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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 unnecessary limit it's 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 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



A typed function can be constructed in two ways:

  • Create from an object with one or multiple signatures:

    typed(signatures: Object.<string, function>) : function
    typed(name: string, signatures: Object.<string, function>) : function
  • Merge multiple typed functions into a new typed function:

    typed(functions: ...function) : function
    typed(name: string, functions: ...function) : function

    Each function in functions can be either a typed function created before, or a plain function having a signature property.


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

    Convert an value to another type. Only applicable when conversions have been defined in typed.conversions (see section Properties). 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
  • typed.find(fn: function, signature: string | Array) : function | null

    Find a specific signature from a typed function. The function currently only finds exact matching signatures.

    For example:

    var fn = typed(...);
    var f = typed.find(fn, ['number', 'string']);
    var f = typed.find(fn, 'number, string');
  • typed.addType(type: {name: string, test: function} [, beforeObjectTest=true])

    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. All types are added to the Array typed.types.


    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.addConversion(conversion: {from: string, to: string, convert: function}

    Add a new conversion. Conversions are added to the Array typed.conversions.

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


  • typed.types: Array.<{name: string, test: function}>

    Array with types. Each object contains a type 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. Custom types can be added like:

    function Person(...) {
    Person.prototype.isPerson = true;
      name: 'Person',
      test: function (x) {
        return x && x.isPerson === true;
  • typed.conversions: Array.<{from: string, to: string, convert: function}>

    An Array with built-in conversions. Empty by default. Can be used for example to defined conversions from boolean to number. For example:

      from: 'boolean',
      to: 'number',
      convert: function (x) {
        return +x;
  • typed.ignore: Array.<string>

    An Array with names of types to be ignored when creating a typed function. This can be useful filter signatures when creating a typed function. Example:

    // a set with signatures maybe loaded from somewhere
    var signatures = {
      'number': function () {...},
      'string': function () {...}
    // we want to ignore a specific type
    typed.ignore = ['string'];
    // the created function fn will only contain the 'number' signature 
    var fn = typed('fn', signatures);


The functions generated with typed({...}) have:

  • A function toString. Returns well readable code which can be used to see what the function exactly does. Mostly for debugging purposes.
  • A property signatures, which holds a map with the (normalized) signatures as key and the original sub-functions as value.
  • A property name containing name of the typed function or an empty string.


Version 2

  • Be able to turn off exception throwing.
  • Extend function signatures:
    • Optional arguments like '[number], array' or like number=, array
    • Nullable arguments like '?Object'
  • Create a good benchmark, to get insight in the overhead.
  • Allow conversions to fail (for example string to number is not always possible). Call this fallible or optional?

Version 3

  • 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
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