Subtraction assignment (-=)

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

Try it

x -= y

x -= y is equivalent to x = x - y.

let bar = 5;

bar -= 2; // 3
bar -= "foo"; // NaN

let foo = 3n;
foo -= 2n; // 1n

Specification
ECMAScript Language Specification # sec-assignment-operators

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