编写一个double函数，将其自身乘以4，格式如下

Question

I want to write a function called twice that takes a function f and a value x as its parameters and returns f(f(x)) . For example,

twice(function (x) { return x * x; }, 3) 

should return 81 . How do I do it?

Answer 1

Call the anonymous function twice which is passed as an argument. On the second call use the returned value of first execution as the argument.

 function twice(funct, a) { return funct(funct(a)); } console.log( twice(function(x) { return x * x; }, 3) ); 

Answer 2

What is stopping you? You already has the answer with you. It is working pretty fine.

 function twice(f, x) { return f(f(x)); } var result = twice(function(n){return n*n;}, 3); console.log(result);