What is meant by 'first class object'?

Question

In a recent question, I received suggestions to talk on, amongst other things, the aspect of JavaScript where functions are \'first class\' objects. What does the \'first c

Answer 1

In javascript functions are first class objects because it can do a lot more than what objects can do.

• A function is an instance of an Object type.

Function instanceof Object //returns true

Like an object a function can have properties and can have a link back to it’s constructor function.

 var o = {}; // empty object 'o'
    o.a = 1 ; 
    o.b = 2 ; 

    console.log(o.a); // 1
    console.log(o.b); // 2 


    function foo(){};
    foo.a = 3 ; 
    foo.b = 4 ; 

    console.log(foo.a);  // logs 3
    console.log(foo.b);  // logs 4 

• Functions can be stored in a variable as a value.

  var foo = function(){}; 
    console.log(foo); // function(){}

• Functions can be passed as arguments to other functions

function callback (foo){
      foo();
}

callback(function(){console.log('Successfuly invoked as an argument inside function callback')})

• You can return a function from a function

 function foo(){
	    return function(){console.log('working!')};
    }

    var bar = foo();
    bar(); // working!

• Can be stored in a variable as a reference.

    var sum = function (a,b){return a+b}  
    sum(4,4);