Functions That Create Objects

A function can also return an object. This is handy when you create many objects of the same shape: you write the keys only once, in the function, and every object it creates has the same properties.

function createBook(title, author) {
    return {
        title: title,
        author: author,
        available: true
    };
}

const book = createBook("Matilda", "Roald Dahl");
console.log(book);

{ title: "Matilda", author: "Roald Dahl", available: true }

title: title may look strange. On the left of the colon is the key, the name of the new property. On the right is its value: the parameter title, which holds "Matilda" in this call. A new book is always available, so available is true in every book the function creates.

In other people's code you'll often see a shorter form: { title, author }. When a property should have the same name as the variable that holds its value, you can write the name just once. { title, author } means exactly the same as { title: title, author: author }. You can use either.

A new object can go straight into an array:

const library = [];
library.push(createBook("Matilda", "Roald Dahl"));
library.push(createBook("Momo", "Michael Ende"));

console.log(library);

[{ title: "Matilda", author: "Roald Dahl", available: true }, { title: "Momo", author: "Michael Ende", available: true }]

A typical mistake is to forget return. The function then returns undefined, and the library would be [undefined, undefined].