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Easy `Function` subclasses





Easy Function subclasses

Build Status

Creates a function instance with specified name and length and prototype. Essentially, it's everything that you need to subclass the ECMAscript Function built-in type.

You use the function-class/invoke Symbol to define the logic that will be executed when the created function instance is invoked.


Install with npm:

$ npm install function-class



var createFunction = require('function-class');
var invoke = require('function-class/invoke');

var fn = createFunction('theName', 6);
assert.equal(, 'theName');
assert.equal(fn.length, 6);

fn[invoke] = function (arg) {
  return + ' ' + this.length + ' ' + arg;

assert.equal('theName 6 foo', fn('foo'));


var inherits = require('util').inherits;
var createFunction = require('function-class');
var invoke = require('function-class/invoke');

function FunctionSubclass (foo) {
  if (typeof this !== 'function')
    return createFunction('subclass', 0, FunctionSubclass, arguments); = foo;
  assert.equal(typeof this, 'function');
  assert.ok(this instanceof FunctionSubclass);
inherits(FunctionSubclass, Function);

FunctionSubclass.prototype[invoke] = function () {

// usage
var f = new FunctionSubclass('foo');
assert.equal(, 'subclass');
assert.equal(f.length, 0);
assert.equal(f(), 'foo');


The main export is the createFunction() function, which returns a new Function instance.

createFunction(name, arity, constructor, arguments) → Function

  • name - String - the name to set for the created function, must be a string (even containing spaces, unicode, emoji, and otherwise invalid JS identifiers)
  • arity - Number - the length to set for the created function, must be a positive integer
  • constructor - Function - the class constructor to use invoke on the created instance, and inherit from the prototype of
  • arguments - Array - array of values to invoke constructor with

All arguemnts are optional. You must specify the invoke function to execute on the created function instance.

var createFunction = require('function-class');

// inherits from Function
var fn = createFunction('foo', 3);

// inherits from FunctionSubclass with constructors invoked with args
var args = [ 'foo', 'bar' ];
var fn = createFunction('name', 0, FunctionSubclass, args);

invoke → Symbol

You use the invoke Symbol to define the function to execute when a created function instance is invoked.

Note that this in the invoke function is bound to the created function instance when invoked without a receiver (i.e. "globally"). Otherwise, this respects the left-hand receiver, or target when used with .call()/.apply().

var invoke = require('function-class/invoke');

fn[invoke] = function () {
  console.log('fn has been invoked!');
  return this;

// fn has been invoked!

assert.equal(fn(), fn);
assert.equal(, global);