typed-function

What is typed-function?

The typed-function npm package allows you to define functions with typed arguments in JavaScript. It provides a way to enforce type checking at runtime, making your code more robust and easier to debug.

What are typed-function's main functionalities?

Define a typed function

This feature allows you to define a function with multiple signatures, each with different types of arguments. The function will execute the appropriate implementation based on the types of the provided arguments.

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

const add = typed({
  'number, number': function (a, b) {
    return a + b;
  },
  'string, string': function (a, b) {
    return a + b;
  }
});

console.log(add(2, 3)); // 5
console.log(add('Hello, ', 'world!')); // 'Hello, world!'

Type checking

This feature enforces type checking at runtime, throwing an error if the provided arguments do not match any of the defined signatures. This helps catch type-related bugs early in the development process.

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

const multiply = typed({
  'number, number': function (a, b) {
    return a * b;
  }
});

try {
  console.log(multiply(2, '3')); // Throws an error
} catch (err) {
  console.error(err.message); // 'TypeError: Unexpected type of argument in function multiply (expected: number, actual: string, index: 1)'
}

Default types

This feature allows you to define default types for your function arguments. If no arguments are provided or if they do not match any specific type, the function will fall back to the default implementation.

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

const greet = typed({
  'string': function (name) {
    return 'Hello, ' + name + '!';
  },
  'any': function () {
    return 'Hello, world!';
  }
});

console.log(greet('Alice')); // 'Hello, Alice!'
console.log(greet()); // 'Hello, world!'

Other packages similar to typed-function

io-ts

io-ts is a runtime type system for IO decoding/encoding in TypeScript. It allows you to define types and validate data at runtime. Compared to typed-function, io-ts is more focused on data validation and transformation rather than function overloading.

runtypes

Runtypes provides a way to define and validate types at runtime in TypeScript. It offers a similar type-checking functionality but is more geared towards defining and validating data structures rather than function signatures.

ts-runtime

ts-runtime is a TypeScript transformer that adds runtime type checks to your TypeScript code. It provides a more integrated approach to type checking in TypeScript, whereas typed-function is a standalone library for JavaScript.

README

typed-function

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.

Features

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+.

Why?

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.

Load

Install via npm:

npm install typed-function

Usage

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) {
  console.log(err.toString());
  // outputs:  TypeError: Unexpected type of argument.
  //           Expected: number or boolean, actual: string, index: 1.
}

Types

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

API

Construction

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.

Methods

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

  Convert a value to another type. Only applicable when conversions have been defined in typed.conversions (see section Properties). Example:

  typed.conversions.push({
    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.find(fn: typed-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]): 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. All types are added to the Array typed.types.

  Example:

  function Person(...) {
    ...
  }
  
  Person.prototype.isPerson = true;
  
  typed.addType({
    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}) : void

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

  typed.addConversion({
    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.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.

Properties

• 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;
  
  typed.types.push({
    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 to define conversions from boolean to number. For example:

  typed.conversions.push({
    from: 'boolean',
    to: 'number',
    convert: function (x) {
      return +x;
  });
  

  Also note the addConversion() method above for simply adding a single conversion at a time.

• typed.ignore: Array.<string>

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

• 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
  

Recursion

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

Output

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 the name of the typed function, if it was assigned one at creation, or an empty string.

Roadmap

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.

Test

To test the library, run:

npm test

Minify

To generate the minified version of the library, run:

npm run minify

Publish

1. Describe the changes in HISTORY.md
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