Statements and Expressions

Purpose

Every line of JavaScript is either an expression — something that evaluates to a value — or a statement — a complete instruction, such as a declaration or a control-flow construct. Knowing which you’re looking at explains why some code can be used inside other code (like passing price * quantity as a function argument) and some can’t (you can’t pass an if block as an argument).

Expressions

An expression always produces a value:

5 + 3; // 8
'hi'.toUpperCase(); // "HI"
isReady ? 'go' : 'wait'; // one of the two strings

Expressions can nest inside other expressions and inside most statements.

Statements

A statement is a complete instruction. Declarations (let, const, function), control flow (if, for, while, switch), and return/break/continue are all statements:

let total = 0;

if (total === 0) {
  console.log('empty cart');
}

An expression statement is an expression used on its own as a statement, most often a function call for its side effect (console.log('hi');) — the value it produces, if any, is simply discarded.

Behavior notes

• A statement can contain expressions (let x = 5 + 3; contains the expression 5 + 3), but a statement itself has no value you can assign or pass around.
• Because if, for, and switch are statements, not expressions, JavaScript has no “if expression” — the closest equivalent for a single value is the conditional (ternary) operator, condition ? a : b.
• Statements are terminated by a semicolon. When one is omitted, Automatic Semicolon Insertion (ASI) inserts one according to specific grammar rules — see Strict mode and the orientation lesson for a common ASI pitfall involving return.

Common mistakes

• Assuming a block like if (x) { ... } produces a value the way an expression does — it doesn’t; nothing can be assigned from it directly.
• Writing a function expecting an object literal to be returned, but leaving a line break between return and the opening { — ASI turns that into return; followed by unreachable code.

Runtime notes

Statements and expressions are defined by ECMAScript itself, so this behavior is identical in every JavaScript engine, browser or Node.