+= symbol is used for addition assignment operator.
a += 5 //equivalent to a = a + 5 a += b //equivalent to a = a + b
1. Addition assign
a with 5.
var a = 4; a += 5; console.log(a);
a with value in
b and assign the result back to
var b = 7; var a = 4; a += b; console.log(a);