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Move type checking logic and type conversions outside of your function in a flexible, organized way. Automatically throw informative errors in case of wrong input arguments.


typed-function has the following features:

  • Runtime type-checking of input arguments.
  • Automatic type conversion of arguments.
  • Compose typed functions with multiple signatures.
  • Supports union types, any type, and variable arguments.
  • Detailed error messaging.

Supported environments: node.js, Chrome, Firefox, Safari, Opera, IE11+.


In JavaScript, functions can be called with any number and any type of arguments. When writing a function, the easiest way is to just assume that the function will be called with the correct input. This leaves the function's behavior on invalid input undefined. The function may throw some error, or worse, it may silently fail or return wrong results. Typical errors are TypeError: undefined is not a function or TypeError: Cannot call method 'request' of undefined. These error messages are not very helpful. It can be hard to debug them, as they can be the result of a series of nested function calls manipulating and propagating invalid or incomplete data.

Often, JavaScript developers add some basic type checking where it is important, using checks like typeof fn === 'function', date instanceof Date, and Array.isArray(arr). For functions supporting multiple signatures, the type checking logic can grow quite a bit, and distract from the actual logic of the function.

For functions dealing with a considerable amount of type checking and conversion logic, or functions facing a public API, it can be very useful to use the typed-function module to handle the type-checking logic. This way:

  • Users of the function get useful and consistent error messages when using the function wrongly.
  • The function cannot silently fail or silently give wrong results due to invalid input.
  • Correct type of input is assured inside the function. The function's code becomes easier to understand as it only contains the actual function logic. Lower level utility functions called by the type-checked function can possibly be kept simpler as they don't need to do additional type checking.

It's important however not to overuse type checking:

  • Locking down the type of input that a function accepts can unnecessarily limit its flexibility. Keep functions as flexible and forgiving as possible, follow the robustness principle here: "be liberal in what you accept and conservative in what you send" (Postel's law).
  • There is no need to apply type checking to all functions. It may be enough to apply type checking to one tier of public facing functions.
  • There is a performance penalty involved for all type checking, so applying it everywhere can unnecessarily worsen the performance.


Install via npm:

npm install typed-function


Here are some usage examples. More examples are available in the /examples folder.

var typed = require('typed-function');

// create a typed function
var fn1 = typed({
  'number, string': function (a, b) {
    return 'a is a number, b is a string';

// create a typed function with multiple types per argument (type union)
var fn2 = typed({
  'string, number | boolean': function (a, b) {
    return 'a is a string, b is a number or a boolean';

// create a typed function with any type argument
var fn3 = typed({
  'string, any': function (a, b) {
    return 'a is a string, b can be anything';

// create a typed function with multiple signatures
var fn4 = typed({
  'number': function (a) {
    return 'a is a number';
  'number, boolean': function (a, b) {
    return 'a is a number, b is a boolean';
  'number, number': function (a, b) {
    return 'a is a number, b is a number';

// create a typed function from a plain function with signature
function fnPlain(a, b) {
  return 'a is a number, b is a string';
fnPlain.signature = 'number, string';
var fn5 = typed(fnPlain);

// use the functions
console.log(fn1(2, 'foo'));      // outputs 'a is a number, b is a string'
console.log(fn4(2));             // outputs 'a is a number'

// calling the function with a non-supported type signature will throw an error
try {
  fn2('hello', 'world');
catch (err) {
  // outputs:  TypeError: Unexpected type of argument.
  //           Expected: number or boolean, actual: string, index: 1.


typed-function has the following built-in types:

  • null
  • boolean
  • number
  • string
  • Function
  • Array
  • Date
  • RegExp
  • Object

The following type expressions are supported:

  • Multiple arguments: string, number, Function
  • Union types: number | string
  • Variable arguments: ...number
  • Any type: any


When a typed function is called, an implementation with a matching signature is called, where conversions may be applied to actual arguments in order to find a match.

