Perform a mathematical operation on a variable with another constant or variable. The += (et al) operators are just a convenient shorthand for the expanded syntax, listed below.

x += y;   // equivalent to the expression x = x + y;
x -= y;   // equivalent to the expression x = x - y; 
x *= y;   // equivalent to the expression x = x * y; 
x /= y;   // equivalent to the expression x = x / y; 

x: any variable type

y: any variable type or constant

x = 2;
x += 4;      // x now contains 6
x -= 3;      // x now contains 3
x *= 10;     // x now contains 30
x /= 2;      // x now contains 15

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