Typescript Introduction III

JavaScript
1

Functions

Function Types

Function types can be defined in three ways.

Method Signature


// function declaration
function addWithMethodSignature(x: number, y: number): number {
  // method signature
  return x + y;
}

// function expression
let anotherAddWithMethodSignature = function(x: number, y: number): number { 
  // method signature
  return x + y;
};

Function type literal

A function type literal specifies the type parameters, regular parameters, and return type of a call signature.

let addWithFuncLiteral: (x: number, y: number) => number = function(x, y) {
  return x + y;
};

Object literal

An object type containing one or more call signatures is said to be a function type.

let addWithObjLiteral: { (x: number, y: number) : number } = function(x, y) {
  return x + y;
};

Function type literal vs Object literal

  • Function types may be written using function type literals or by including call signatures in object type literals. i.e.
< T1, T2, ... > ( p1, p2, ... ) => R

is exactly equivalent to the object type literal

{ < T1, T2, ... > ( p1, p2, ... ) : R }
  • To differentiate function types defined with object literal, check to see whether the object literal/interface contains a call signature
interface A { // an object with a property called `log` which is function type
  log(num: number): number; // a property
}

let obj: A = {
  log(n) {
    return n;
  }
}

interface A1 { // a function type.
  (num: number): number // call signature
}

let func: A1 = (n: number) => n

Optional parameters

Optional property can be marked with ? following the parameter name. E.g.

function parseInt(s: string, radix?: number): number {
  // ...
}

let result1 = parseInt('3'); // ok 
let result2 = parseInt('3', 2);  // ok
let result3 = parseInt('3', 2 , 'too many'); // ok

Overloads

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 {
  if (typeof x == "object") {
    let pickedCard = Math.floor(Math.random() * x.length);
    return pickedCard;
  }
  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);

Notice

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