Programming Fundamentals › Programming Basics

Expression vs Statement

An expression produces a value; a statement performs an action.

Also known as: expressions and statements, statement vs expression, expression, statement

• An expression produces a value: 2 + 3, price * quantity, len(items), user.name.
• A statement performs an action: assigning a variable, running a loop, returning, importing.

x = 2 + 3          # statement: assignment; "2 + 3" is the expression inside
print(x * 2)       # statement; "x * 2" is an expression

if x > 3:          # statement
    ...

A simple test: can you put it where a value is expected, such as inside parentheses, as a function argument or on the right of =?

foo(2 + 3)         # fine: an expression
foo(x = 3)         # a statement can't go here (in Python)

Where languages differ

• In JavaScript and Python, if is a statement. To choose a value, use the ternary expression: a if cond else b (Python) or cond ? a : b (JavaScript).
• In Rust, Ruby and many functional languages, if and match are expressions that return values, so let x = if cond { 1 } else { 2 }; works.
• A function call is an expression. Its definition is a statement (but a lambda or arrow function is an expression).

const label = isAdmin ? "Admin" : "User";    // choose a value: expression

Why it matters

• Syntax errors: “invalid syntax” often means a statement was placed where an expression was expected.
• Arrow functions and lambdas can only contain expressions (in Python’s lambda).
• Templates and JSX ({ ... }) accept only expressions, so you use ternaries and .map(), not if and for.
• Readable code: prefer expressions for computing values, statements for actions.

An expression can also have a side effect (i++, a function call that writes a file), so “produces a value” is not the same as “pure”.