robertcoopercode

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

let dog: Animal;

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

robertcoopercode

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

let dog: Animal;

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

robertcoopercode

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

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

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

// 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

type Student = {
    id: string;
    age: number;
};

type Employee = {
    companyId: string;
};

let person: Student & Employee;

robertcoopercode

let list: [string, string, number];

list = ['apple', 'banana', 8.75]; // ✅
list = ['apple', true, 8.75]; // ❌ - the second argument should be of type string
list = ['apple', 'banana', 10.33, 3]; // ❌ - the tuple specifies a length of 3, not 4

robertcoopercode

// Optional function parameter
function callMom(message?: string) {
    if (!message) {
        console.log('Hi mom. Love you. Bye.');
    } else {
        console.log(message);
    }
}

// Interface describing an object containing an optional property

robertcoopercode

enum ThemeColors {
    Primary = 'primary',
    Secondary = 'secondary',
    Dark = 'dark',
    DarkSecondary = 'darkSecondary',
};

robertcoopercode

const fillArray = <ArrayElementType>(len: number, elem: ArrayElementType) => {
    return new Array<ArrayElementType>(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