How to add key-value pair in javascript object?

Solution: In javascript, the object is almost everything. If you understand the objects, you understand javascript.

• Booleans can be objects if they defined with a new keyword
• Strings can be objects if they defined with a new keyword
• Numbers can be objects if they defined with a new keyword
• Arrays are always objects
• Functions are always objects  
• Dates are always objects
• Regular expressions are always objects
• Maths are always objects

All javascript values except primitive are objects.

Basically, Javascript objects are key-value pairs and We can add key-value pairs in multiples ways but here I will explain the most popular 4 ways:-

1) Using Dot Notation:

We can easily add key-value pairs using dot notation, like this:

2) Using Square bracket:

Dot notation has some limitations. If keys are dynamic or if it starts with a number or if it contains spaces or hyphens, you will get an error. So in that case, we can use a Square bracket method:

3) Using Spread(...) operator:

The Spread operator creates a new object and it is useful when you don't want to change the original object.

4) Using Object.assign() method:

This method takes two parameters, the first is the target object, and the second is the key-value pairs that you want to add to the target object(called source object). This method changes the original object i.e. Object.assign method does not create a new object.

For example: