The -- operator decrements (subtracts one from) its operand and returns the value before or after the decrement, depending on where the operator is placed.

Try it

let x = 3;
const y = x--;

console.log(`x:${x}, y:${y}`);
// Expected output: "x:2, y:3"

let a = 3;
const b = --a;

console.log(`a:${a}, b:${b}`);
// Expected output: "a:2, b:2"

Syntax

x--
--x

Description

The -- operator is overloaded for two types of operands: number and BigInt. It first coerces the operand to a numeric value and tests the type of it. It performs BigInt decrement if the operand becomes a BigInt; otherwise, it performs number decrement.

If used postfix, with operator after operand (for example, x--), the decrement operator decrements and returns the value before decrementing.

If used prefix, with operator before operand (for example, --x), the decrement operator decrements and returns the value after decrementing.

The decrement operator can only be applied on operands that are references (variables and object properties; i.e., valid assignment targets). --x itself evaluates to a value, not a reference, so you cannot chain multiple decrement operators together.

--(--x); // SyntaxError: Invalid left-hand side expression in prefix operation

Examples

Postfix decrement

let x = 3;
const y = x--;
// x is 2; y is 3

let x2 = 3n;
const y2 = x2--;
// x2 is 2n; y2 is 3n

Prefix decrement

let x = 3;
const y = --x;
// x is 2; y = 2

let x2 = 3n;
const y2 = --x2;
// x2 is 2n; y2 is 2n

Specifications

Browser compatibility

See also

• Addition (+)
• Subtraction (-)
• Division (/)
• Multiplication (*)
• Remainder (%)
• Exponentiation (**)
• Increment (++)
• Unary negation (-)
• Unary plus (+)