JavaScript Syntax

JavaScript syntax refers to the rules and guidelines that define how JavaScript programs are written and structured.

// Declaring variables:
let a;
let b;

// Assigning values to variables:
a = 10;
b = 15;

// Declaring and assigning a variable in one step:
let sum = a + b;

JavaScript Syntax Categories

The JavaScript syntax categorizes values into two types:

• Fixed values, also known as Literals.
• Changeable values, also known as Variables.

JavaScript Literals

The two key syntax rules for fixed values are:

1. Numbers can be written with or without decimal points.

10.50
1001

2. Strings are text enclosed in either double quotes (") or single quotes (').

"William Wilson"
'William Wilson'

JavaScript Variables

Variables store data values and can be declared using the let, const, or var keywords. The key difference is that const creates a constant variable, which cannot be reassigned.

let x = 10;      // Variable with `let` (can be reassigned)
const y = 20;    // Constant variable with `const` (cannot be reassigned)

JavaScript Operators

JavaScript uses arithmetic operators (+, -, *, /) to perform calculations and compute values.

(5 + 6) * 10

The assignment operator (=) is used to set a value to a variable.

let x, y;
x = 5;
y = 6;

JavaScript Expressions

An expression is a combination of values, variables, and operators that results in a computed value.

5 * 10 // Evaluates to 50

Expressions can also include values stored in variables.

x * 10

Values can be of different types, including numbers and strings.

For example, the expression "Alice" + " " + "Smith" evaluates to "Alice Smith".

"Alice" + " " + "Smith" // + is a concatination operator for strings

JavaScript Keywords

JavaScript keywords are used to define actions that the program should carry out.

The let keyword instructs the browser to create variables.

let a, b;
a = 10 + 5;
b = a * 5;
document.getElementById("key").innerHTML = b;

/*
Output:
75
*/

The var keyword is also used to instruct the browser to create variables.

var a, b;
a = 10 + 5;
b = a * 5;
document.getElementById("key").innerHTML = b;

/*
Output:
75
*/

JavaScript Comments

Text following double slashes (//) or enclosed between /* and */ is treated as a comment.

Comments are ignored by the JavaScript engine and do not execute.

let a;
a = 10;
// a = 5; It will not be executed
document.getElementById("comment").innerHTML = a;

JavaScript Identifiers

Identifiers are the names used in JavaScript to label variables, keywords, and functions.

A JavaScript identifier must start with one of the following:

• A letter (A-Z or a-z)
• A dollar sign ($)
• An underscore (_)

After the first character, it can include letters, digits (0-9), underscores, or dollar signs.

Note: Identifiers cannot begin with a number. This ensures JavaScript can easily differentiate between identifiers and numeric values.

JavaScript is Case Sensitive

JavaScript identifiers are case-sensitive.

For example, firstname and FirstName are considered two distinct variables.

let firstname, FirstName;
firstname = "William";
FirstName = "James";
document.getElementById("sensitive").innerHTML = firstname;

/*
Output:
William
*/

JavaScript treats LET and Let differently from the keyword let, as it is case-sensitive.