The increment (++) operator increments (adds one to) its operand and returns the value before or after the increment, depending on where the operator is placed.
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 increment if the operand becomes a BigInt; otherwise, it performs number increment.
If used postfix, with operator after operand (for example, x++), the increment operator increments and returns the value before incrementing.
If used prefix, with operator before operand (for example, ++x), the increment operator increments and returns the value after incrementing.
The increment 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 increment operators together.
++(++x); // SyntaxError: Invalid left-hand side expression in prefix operation
Examples
Postfix increment
let x = 3;
const y = x++;
// x is 4; y is 3
let x2 = 3n;
const y2 = x2++;
// x2 is 4n; y2 is 3n
Prefix increment
let x = 3;
const y = ++x;
// x is 4; y is 4
let x2 = 3n;
const y2 = ++x2;
// x2 is 4n; y2 is 4n