robertcoopercode

// The `name` parameter can be either a string or null
const sayHappyBirthdayOnFacebook = (name: string | null) => {
    if (name === null) {
        console.log('Happy birthday!');
    } else {
        console.log(`Happy birthday ${name}!`);
    }
};

sayHappyBirthdayOnFacebook(null); // => "Happy birthday!"

robertcoopercode

const fillArray = <T>(len: number, elem: T) => {
    return new Array<T>(len).fill(elem);
};

const newArray = fillArray<string>(3, 'hi'); // => ['hi', 'hi', 'hi']

newArray.push('bye'); // ✅
newArray.push(true); // ❌ - only strings can be added to the array

robertcoopercode

let dog: {
    kind: string;
    weight: number;
};

dog = {
    kind: 'mammal',
    weight: 10,
}; // ✅

robertcoopercode

type Animal = {
    kind: string;
    weight: number;
}

let dog: Animal;

dog = {
    kind: 'mammal',
    weight: 10

robertcoopercode

interface Animal {
    kind: string;
    weight: number;
}

let dog: Animal;

dog = {
    kind: 'mammal',
    weight: 10,

robertcoopercode

// First method is using the square bracket notation
let messageArray: string[] = ['hello', 'my name is fred', 'bye'];

// Second method uses the Array keyword notation
let messageArray: Array<string> = ['hello', 'my name is fred', 'bye'];

robertcoopercode

let aBoolean: boolean = true;
let aNumber: number = 10;
let aString: string = 'woohoo';

robertcoopercode

let list = [10, 22, 4, null, 5];

list.push(6); // ✅
list.push(null); // ✅
list.push('nope'); // ❌ - type 'string' is neither of type 'number' or 'null'

robertcoopercode

function add(a: number, b: number) {
    return a + b;
}

const result = add(2, 4);

result.toFixed(2); // ✅
result.length; // ❌ - length is not a property of number types

robertcoopercode

// Default function parameters
const tweetLength = (message = 'A default tweet') => {
    return message.length;
}