Summary

In this lesson, you learned that:

• An object keeps together values that describe one thing: { name: "Rex", species: "dog", age: 3 }. It is made of properties: a key, a colon and a value. Properties are separated by commas.
• console.log(pet) prints an object the way it's written in code. To add a label, separate it with a comma: console.log("Pet:", pet). With +, the object would turn into [object Object].
• You read a property with a dot: pet.name. A property the object doesn't have gives undefined. Uppercase and lowercase letters in keys matter.
• In square brackets, the key can be in a variable: stock[fruit]. stock["peaches"] is the same as stock.peaches.
• Assigning changes a property (pet.age = 3;) or adds a new one (pet.owner = "Eva";). This works on an object in a constant too; you just can't put a different object in the constant.
• An array of objects holds many objects of the same kind. pets[1].name is the name of the second pet.
• A for...of loop goes through such an array one object at a time. You can print the objects, add up a property or pick some of them with if.
• A function can take an object or an array of objects as an argument, and it can return a new object.
• A function stored in an object is called a method, like log in console.log().

In the next lesson, JavaScript on Web Pages, your code will run on a web page: it will change what the page shows and react to clicks. And you'll see that the parts of a web page are objects too, with properties you can read and change.

Now some practice. The library in the editor keeps a list of its books.