Joe Wood Notes

Functions

Change Topic
  • Function definition with arguments and a return type - how to pass arguments to a function and express the return type.
  • Function overloading - how to provide multiple different implementations with different sets of arguments
  • Default or Optional Arguments - how to provide defaults or express optionality in arguments
  • Lambda Functions - defining a lambda function
  • Lambda Function Capture - using local in-scope variables inside the lambda function
  • Function Type Parameter (Generics) - how to define a function with one or more types as an argument
  • Function Type Parameter Constraints - how to define a constraint on the type parameter

TypeScript Functions

Select Language

Change Language

Function Definition with Arguments and Return Type

TypeScript allows specifying argument types and return types explicitly in function definitions.

function add(x: number, y: number): number {
    return x + y;
}

const greet = (name: string): string => `Hello, ${name}!`;
  • x: number, y: number: Defines parameter types.
  • : number after () specifies the return type.

Function Overloading

Function overloading processed at runtime

function add(x: number, y: number): number;
function add(x: string, y: string): string;
function add(x: any, y: any): any {
    return x + y;
}

console.log(add(5, 10)); // 15
console.log(add("Hello, ", "World!")); // "Hello, World!"

Function Default Arguments

function doIt(b: number, a = 1) {
    return b / a;
}

console.log("DoIt " + doIt(10) + " " + doIt(10, 2));

Lambda Functions

const doIt = (b: number, c: number) => a * b;

console.log("DoIt " + doIt(10, 5));

Lambda Function Capture

function createCounter(): () => number {
    let count = 0; // This variable is captured by the closure

    return () => {
        count += 1;
        return count;
    };
}

const counterA = createCounter();
console.log(counterA()); // 1
console.log(counterA()); // 2

const counterB = createCounter();
console.log(counterB()); // 1 (separate closure)
console.log(counterA()); // 3 (original closure still intact)
function makeMultiplier(factor: number): (input: number) => number {
    // factor is captured by the inner function
    return (input: number) => input * factor;
}

const double = makeMultiplier(2);
const triple = makeMultiplier(3);

console.log(double(5)); // 10
console.log(triple(5)); // 15

Function Generics

function identity<T>(value: T): T {
    return value;
}

console.log(identity<number>(42)); // 42
console.log(identity<string>("Hello")); // "Hello"

Function Generics Constraint

And with a constraint:

function getLength<T extends { length: number }>(arg: T): number {
    return arg.length;
}

console.log(getLength("Hello")); // 5
console.log(getLength([1, 2, 3])); // 3