Among all matching signatures, the one to execute is chosen by the following preferences, in order of priority:

  • one that does not have an ...any parameter
  • one with the fewest any parameters
  • one that does not use conversions to match a rest parameter
  • one with the fewest conversions needed to match overall
  • one with no rest parameter
  • If there's a rest parameter, the one with the most non-rest parameters
  • The one with the largest number of preferred parameters
  • The one with the earliest preferred parameter

When this process gets to the point of comparing individual parameters, the preference between parameters is determined by the following, in priority order:

  • All specific types are preferred to the 'any' type
  • All directly matching types are preferred to conversions
  • Types earlier in the list of known types are preferred
  • Among conversions, ones earlier in the list are preferred

If none of these aspects produces a preference, then in those contexts in which Array.sort is stable, the order implementations were listed when the typed-function was created breaks the tie. Otherwise the dispatch may select any of the "tied" implementations.



typed([name: string], ...Object.<string, function>|function)

A typed function can be constructed from an optional name and any number of (additional) arguments that supply the implementations for various signatures. Each of these further arguments must be one of the following:

  • An object with one or multiple signatures, i.e. a plain object with string keys, each of which names a signature, and functions as the values of those keys.

  • A previously constructed typed function, in which case all of its signatures and corresponding implementations are merged into the new typed function.

  • A plain function with a signature property whose value is a string giving that function's signature.

The name, if specified, must be the first argument. If not specified, the new typed-function's name is inherited from the arguments it is composed from, as long as any that have names agree with one another.

If the same signature is specified by the collection of arguments more than once with different implementations, an error will be thrown.

Properties and methods of a typed function fn

  • : string

    The name of the typed function, if one was assigned at creation; otherwise, the value of this property is the empty string.

  • fn.signatures : Object.<string, function>

    The value of this property is a plain object. Its keys are the string signatures on which this typed function fn is directly defined (without conversions). The value for each key is the function fn will call when its arguments match that signature. This property may differ from the similar object used to create the typed function, in that the originally provided signatures are parsed into a canonical, more usable form: union types are split into their constituents where possible, whitespace in the signature strings is removed, etc.

  • fn.toString() : string

    Returns human-readable code showing exactly what the function does. Mostly for debugging purposes.

Methods of the typed package

  • typed.convert(value: *, type: string) : *

    Convert a value to another type. Only applicable when conversions have been added with typed.addConversion() and/or typed.addConversion() (see below in the method list). Example:

      from: 'number',
      to: 'string',
      convert: function (x) {
        return +x;
    var str = typed.convert(2.3, 'string'); // '2.3' 
  • typed.create() : function

    Create a new, isolated instance of typed-function. Example:

    var typed = require('typed-function');  // default instance
    var typed2 = typed.create();            // a second instance

    This would allow you, for example, to have two different type hierarchies for different purposes.

  • typed.resolve(fn: typed-function, argList: Array<any>): signature-object

    Find the specific signature and implementation that the typed function fn will call if invoked on the argument list argList. Returns null if there is no matching signature. The returned signature object has properties params, test, fn, and implementation. The difference between the last two properties is that fn is the original function supplied at typed-function creation time, whereas implementation is ready to be called on this specific argList, in that it will first perform any necessary conversions and gather arguments up into "rest" parameters as needed.

    Thus, in the case that arguments a0,a1,a2 (say) do match one of the signatures of this typed function fn, then fn(a0, a1, a2) (in a context in which this will be, say, t) does exactly the same thing as

    typed.resolve(fn, [a0,a1,a2]).implementation.apply(t, [a0,a1,a2]).

    But resolve is useful if you want to interpose any other operation (such as bookkeeping or additional custom error checking) between signature selection and execution dispatch.

  • typed.findSignature(fn: typed-function, signature: string | Array, options: object) : signature-object

    Find the signature object (as returned by typed.resolve above), but based on the specification of a signature (given either as a comma-separated string of parameter types, or an Array of strings giving the parameter types), rather than based on an example argument list.

