Objects JS

Question 1

In objects, what is a property?

Answer 1

A “key:value” pair, where key is a string (the name of the property) and the value van be anything

Question 2

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

What is happening here ?

Answer 2

Empty object is being created

Question 3

Let’s create an object for an user, where we know the name and the age of the user.

Answer 3

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

Question 4

How to access property values?

Answer 4

Property values are accessible using the dot notation:

alert( user.name ); // John

Question 5

How to remove a property?

Answer 5

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

delete user.age;

Question 6

What is a multiword property name?

Answer 6

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

“likes birds”: true

Question 7

For multiword properties, the dot access doesn’t work. What do we use instead?

Answer 7

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

user[“likes birds”] = true;

Question 8

What is a computed property?

Answer 8

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

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}.

Question 9

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. How would that be ?

Answer 9

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
// …
};
}

Question 10

Can we use both normal properties and shorthands in the same object?

Answer 10

Yes!

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

Question 11

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

Does the same apply for object properties?

Answer 11

No! for an object property, there’s no such restriction, there are no limitations on property names.

Other types are automatically converted to strings.

For instance, a number 0 becomes a string “0” when used as a property key.

Question 12

What is the Property existence test, “in” operator?

Answer 12

So we can easily test whether the property exists:

There’s also a special operator “in” for that.

The syntax is:

“key” in object

let user = { name: “John”, age: 30 };

alert( “age” in user ); // true, user.age exists

Question 13

To walk over all keys of an object, we use?

Answer 13

The “for..in” loop

The syntax:

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

Question 14

If we loop over an object, do we get all properties in the same order they were added?

Answer 14

“ordered in a special fashion”: integer properties are sorted, others appear in creation order.

Question 15

Integer properties? What’s that?

Answer 15

The “integer property” term here means a string that can be converted to-and-from an integer without a change.