Programming Fundamentals › Programming Basics

Block Scope vs Function Scope

Whether a variable lives until the end of its block or of the whole function.

Also known as: function scope, block scoped variables, scope of a variable

Scope is the part of a program where a name can be used. Languages differ in how big that part is.

• Block scope: a variable lives only inside the { ... } block (an if, a loop, a plain block) where it’s declared.
• Function scope: a variable lives for the whole function, no matter which block declared it.

function demo() {
  if (true) {
    var a = 1;      // function scope
    let b = 2;      // block scope
  }
  console.log(a);   // 1
  console.log(b);   // ReferenceError: b is not defined
}

By language

• JavaScript: let and const are block-scoped. var is function-scoped, which causes surprises (var, let and const).
• Python: function scope. Variables created inside an if or for are still visible afterward in the same function. Only functions, classes and modules create new scopes.
• Java, C, C#, Go: block scope.

for i in range(3):
    last = i
print(last)       # 2: it still exists after the loop

Why it matters

• Fewer accidents: a block-scoped variable can’t leak into code that shouldn’t see it.
• Loop bugs: closures created in a loop capture a shared variable under function scope, but a fresh one under block scope.
• Use before definition: knowing where a name exists explains “not defined” errors.

Declare variables in the smallest scope that needs them. See scope and shadowing.