Identifiers and Reserved Words in JavaScript

Last modified: March 24, 2024 by Atrowel

In the Literals in JavaScript blog, we have learned about the Literals. In JavaScript, literals are like straightforward shortcuts for expressing values directly in code, such as numbers or text. They serve as constants that stay fixed throughout the program. In this blog, we will learn about Identifiers and Reserved Words in JavaScript.

What are Identifiers?

The name itself indicates that it identifies the memory location. The names given to the memory locations are called Identifiers. For example, variables can be identifiers. The variables that we use in programs are identifiers. Examples of Identifiers are variables, function names, constants, etc.

Rules for Identifiers

The identifier must begin with a letter, an underscore (_), or a dollar symbol ($), followed by the letters, digits, underscores, or dollar signs.
The identifier's name should not begin with numbers or any special symbols except for underscore (_) and dollar sign ($).
The identifier name cannot be a keyword. For example, if, else, while, for, etc cannot be used as an identifier name.
Identifier names are case-sensitive. For example, name, NAME, Name are treated as different variables. Even though the names given are the same, the casing differs.

Valid Identifiers

Let’s see with the help of example Valid Identifiers

let name = "Smith"
let Name = "Smith"        //variables are case sensitive
let firstName = "John"    //combination of characters
let _name="john"          //starts with underscore(_)
let first_name = "john"   //contains underscore(_)
let name_="john"          //ends with underscore(_)
let $name = "john"        //starts with dollar($)
let first$name = "john"   //contains dollar in between of the variable
let name$ = "john"        //ends with dollar($)
let first765name = "john" //contains number
let name869 = "john"      //ends with number

Invalid Identifiers

Let’s take an example of the identifiers that are invalid

let 989abc = "Invalid";           // Identifier cannot start with a digit
let string variable = "Invalid";  // Identifier cannot contain spaces
let while = "Invalid";            // Identifier cannot be a reserved keyword
let string-variable = "Invalid";  // Identifier cannot contain a hyphen
let string@variable = "Invalid";  // Identifier cannot contain the @ symbol
let stringvariable = "Invalid";   // Case matters, so this is a different identifier
let my#variable = "Invalid";      // Contains an invalid character

Reserved Words

In JavaScript, reserved words, also called keywords, have specific meanings reserved for special purposes in the language. Due to their crucial roles in the language's structure and operation, these words cannot be used as identifiers, like variable or function names. Trying to use reserved words as identifiers will cause syntax problems. Here is a list of words that you can't use as identifiers:

Reserved Word	Description
break	Terminates a loop or switch statement
case	Defines a specific case in a switch statement
catch	Catches exceptions thrown in try blocks
class	Declares a class in JavaScript
const	Declares a constant variable
continue	Skips the rest of a loop and continues with the next iteration
debugger	Pauses the execution of JavaScript code
default	Defines the default case in a switch statement
delete	Deletes a property from an object
do	Starts a do...while loop
else	Defines an alternative block of code in an if statement
export	Exports a module or a part of it for use in other scripts
extends	Extends a class to create a new class
false	Represents the Boolean value false
finally	Specifies a block of code to be executed after a try block
for	Initiates a for loop
function	Declares a function
if	Creates a conditional statement
import	Imports modules or specific members from them
in	Checks if a specified property is in an object
instanceof	Checks if an object is an instance of a specific class
new	Creates an instance of a user-defined object type
null	Represents the absence of a value or a null object
return	Exits a function and specifies a value to be returned
super	Calls the parent constructor in an extended class
switch	Evaluates an expression and executes the corresponding case
this	Refers to the current object
throw	Throws an exception
true	Represents the Boolean value true
try	Encloses a block of code that might throw an exception
typeof	Returns a string indicating the type of an operand
undefined	Represents an undefined value
var	Declares a variable
void	Evaluates an expression and returns undefined
while	Creates a while loop
with	Extends the scope chain for a statement
yield	Pauses and resumes a generator function

Semicolon

In JavaScript, we use semicolons (;) to separate statements. If statements are on separate lines, we can skip the semicolon between them. For instance:

x = 10
y = 20;

Here, since the statements are on different lines, we can omit the semicolon in the first statement. However, if statements are on the same line, like:

x=10; y =20;

We cannot omit the semicolon, as both statements are on the same line.

JavaScript code involves choosing unique identifiers, steering clear of reserved words like "if" or "function," and punctuating statements with semicolons for flawless execution.