In Javascript, if there is an object with a lot of properties that are functions, how do you convert them to array of strings (of the function names)?

Question

In Javascript, if an object has lots of properties that are functions:

var obj = { foo: function() { ... },
            bar: function() { ... },
              ...
          }

then how can you get an array of names of those functions? That is, an array

["foo", "bar", ... ]

thanks.

Answer 1

var names = [];
for( var k in obj ) {
   if(obj.hasOwnProperty(k) && typeof obj[k] == 'function') {
      names.push(k);
   }
}

Answer 2

var functions = [];
for (var prop in obj) {
    if ((typeof obj[prop]) == 'function') {
        // it's a function
        functions.push(prop);
    }
}

Answer 3

Edit: I've slightly misread the question, you want to extract the names of only the properties that are function objects:

function methods(obj) {
  var result = [];
  for (var prop in obj) {
    if (obj.hasOwnProperty(prop) && typeof obj[prop] == 'function') {
      result.push(prop);
    }
  }
  return result;
}

var obj = {
  foo: function() { },
  bar: function() { },
};

methods(obj); // ["foo", "bar"]

I'm using the hasOwnProperty method, to ensure that the enumerated properties in fact exist physically in the object.

Note that this approach and all other answers have a small problem IE.

The JScript's DontEnum Bug, custom properties that shadow non-enumerable properties (DontEnum) higher in the prototype chain, are not enumerated using the for-in statement, for example :

var foo = {
  constructor : function() { return 0; },
  toString : function() { return "1"; },
  valueOf : function() { return 2; }
  toLocaleString : function() { return "3"; }
};

for (var propName in foo ) { alert(propName); }

The object foo clearly has defined four own properties, but those properties exist in Object.prototype marked as DontEnum, if you try to enumerate the properties of that object with the for-in statement in IE, it won't find any.

This bug is present on all IE versions, and has been recently fixed in IE9 Platform Preview.

Answer 4

To complete other answers: you can also use instanceof:

var obj = { foo: function() { ... },
            bar: function() { ... },
              ...
          },
    fnArr = [];

for (var label in obj){
  if (obj[label] instanceof Function){
     fnArr.push(label)
  }
}

Answer 5

With ES5:

var obj = {
    foo: function() {},
    bar: function() {},
    baz: true
};

function getMethods(object) {
    return Object.keys(object).filter(function(key) {
        return typeof object[key] == 'function';
    });
}

getMethods(obj); // [foo, bar]

Object.keys(<object>) returns the names of all enumerable properties of an object as an array, of which the non-functions are filtered out.

Example - works on Chrome release and nightly builds of Webkit and Tracemonkey (Firefox).