reads {String}, {Number} and {Boolean} in JSDoc as the lowercase types, but new String(…) still creates a wrapper object, which is not a string: the third example assigns a new String(…) to a string and gets TS2322 with the hint "'string' is a primitive, but 'String' is a wrapper object". Write the lowercase names.

Values from forms and URLs

Input fields, query strings and localStorage give you strings. Passing one where a number is expected is the mistake the second example catches (TS2345). Convert at the boundary with Number(…) or parseInt(…, 10) and check the result with Number.isFinite before using it.

Gotchas

  • A number parameter accepts Infinity and the not-a-number value; the type cannot rule them out, so check with Number.isFinite where it matters.
  • Template literals and + with a string produce strings, so price + "" has type string, not number.
  • Without strict, null and undefined are assignable to every primitive type.

Questions

Should I write {String} or {string} in JSDoc?

{string}. In a .js file TypeScript reads {String} as string too, but in a .ts file String is the wrapper object type, and TypeScript's own messages and the handbook use the lowercase names.

  • Type expression

    Literal types

    A literal type is one exact value, such as "GET" or 5, used as a type. In JSDoc it goes inside the braces as in TypeScript, usually in a union of values.

  • Type expression

    Union types

    A union type says a value is one of several types: {string | number} in JSDoc, written as in TypeScript. How to write one, and how to narrow it before use.

  • JSDoc tag

    @type

    The JSDoc @type tag gives a variable, property or expression a type that TypeScript checks: the syntax, casts with parentheses, and whole function types.

  • JSDoc tag

    @param

    The JSDoc @param tag types a function parameter for TypeScript: optional and default values, rest parameters, options objects, and the errors it catches.

Sources

Last reviewed 20 October 2026.