JS Statements

JavaScript Statements: Syntax, Structure, & Execution

JavaScript programs consist of a sequence of statements executed by the engine in the order they are written. A statement performs an action, such as declaring variables, calling functions, evaluating conditions, or looping through data.

Anatomy of a JavaScript Statement

flowchart LR
    Stmt["Statement: let total = price * quantity;"] --> Key["Keyword: let"]
    Stmt --> Decl["Variable: total"]
    Stmt --> Op["Assignment Operator: ="]
    Stmt --> Expr["Expression: price * quantity"]
    Stmt --> Term["Terminator: ;"]

Expression vs Statement

Practical Code Example

// Demonstrating statements, expressions, and block statements

// 1. Variable Declaration & Assignment Statements
let unitPrice = 49.99;
let quantity = 3;
let taxRate = 0.08;

// 2. Expression Statement evaluating total cost
let subtotal = unitPrice * quantity; // Expression: unitPrice * quantity
let grandTotal = subtotal + (subtotal * taxRate);

// 3. Conditional Control Statement Block
if (grandTotal > 100) {
  // Block statement grouped by curly braces {}
  let discount = grandTotal * 0.10;
  grandTotal -= discount;
  console.log(`Discount Applied! New Total: \$${grandTotal.toFixed(2)}`);
} else {
  console.log(`Standard Total: \$${grandTotal.toFixed(2)}`);
}

// 4. Function Declaration Statement
function calculateShipping(weightKg) {
  if (weightKg <= 0) return 0;
  return weightKg * 2.50;
}

console.log(`Shipping Cost: \$${calculateShipping(4)}`);

Best Practices & Gotchas

• Use Explicit Semicolons: JavaScript has Automatic Semicolon Insertion (ASI), but relying on ASI can introduce subtle bugs when lines start with [, (, or backticks.
• Group Related Statements in Blocks {}: Always use curly braces for if, for, and while blocks even if they contain only a single statement.
• Keep Statements Focused: Avoid chaining multiple side-effect operations within a single line statement.

Self-Check Challenge

Identify which line is an expression and which is a statement:
Line A: x = y + 10;
Line B: y + 10