A variable is a "named storage" location for data.

1.Creating a variable in JS (Declaring a variable).

let myVariable;

2.Use assignment operator ( = ) to store data into the variable.

myVariable = 5;

Now the value 5 is stored in variable "myVariable", we can access it using the variable name.

3.Accessing value stored in Variable.

console.log(myVariable);

We could combine the variable declaration and assignment into a single line as well.

let message = "Hello";

// also
let var1=5, var2="string", var3="second string";

Variable Naming

Rules for variable names in JS:

• Must contain only letters, digits or symbols $ and _ (underscore).
• First character must not be a digit.

  // valid variable names
  let userName;
  let test_123;
  

Note: Case matters (apple and Apple are two different variables).

Keywords

Keywords are the reserved words that cannot be used as a variable name because they are used by the language itself. Example: let, class, return etc.

Usually, we need to define a variable before using it. But in earlier versions of JS it was possible to declare a variable by just assigning a value to it. But if we put "use strict" it would cause an error.

num = 5; // works

"use strict"
num2 = 10; // throws an error ( num2 is not defined)

Constants

The value of a variable could be changed at any point, but in the case of constants once declared they cannot be reassigned. Any attempt to do so would cause an error. To declare a constant variable we use "const" instead of "let".

const myConstant = 100;

Naming Practices

• When the name contains multiple words, camelCase is commonly used.
• Constants are named using capital letters and underscores.
• Use human-readable name.