JavaScript Function Generator
Describe a function, its parameters and its return type, and get a ready-to-fill JavaScript function. The generator writes the signature with default values, a JSDoc block with types and optional parameters, runtime checks that throw TypeError for wrong arguments, a placeholder return value of the right type, and the export statement for ES modules or CommonJS.
- Runs in your browser
- No sign-up
- Free to use
How to use JavaScript Function Generator
- Enter the function name and a short description.
- List the parameters as name: type = default.
- Choose the return type, style and export.
- Copy the function and fill in the TODO.
JavaScript Function Generator features
Six styles
Declaration, arrow, async, async arrow, generator or class method.
JSDoc
Types, optional parameters, defaults, return type and @throws.
Runtime validation
typeof, Number.isInteger, Array.isArray and instanceof checks.
Defaults
Default parameter values written as literals.
Exports
Named, default, CommonJS or none.
Design hints
Warnings about parameter order and long parameter lists.
When to use JavaScript Function Generator
- Scaffolding API helpers and utility functions.
- Adding JSDoc to plain JavaScript projects for editor support.
- Writing defensive functions for public libraries.
- Teaching function syntax and documentation conventions.
JavaScript Function Generator FAQ
Why validate arguments at runtime?
JavaScript does not check types. Functions in libraries or at module boundaries often receive wrong values; failing early with a clear TypeError is easier to debug than a crash deeper in the code.
How are optional parameters documented?
JSDoc writes them in square brackets, with the default if there is one: @param {number} [limit=20].
What placeholder is returned?
A value of the declared type, such as '' for strings, 0 for numbers or [] for arrays, so the function can be called before it is implemented.
Which style should I choose?
Declarations are hoisted and easy to read; arrow functions suit callbacks and keep this from the surrounding scope; async functions return promises; generators produce sequences lazily.
Is the output TypeScript?
No, it is JavaScript with JSDoc. For TypeScript, use the TypeScript generators.
Is anything uploaded?
No. The function is generated in your browser.
Well-formed JavaScript functions
Most functions follow the same pattern: a name that says what they do, parameters with sensible defaults, checks that reject bad input, the actual work and a return value. Writing that frame by hand is repetitive, and the details, such as JSDoc syntax for optional parameters, are easy to get wrong. The generator produces the frame so you can focus on the logic.
Parameters are written in a compact neutral notation: name: type = default, with a question mark for optional parameters and a comment for a description. Types map to JavaScript and JSDoc naturally, so int and float become number, T[] becomes Array<T> and map<string, T> becomes Object<string, T>. Defaults are emitted as literals in the signature, where JavaScript evaluates them on each call.
With validation on, every typed parameter gets a check at the top of the body. Strings use typeof, integers Number.isInteger, floats a number check that also rejects NaN, dates instanceof Date with a valid time, and arrays Array.isArray with a check of each element. Optional parameters pass when undefined, and nullable types accept null.
The JSDoc block documents everything that callers need: the description, each parameter with its type, optional parameters in brackets with their defaults, the return type, wrapped in Promise for async functions, and the TypeError that validation may throw. Editors use it for autocompletion and inline documentation.
Finally the function is wrapped in the requested style and export. The notes point out design issues worth fixing early, such as a required parameter after an optional one or a long list of positional parameters that would read better as one options object.