    The optional third argument, is a plain object giving options controlling the search. Currently, the only implemented option is exact, which if true (defaults to false), limits the search to exact type matches, i.e. signatures for which no conversion functions need to be called in order to apply the function.

    Throws an error if the signature is not found.

  • typed.find(fn: typed-function, signature: string | Array, options: object) : function

    Convenience method that returns just the implementation from the signature object produced by typed.findSignature(fn, signature, options).

    For example:

    var fn = typed(...);
    var f = typed.find(fn, ['number', 'string']);
    var f = typed.find(fn, 'number, string', 'exact');
  • typed.referTo(...string, callback: (resolvedFunctions: ...function) => function)

    Within the definition of a typed-function, resolve references to one or multiple signatures of the typed-function itself. This looks like:

    typed.referTo(signature1, signature2, ..., function callback(fn1, fn2, ...) {
      // ... use the resolved signatures fn1, fn2, ...

    Example usage:

    const fn = typed({
      'number': function (value) {
        return 'Input was a number: ' + value;
      'boolean': function (value) {
        return 'Input was a boolean: ' + value;
      'string': typed.referTo('number', 'boolean', (fnNumber, fnBoolean) => {
        return function fnString(value) {
          // here we use the signatures of the typed-function directly:
          if (value === 'true') {
            return fnBoolean(true);
          if (value === 'false') {
            return fnBoolean(false);
          return fnNumber(parseFloat(value));

    See also typed.referToSelf(callback).

  • typed.referToSelf(callback: (self) => function)

    Refer to the typed-function itself. This can be used for recursive calls. Calls to self will incur the overhead of fully re-dispatching the typed-function. If the signature that needs to be invoked is already known, you can use typed.referTo(...) instead for better performance.

    In typed-function@2 it was possible to use this(...) to reference the typed-function itself. In typed-function@v3, such usage is replaced with the typed.referTo(...) and typed.referToSelf(...) methods. Typed-functions are unbound in typed-function@v3 and can be bound to another context if needed.

  • typed.isTypedFunction(entity: any): boolean

    Return true if the given entity appears to be a typed function (created by any instance of typed-function), and false otherwise. It tests for the presence of a particular property on the entity, and so could be deceived by another object with the same property, although the property is chosen so that's unlikely to happen unintentionally.

  • typed.addType(type: {name: string, test: function, [, beforeObjectTest=true]): void

    Add a new type. A type object contains a name and a test function. The order of the types determines in which order function arguments are type-checked, so for performance it's important to put the most used types first. Also, if one type is contained in another, it should likely precede it in the type order so that it won't be masked in type testing.


    function Person(...) {
    Person.prototype.isPerson = true;
      name: 'Person',
      test: function (x) {
        return x && x.isPerson === true;

    By default, the new type will be inserted before the Object test because the Object test also matches arrays and classes and hence typed-function would never reach the new type. When beforeObjectTest is false, the new type will be added at the end of all tests.

  • typed.addTypes(types: TypeDef[] [, before = 'any']): void

    Adds an list of new types. Each entry of the types array is an object like the type argument to typed.addType. The optional before argument is similar to typed.addType as well, except it should be the name of an arbitrary type that has already been added (rather than just a boolean flag)

  • typed.clear(): void

    Removes all types and conversions from the typed instance. Note that any typed-functions created before a call to clear will still operate, but they may prouce unintelligible messages in case of type mismatch errors.

  • typed.addConversion(conversion: {from: string, to: string, convert: function}) : void

    Add a new conversion.

      from: 'boolean',
      to: 'number',
      convert: function (x) {
        return +x;

    Note that any typed functions created before this conversion is added will not have their arguments undergo this new conversion automatically, so it is best to add all of your desired automatic conversions before defining any typed functions.

  • typed.addConversions(conversions: ConversionDef[]): void

    Convenience method that adds a list of conversions. Each element in the conversions array should be an object like the conversion argument of typed.addConversion.

