JavaScript: function returning an object
In JavaScript, most functions are both callable and instantiable: they have both a [[Call]] and [[Construct]] internal methods.
As callable objects, you can use parentheses to call them, optionally passing some arguments. As a result of the call, the function can return a value.
var player = makeGamePlayer("John Smith", 15, 3);
The code above calls function makeGamePlayer
and stores the returned value in the variable player
. In this case, you may want to define the function like this:
function makeGamePlayer(name, totalScore, gamesPlayed) {
// Define desired object
var obj = {
name: name,
totalScore: totalScore,
gamesPlayed: gamesPlayed
};
// Return it
return obj;
}
Additionally, when you call a function you are also passing an additional argument under the hood, which determines the value of this
inside the function. In the case above, since makeGamePlayer
is not called as a method, the this
value will be the global object in sloppy mode, or undefined in strict mode.
As constructors, you can use the new
operator to instantiate them. This operator uses the [[Construct]] internal method (only available in constructors), which does something like this:
- Creates a new object which inherits from the
.prototype
of the constructor - Calls the constructor passing this object as the
this
value - It returns the value returned by the constructor if it's an object, or the object created at step 1 otherwise.
var player = new GamePlayer("John Smith", 15, 3);
The code above creates an instance of GamePlayer
and stores the returned value in the variable player
. In this case, you may want to define the function like this:
function GamePlayer(name,totalScore,gamesPlayed) {
// `this` is the instance which is currently being created
this.name = name;
this.totalScore = totalScore;
this.gamesPlayed = gamesPlayed;
// No need to return, but you can use `return this;` if you want
}
By convention, constructor names begin with an uppercase letter.
The advantage of using constructors is that the instances inherit from GamePlayer.prototype
. Then, you can define properties there and make them available in all instances
You can simply do it like this with an object literal:
function makeGamePlayer(name,totalScore,gamesPlayed) {
return {
name: name,
totalscore: totalScore,
gamesPlayed: gamesPlayed
};
}
The latest way to do this with ES2016 JavaScript
let makeGamePlayer = (name, totalScore, gamesPlayed) => ({
name,
totalScore,
gamesPlayed
})