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