  • typed.removeConversion(conversion: ConversionDef): void

    Removes a single existing conversion. An error is thrown if there is no conversion from and to the given types with a strictly equal convert function as supplied in this call.

  • typed.clearConversions(): void

    Removes all conversions from the typed instance (leaving the types alone).

  • typed.createError(name: string, args: Array.<any>, signatures: Array.<Signature>): TypeError

    Generates a custom error object reporting the problem with calling the typed function of the given name with the given signatures on the actual arguments args. Note the error object has an extra property data giving the details of the problem. This method is primarily useful in writing your own handler for a type mismatch (see the typed.onMismatch property below), in case you have tried to recover but end up deciding you want to throw the error that the default handler would have.


  • typed.onMismatch: function

    The handler called when a typed-function call fails to match with any of its signatures. The handler is called with three arguments: the name of the typed function being called, the actual argument list, and an array of the signatures for the typed function being called. (Each signature is an object with property 'signature' giving the actual signature and
    property 'fn' giving the raw function for that signature.) The default value of onMismatch is typed.throwMismatchError.

    This can be useful if you have a collection of functions and have common behavior for any invalid call. For example, you might just want to log the problem and continue:

    const myErrorLog = [];
    typed.onMismatch = (name, args, signatures) => {
      myErrorLog.push(`Invalid call of ${name} with ${args.length} arguments.`);
      return null;
    typed.sqrt(9); // assuming definition as above, will return 3
    typed.sqrt([]); // no error will be thrown; will return null.
    console.log(`There have been ${myErrorLog.length} invalid calls.`)

    Note that there is only one onMismatch handler at a time; assigning a new value discards the previous handler. To restore the default behavior, just assign typed.onMismatch = typed.throwMismatchError.

    Finally note that this handler fires whenever any typed function call does not match any of its signatures. You can in effect define such a "handler" for a single typed function by simply specifying an implementation for the ... signature:

    const lenOrNothing = typed({
      string: s => s.length,
      '...': () => 0
    console.log(lenOrNothing('Hello, world!')) // Output: 13
    console.log(lenOrNothing(57, 'varieties')) // Output: 0
  • typed.warnAgainstDeprecatedThis: boolean

    Since typed-function v3, self-referencing a typed function using this(...) or this.signatures has been deprecated and replaced with the functions typed.referTo and typed.referToSelf. By default, all function bodies will be scanned against this deprecated usage pattern and an error will be thrown when encountered. To disable this validation step, change this option to false.


The this keyword can be used to self-reference the typed-function:

var sqrt = typed({
  'number': function (value) {
    return Math.sqrt(value);
  'string': function (value) {
    // on the following line we self reference the typed-function using "this"
    return this(parseInt(value, 10));

// use the typed function
console.log(sqrt('9')); // output: 3


Version 4

  • Extend function signatures:
    • Optional arguments like '[number], array' or like number=, array
    • Nullable arguments like '?Object'
  • Allow conversions to fail (for example string to number is not always possible). Call this fallible or optional?

Version 5

  • Extend function signatures:
    • Constants like '"linear" | "cubic"', '0..10', etc.
    • Object definitions like '{name: string, age: number}'
    • Object definitions like 'Object.<string, Person>'
    • Array definitions like 'Array.<Person>'
  • Improve performance of both generating a typed function as well as the performance and memory footprint of a typed function.


To test the library, run:

npm test


To generate the minified version of the library, run:

npm run minify


  1. Describe the changes in
  2. Increase the version number in package.json
  3. Test and build:
    npm install
    npm run build
    npm test
  4. Verify whether the bundle and minified bundle works correctly by opening ./test/browser.html and ./test/browser.min.html in your browser.
  5. Commit the changes
  6. Merge develop into master, and push master
  7. Create a git tag, and pus this
  8. publish the library:
    npm publish


Runtime type-checking for JavaScript functions







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