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So I'm working on some code right now and there is this code I'm trying to understand. I've read a bit about it and it seems that the code is using object literals. So here is the code.

var car = function(){

    // ...
    function drive(model, color){
        // ... do something
    }
    // ...

    return {
        start: drive
    }

}();

Called in another file elsewhere

car.start(audi, black);

So how does this work. First it seems that in javascript a class can have a return method and not just a method. Also the return method is calling a method? using object literals? I'm a bit confused here.

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  • I am confused about what it is that you are confused about. Commented Dec 28, 2024 at 7:54

3 Answers 3

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That's an encapsulation pattern.

  1. Objects: There are no classes in Javascript, only objects. car holds an object with the structure seen in the return statement.
  2. Functions: The outer function is anonymous (no name) and directly called after definition (the parantheses at the end).
  3. Local scope: The drive function only exists in the scope of the surrounding function. You cannot call drive from outside.
  4. Encapsulation: With the return statement you can control which functionality will be accessable from the outside and how it is labeled. Therefore, car is an object with one attributed named start which is the function definition of drive.

If you call car.start without parentheses you get the function definition itself:

function (model, color) {
    ...
}

This is because functions can be assigned to variables (first-class) without actually calling them, so that you can call predefined functions at a later time with a scope of your choice.

If you call car.start(audi, black) you execute the function with the given parameters and get whatever is returned by the drive function.

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2

First it seems that in javascript a class

JavaScript doesn't have classes. The closest thing it has to them is constructor functions, but you don't have any of those here.

So how does this work

car is the return value of an anonymous function that is called as soon as it is defined (it has () after the closing }).

That return value is an object created using an object literal.

The value of the start property of that object literal is the drive function. Functions are first class objects in JavaScript so can be passed around just like any other kind of object.

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Thanks Quentin, this makes more sense now.
@MichaelNana - see it in action in your browser console here: jsfiddle.net/pratik136/Gbg4R
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From what it looks like you are establishing the object of car, then you are calling the drive function from another file and passing the values audi and black through the variables model and color. Then you are returning the instance of the object ("start") with the variables and actions held inside it.

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