What is meant by 'first class object'?

To quote Wikipedia:

In computer science, a programming language is said to support first-class functions (or function literal) if it treats functions as first-class objects. Specifically, this means that the language supports constructing new functions during the execution of a program, storing them in data structures, passing them as arguments to other functions, and returning them as the values of other functions.

This page also illustrates it beautifully:

Really, just like any other variable

• A function is an instance of the Object type
• A function can have properties and has a link back to its constructor method
• You can store the function in a variable
• You can pass the function as a parameter to another function
• You can return the function from a function

also read TrayMan's comment, interesting...

The notion of "first-class functions" in a programming language was introduced by British computer scientist Christopher Strachey in the 1960s. The most famous formulation of this principle is probably in Structure and Interpretation of Computer Programs by Gerald Jay Sussman and Harry Abelson:

• They may be named by variables.
• They may be passed as arguments to procedures.
• They may be returned as the results of procedures.
• They may be included in data structures.

Basically, it means that you can do with functions everything that you can do with all other elements in the programming language. So, in the case of JavaScript, it means that everything you can do with an Integer, a String, an Array or any other kind of Object, you can also do with functions.

Simple test. If you can do this in your language (Python as example):

def double(x):
    return x*x

f = double

print f(5) #prints 25

Your language is treating functions as first class objects.

Definition on the Mozilla site is concise and clear. According to them,

In JavaScript, functions are first-class objects, because they can have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called. In brief, they are Function objects.

And

a function is a "subprogram" that can be called by code external (or internal in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function, and the function will return a value.

It means that functions are objects, with a type and a behaviour. They can be dynamically built, passed around as any other object, and the fact that they can be called is part of their interface.

It means that function actually inherits from Object. So that you can pass it around and work with it like with any other object.

In c# however you need to refrain to delegates or reflection to play around with functions. (this got much better recently with lambda expressions)

i guess when something is first class in a language, it means that it's supported by its syntax rather than a library or syntactic sugar. for example, classes in C are not first class

Simple in JavaScript, functions are first-class objects that is, functions are of the type Object and they can be used in a first-class manner like any other object (String, Array, Number, etc.) since they are in fact objects themselves. They can be “stored in variables, passed as arguments to functions, created within functions, and returned from functions