Subtraction assignment (-=)

The subtraction assignment (-=) operator performs subtraction on the two operands and assigns the result to the left operand.

Try it

let a = 2;

console.log((a -= 3));
// Expected output: -1

console.log((a -= "Hello"));
// Expected output: NaN

x -= y

x -= y is equivalent to x = x - y, except that the expression x is only evaluated once.

undefined

let bar = 5;

bar -= 2; // 3

Other non-BigInt values are coerced to numbers:

bar -= "foo"; // NaN

let foo = 3n;
foo -= 2n; // 1n
foo -= 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions