Objects

As we know from the chapter Data types, there are eight data types in JavaScript. Seven of them are called “primitive”, because their values contain only a single thing (be it a string or a number or whatever).

In contrast, objects are used to store keyed collections of various data and more complex entities. In JavaScript, objects penetrate almost every aspect of the language. So we must understand them first before going in-depth anywhere else.

An object can be created with figure brackets {…} with an optional list of properties. A property is a “key: value” pair, where key is a string (also called a “property name”), and value can be anything.

We can imagine an object as a cabinet with signed files. Every piece of data is stored in its file by the key. It’s easy to find a file by its name or add/remove a file.

An empty object (“empty cabinet”) can be created using one of two syntaxes:

let user = new Object(); // "object constructor" syntax
let user = {};  // "object literal" syntax

Usually, the figure brackets {...} are used. That declaration is called an object literal.

Literals and properties

We can immediately put some properties into {...} as “key: value” pairs:

let user = {     // an object
  name: "John",  // by key "name" store value "John"
  age: 30        // by key "age" store value 30
};

A property has a key (also known as “name” or “identifier”) before the colon ":" and a value to the right of it.

In the user object, there are two properties:

1. The first property has the name "name" and the value "John".
2. The second one has the name "age" and the value 30.

The resulting user object can be imagined as a cabinet with two signed files labeled “name” and “age”.

We can add, remove and read files from it at any time.

Property values are accessible using the dot notation:

// get property values of the object:
alert( user.name ); // John
alert( user.age ); // 30

The value can be of any type. Let’s add a boolean one:

user.isAdmin = true;

To remove a property, we can use the delete operator:

delete user.age;

We can also use multiword property names, but then they must be quoted:

let user = {
  name: "John",
  age: 30,
  "likes birds": true  // multiword property name must be quoted
};

The last property in the list may end with a comma:

let user = {
  name: "John",
  age: 30,
}

That is called a “trailing” or “hanging” comma. Makes it easier to add/remove/move around properties, because all lines become alike.

Square brackets

For multiword properties, the dot access doesn’t work:

// this would give a syntax error
user.likes birds = true

JavaScript doesn’t understand that. It thinks that we address user.likes, and then gives a syntax error when comes across unexpected birds.

The dot requires the key to be a valid variable identifier. That implies: contains no spaces, doesn’t start with a digit and doesn’t include special characters ($ and _ are allowed).

There’s an alternative “square bracket notation” that works with any string:

let user = {};

// set
user["likes birds"] = true;

// get
alert(user["likes birds"]); // true

// delete
delete user["likes birds"];

Now everything is fine. Please note that the string inside the brackets is properly quoted (any type of quotes will do).

Square brackets also provide a way to obtain the property name as the result of any expression – as opposed to a literal string – like from a variable as follows:

let key = "likes birds";

// same as user["likes birds"] = true;
user[key] = true;

Here, the variable key may be calculated at run-time or depend on the user input. And then we use it to access the property. That gives us a great deal of flexibility.

For instance:

let user = {
  name: "John",
  age: 30
};

let key = prompt("What do you want to know about the user?", "name");

// access by variable
alert( user[key] ); // John (if enter "name")

The dot notation cannot be used in a similar way:

let user = {
  name: "John",
  age: 30
};

let key = "name";
alert( user.key ) // undefined

Computed properties

We can use square brackets in an object literal, when creating an object. That’s called computed properties.

For instance:

let fruit = prompt("Which fruit to buy?", "apple");

let bag = {
  [fruit]: 5, // the name of the property is taken from the variable fruit
};

alert( bag.apple ); // 5 if fruit="apple"

The meaning of a computed property is simple: [fruit] means that the property name should be taken from fruit.

So, if a visitor enters "apple", bag will become {apple: 5}.

Essentially, that works the same as:

let fruit = prompt("Which fruit to buy?", "apple");
let bag = {};

// take property name from the fruit variable
bag[fruit] = 5;

…But looks nicer.

We can use more complex expressions inside square brackets:

let fruit = 'apple';
let bag = {
  [fruit + 'Computers']: 5 // bag.appleComputers = 5
};

Square brackets are much more powerful than dot notation. They allow any property names and variables. But they are also more cumbersome to write.

So most of the time, when property names are known and simple, the dot is used. And if we need something more complex, then we switch to square brackets.

Property value shorthand

In real code, we often use existing variables as values for property names.

For instance:

function makeUser(name, age) {
  return {
    name: name,
    age: age,
    // ...other properties
  };
}

let user = makeUser("John", 30);
alert(user.name); // John

In the example above, properties have the same names as variables. The use-case of making a property from a variable is so common, that there’s a special property value shorthand to make it shorter.

Instead of name:name we can just write name, like this:

function makeUser(name, age) {
  return {
    name, // same as name: name
    age,  // same as age: age
    // ...
  };
}

We can use both normal properties and shorthands in the same object:

let user = {
  name,  // same as name:name
  age: 30
};

Property names limitations

As we already know, a variable cannot have a name equal to one of the language-reserved words like “for”, “let”, “return” etc.

But for an object property, there’s no such restriction:

// these properties are all right
let obj = {
  for: 1,
  let: 2,
  return: 3
};

alert( obj.for + obj.let + obj.return );  // 6

In short, there are no limitations on property names. They can be any strings or symbols (a special type for identifiers, to be covered later).

Other types are automatically converted to strings.

For instance, a number 0 becomes a string "0" when used as a property key:

let obj = {
  0: "test" // same as "0": "test"
};

// both alerts access the same property (the number 0 is converted to string "0")
alert( obj["0"] ); // test
alert( obj[0] ); // test (same property)

There’s a minor gotcha with a special property named __proto__. We can’t set it to a non-object value:

let obj = {};
obj.__proto__ = 5; // assign a number
alert(obj.__proto__); // [object Object] - the value is an object, didn't work as intended

As we see from the code, the assignment to a primitive 5 is ignored.

We’ll cover the special nature of __proto__ in subsequent chapters, and suggest the ways to fix such behavior.

Property existence test, “in” operator

A notable feature of objects in JavaScript, compared to many other languages, is that it’s possible to access any property. There will be no error if the property doesn’t exist!

Reading a non-existing property just returns undefined. So we can easily test whether the property exists:

let user = {};

alert( user.noSuchProperty === undefined ); // true means "no such property"

There’s also a special operator "in" for that.

The syntax is:

"key" in object

For instance:

let user = { name: "John", age: 30 };

alert( "age" in user ); // true, user.age exists
alert( "blabla" in user ); // false, user.blabla doesn't exist

Please note that on the left side of in there must be a property name. That’s usually a quoted string.

If we omit quotes, that means a variable should contain the actual name to be tested. For instance:

let user = { age: 30 };

let key = "age";
alert( key in user ); // true, property "age" exists

Why does the in operator exist? Isn’t it enough to compare against undefined?

Well, most of the time the comparison with undefined works fine. But there’s a special case when it fails, but "in" works correctly.

It’s when an object property exists, but stores undefined:

let obj = {
  test: undefined
};

alert( obj.test ); // it's undefined, so - no such property?

alert( "test" in obj ); // true, the property does exist!

In the code above, the property obj.test technically exists. So the in operator works right.

Situations like this happen very rarely, because undefined should not be explicitly assigned. We mostly use null for “unknown” or “empty” values. So the in operator is an exotic guest in the code.

The "for..in" loop

To walk over all keys of an object, there exists a special form of the loop: for..in. This is a completely different thing from the for(;;) construct that we studied before.

The syntax:

for (key in object) {
  // executes the body for each key among object properties
}

For instance, let’s output all properties of user:

let user = {
  name: "John",
  age: 30,
  isAdmin: true
};

for (let key in user) {
  // keys
  alert( key );  // name, age, isAdmin
  // values for the keys
  alert( user[key] ); // John, 30, true
}

Note that all “for” constructs allow us to declare the looping variable inside the loop, like let key here.

Also, we could use another variable name here instead of key. For instance, "for (let prop in obj)" is also widely used.

Ordered like an object

Are objects ordered? In other words, if we loop over an object, do we get all properties in the same order they were added? Can we rely on this?

The short answer is: “ordered in a special fashion”: integer properties are sorted, others appear in creation order. The details follow.

As an example, let’s consider an object with the phone codes:

let codes = {
  "49": "Germany",
  "41": "Switzerland",
  "44": "Great Britain",
  // ..,
  "1": "USA"
};

for (let code in codes) {
  alert(code); // 1, 41, 44, 49
}

The object may be used to suggest a list of options to the user. If we’re making a site mainly for a German audience then we probably want 49 to be the first.

But if we run the code, we see a totally different picture:

• USA (1) goes first
• then Switzerland (41) and so on.

The phone codes go in the ascending sorted order, because they are integers. So we see 1, 41, 44, 49.

// Number(...) explicitly converts to a number
// Math.trunc is a built-in function that removes the decimal part
alert( String(Math.trunc(Number("49"))) ); // "49", same, integer property
alert( String(Math.trunc(Number("+49"))) ); // "49", not same "+49" ⇒ not integer property
alert( String(Math.trunc(Number("1.2"))) ); // "1", not same "1.2" ⇒ not integer property

…On the other hand, if the keys are non-integer, then they are listed in the creation order, for instance:

let user = {
  name: "John",
  surname: "Smith"
};
user.age = 25; // add one more

// non-integer properties are listed in the creation order
for (let prop in user) {
  alert( prop ); // name, surname, age
}

So, to fix the issue with the phone codes, we can “cheat” by making the codes non-integer. Adding a plus "+" sign before each code is enough.

Like this:

let codes = {
  "+49": "Germany",
  "+41": "Switzerland",
  "+44": "Great Britain",
  // ..,
  "+1": "USA"
};

for (let code in codes) {
  alert( +code ); // 49, 41, 44, 1
}

Now it works as intended.

Summary

Objects are associative arrays with several special features.

They store properties (key-value pairs), where:

• Property keys must be strings or symbols (usually strings).
• Values can be of any type.

To access a property, we can use:

• The dot notation: obj.property.
• Square brackets notation obj["property"]. Square brackets allow taking the key from a variable, like obj[varWithKey].

Additional operators:

• To delete a property: delete obj.prop.
• To check if a property with the given key exists: "key" in obj.
• To iterate over an object: for (let key in obj) loop.

What we’ve studied in this chapter is called a “plain object”, or just Object.

There are many other kinds of objects in JavaScript:

• Array to store ordered data collections,
• Date to store the information about the date and time,
• Error to store the information about an error.
• …And so on.

They have their special features that we’ll study later. Sometimes people say something like “Array type” or “Date type”, but formally they are not types of their own, but belong to a single “object” data type. And they extend it in various ways.

Objects in JavaScript are very powerful. Here we’ve just scratched the surface of a topic that is really huge. We’ll be closely working with objects and learning more about them in further parts of the tutorial.