3. Programs, Statements and Expressions

JavaScript programs are built from statements and expressions, the core components that define what your program does and how it behaves. This page will explain what statements and expressions are and how they differ.

3.1. What is a Statement?

A statement is a complete instruction that JavaScript executes.

Examples of statements:

let name = "Alice"; // Declares a variable and assigns a value
console.log(name);  // Prints the value of 'name' to the console

3.2. Semicolons

In JavaScript, the semicolon (;) is used to signal the end of a statement. It helps the JavaScript engine know where one instruction ends and the next begins.

Example:

let greeting = "Hello"; // Correctly ended with ;
let greeting = "Hello"  // Incorrect

Although JavaScript has a feature called Automatic Semicolon Insertion (ASI) that tries to add missing semicolons for you, relying on it can lead to unexpected errors. It’s a good habit to include semicolons explicitly.

4. Expressions

An expression is a piece of code that produces a value. Expressions can be as simple as a number or more complex like a calculation.

Examples of expressions:

5 + 3        // Produces the value 8
"x" + "y"    // Produces the value "xy"
Math.sqrt(9) // Produces the value 3

Expressions can be used inside statements. For example:

let result = 5 + 3;    // The expression '5 + 3' is part of the statement
console.log(result);   // 'result' is an expression that evaluates to 8

5. Multiple Statements on One Line

You can write multiple statements on the same line if you separate them with semicolons:

let a = 1; let b = 2; console.log(a + b);

However, for readability, it’s better to write one statement per line.