Question

I saw this code before, but I don’t know what the meaning:

var person1 = {
    toLocaleString : function(){
      return "Nikolaos";
     },
    toString : function(){
      return "Nicholas";
    }
}

var person2 = {
   toLocaleString : function(){
      return "bum";
   },
   toString : function(){
       return "Greg";
   } 

}

var people = [person1, person2];
alert(people.toString());
alert(people.toLocaleString());

does the function create an object with the method of toLocaleString and toString??or…??

Answer 1

That code is doing three things:

1. Using object literal syntax to create object instances
2. Using anonymous function expressions to create functions and bind them to properties on the objects. (Functions are first-class objects in JavaScript, so you can keep references to them, pass the references around, etc.)
3. Specifically, it’s overriding two standard functions that all JavaScript objects inherit from the Object prototype.

Let’s break it down a bit.

1) Object literal notation:

var obj = {propName: propValue};

The { and } in this case denote an object literal. Within an object literal, you can write propName: propValue to assign propValue to the property with the name propName on the object. This is the same as:

var obj = {};             // Get an empty object
obj.propName = propValue; // Add a property to it

You can do multiple properties separated with commas. So for instance:

var obj = {
    author: "Douglas Adams",
    title:  "The Hitchhiker's Guide to the Galaxy",
    answer: 42
};

That creates an object with three properties, two with string values and one with a number value.

Note that the right-hand side are processed just like an assignment, and so can be anything that can appear on the right-hand side of an assignment statement:

var x = "bar";
var obj = {
    three: 1 + 2,
    fubar: "foo " + x
};

The property names can be put in quotes if you like:

var x = "bar";
var obj = {
    "three": 1 + 2,
    "fubar": "foo " + x
};

…which is handy for specifying properties that have the names of reserved tokens (like “if”, or “return”) or formerly-reserved tokens (like “class”) where it would be a syntax error if they weren’t in quotes.

2) Now let’s look at function expressions:

var f = function() { /* your code here */ };

That’s a function expression. It creates a new function and assigns a reference to it to the variable f. You can call it by calling f().

var f = function(name) {
    alert("Hi " + name);
};
f("Fred"); // alerts "Hi Fred"

1 + 2) So putting it together with object literal notation:

var obj = {
    foo: function(name) {
        alert("Hi " + name);
    }
};
obj.foo("Fred"); // alerts "Hi Fred"

(I don’t like anonymous functions, I prefer my functions to have names, but that’s another topic.)

3) And finally: As maerics pointed out, the specific functions that are being used in that code are toString and toLocaleString, both of which are standard functions of JavaScript objects. That means that those will override the standard version and so return the given values whenever the standard function would have been called.