Variables, Keywords and Comments

Last Updated: 01 Oct 2025

Variables

  • Variables are containers for storing data values.
  • They are used to hold values of different data types.
  • Variables are declared using the var, let, or const keywords.
var a = 10;
let b = 20;
const c = 30;

Note , Difference between var, let and const is that

  • var is function scoped and let and const are block scoped
  • var can be redeclared and updated but let and const cannot be redeclared
  • const cannot be updated

Rules for variable names:

  • Must start with a letter or an underscore.
  • Can contain letters, numbers, and underscores.
  • Cannot start with a number.
  • Reserved keywords cannot be used as variable names.
  • Case-sensitive (a and A are different variables).

Naming Conventions:

  • Use descriptive names (e.g. user_name, user_age, user_height)
  • Avoid abbreviations (e.g. u_name, u_age, u_height)
  • Avoid Single Letter Variables (e.g. a, b, c)

Naming Standard:

  • camelCase → userName (common in JS)
  • PascalCase → UserName (used in classes)
  • snake_case → user_name (less common but okay)
  • UPPERCASE → USER_NAME (for constants)

2. Keywords

Keywords are reserved words in JavaScript that cannot be used as variable names. Example: let, const, var, if, else, return, function, etc.

3. Comments

Comments are used to explain code or provide additional information.

  • Single-line comments start with // and end at the end of the line.
  • Multi-line comments start with /* and end with */.
// This is a single-line comment

/* This is 
a multi-line
comment */
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