JavaScript Reserved Words

JavaScript, a versatile and widely-used programming language, has a set of reserved words that play a crucial role in defining its syntax and structure. These reserved words have specific meanings and functionalities within the language, and developers need to be aware of them to write correct and effective JavaScript code. In this detailed guide, we will explore JavaScript reserved words, categorizing them and providing insights into their usage.

What Are Reserved Words?

Reserved words in JavaScript are words that have predefined meanings and cannot be used as identifiers (such as variable names, function names, or labels) in the code. They are part of the language’s syntax and serve various purposes, including defining control flow, declaring variables, and handling exceptions.

Categories of Reserved Words

JavaScript reserved words can be categorized into different groups based on their functionalities. Let’s explore these categories:

1. Primitive Types:

  • undefined: Represents an uninitialized variable or an object property without a value.
  • null: Represents the intentional absence of any object value.

2. Data Types:

  • boolean: Represents a Boolean value (true or false).
  • number: Represents numeric values.
  • string: Represents textual data.

3. Control Flow:

  • if, else: Used for conditional statements.
  • switch, case, default: Used for switch statements.
  • while, do, for: Used for loop constructs.
  • break, continue: Used to control loop execution.

4. Functions and Objects:

  • function: Declares a function.
  • return: Specifies the value to be returned from a function.
  • new: Creates an instance of a user-defined object type or one of the built-in object types.
  • this: Refers to the current object.

5. Error Handling:

  • try, catch, finally: Used for exception handling.

6. Variables and Constants:

  • var: Declares a variable.
  • let, const: Declare block-scoped variables and constants, introduced in ECMAScript 6.

7. Operators:

  • typeof: Returns a string indicating the type of an operand.
  • instanceof: Tests whether an object’s prototype chain includes a specific constructor.

8. Reserved for Future Use:

  • enum: Reserved for possible future use.

Best Practices with Reserved Words

  1. Avoid Using Reserved Words as Identifiers:
    When naming variables, functions, or other identifiers, avoid using reserved words to prevent confusion and potential errors.
   // Incorrect
   let undefined = 'Hello';

   // Correct
   let myVariable = 'Hello';
  1. Use Meaningful Identifiers:
    Choose descriptive and meaningful names for variables and functions. This enhances code readability and makes it easier for others (or your future self) to understand the purpose of each identifier.
   // Less readable
   function fn(x, y) {
     return x + y;

   // More readable
   function calculateSum(num1, num2) {
     return num1 + num2;
  1. Stay Informed about ECMAScript Updates:
    JavaScript is an evolving language, and new features or changes may be introduced in later ECMAScript specifications. Stay informed about updates to the language to adopt new functionalities and best practices.


JavaScript reserved words are an integral part of the language’s syntax and structure. By understanding their meanings and functionalities, developers can write more effective and error-free code. Adhering to best practices, such as avoiding the use of reserved words as identifiers and choosing meaningful names, contributes to code readability and maintainability. As JavaScript continues to evolve, staying informed about language updates ensures that developers can leverage the latest features and follow recommended coding practices.

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