Subtraction assignment (-=)

The subtraction assignment operator (-=) subtracts the value of the right operand from a variable and assigns the result to the variable.

Operator: x -= y
Meaning:  x  = x - y

// Assuming the following variable
//  bar = 5

bar -= 2     // 3
bar -= 'foo' // NaN

• Assignment operators in the JS guide
• Subtraction operator