Overloads
JavaScript is inherently a very dynamic language. It’s not uncommon for a single JavaScript function to return different types of objects based on the shape of the arguments passed in.
let suits = ["hearts", "spades", "clubs", "diamonds"];
function pickCard(x): any {
// Check to see if we're working with an object/array
// if so, they gave us the deck and we'll pick the card
if (typeof x == "object") {
let pickedCard = Math.floor(Math.random() * x.length);
return pickedCard;
}
// Otherwise just let them pick the card
else if (typeof x == "number") {
let pickedSuit = Math.floor(x / 13);
return { suit: suits[pickedSuit], card: x % 13 };
}
}
let myDeck = [{ suit: "diamonds", card: 2 }, { suit: "spades", card: 10 }, { suit: "hearts", card: 4 }];
let pickedCard1 = myDeck[pickCard(myDeck)];
alert("card: " + pickedCard1.card + " of " + pickedCard1.suit);
let pickedCard2 = pickCard(15);
alert("card: " + pickedCard2.card + " of " + pickedCard2.suit);
Here the pickCard function will return two different things based on what the user has passed in. If the users passes in an object that represents the deck, the function will pick the card. If the user picks the card, we tell them which card they’ve picked. But how do we describe this to the type system?
The answer is to supply multiple function types for the same function as a list of overloads. This list is what the compiler will use to resolve function calls. Let’s create a list of overloads that describe what our pickCard accepts and what it returns.
let suits = ["hearts", "spades", "clubs", "diamonds"];
function pickCard(x: {suit: string; card: number; }[]): number;
function pickCard(x: number): {suit: string; card: number; };
function pickCard(x): any {
// Check to see if we're working with an object/array
// if so, they gave us the deck and we'll pick the card
if (typeof x == "object") {
let pickedCard = Math.floor(Math.random() * x.length);
return pickedCard;
}
// Otherwise just let them pick the card
else if (typeof x == "number") {
let pickedSuit = Math.floor(x / 13);
return { suit: suits[pickedSuit], card: x % 13 };
}
}
let myDeck = [{ suit: "diamonds", card: 2 }, { suit: "spades", card: 10 }, { suit: "hearts", card: 4 }];
let pickedCard1 = myDeck[pickCard(myDeck)];
alert("card: " + pickedCard1.card + " of " + pickedCard1.suit);
let pickedCard2 = pickCard(15);
alert("card: " + pickedCard2.card + " of " + pickedCard2.suit);
With this change, the overloads now give us type-checked calls to the pickCard function.
In order for the compiler to pick the correct typecheck, it follows a similar process to the underlying JavaScript. It looks at the overload list, and proceeding with the first overload attempts to call the function with the provided parameters. If it finds a match, it picks this overload as the correct overload. For this reason, its customary to order overloads from most specific to least specific.
Note that the function pickCard(x): any piece is not part of the overload list, so it only has two overloads: one that takes an object and one that takes a number. Calling pickCard with any other parameter types would cause an error.
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