JavaScript recursive function

1. What is a recursive function? Is to call the function itself within a function, a very typical example is the use of function evaluation number Feibolaqi columns:

function fibonacciSequence(n) {
            if(n == 1 || n == 2){
                return 1;
            }else if(n == 3){
                return fibonacciSequence(2) + fibonacciSequence(1);
            }else if(n == 4){
                return fibonacciSequence(3)+fibonacciSequence(2);
            }else if(n == 5){
                return fibonacciSequence(4) + fibonacciSequence(3);
            }else if(n == 6){
                return fibonacciSequence(5) + fibonacciSequence(4);
            }
           //·········
            else if(n == n){
                return fibonacciSequence(n-1) + fibonacciSequence(n-2)
            };
        };
        console.log(fibonacciSequence(10));

Recursive function is often used to solve some of the problems the cycle is repeated, though useful, but it is very consumption performance, recursive function should be noted that the conditions must be ended, otherwise it will lead to an endless loop.