How to Pass Parameter to array.map()?

Question

I am trying to map values to a function that will accept two numbers to multiply but am having trouble doing so (if this doesn't make sense, take a look at the examples below).

I have an array of numbers and I would like to double/triple/quadruple... the values of this array. I have created functions that would do this, and am feeding these double() and triple() into map().

var arr = [1, 2, 3, 4, 5];  function double(num) {   return num * 2; }  function triple(num) {   return num * 3; }  console.log( arr.map(double) ); console.log( arr.map(triple) ); 

This solution is not scalable as what if I want to multiply the values by 5, or 10? I need a more abstract function that would take a parameter of what to multiply. I am confused about how to do this. My attempt so far is:

var arr = [1, 2, 3, 4, 5];  function multiply(num, multiplyBy) {   return num * multiplyBy; }  console.log( arr.map(multiplyBy(4) ); // Uncaught TypeError: NaN is not a function 

How would I pass multiply() the multiplyBy parameter?

Answer 1

You can do something called a factory currying. Effectively, you make a function that will return another function which is tailored to your needs. In this case, an anonymous one.

var arr = [1, 2, 3, 4, 5];  function multiplyBy(scale) {     return function(num){         return num * scale;     } }  console.log( arr.map( multiplyBy(4) )); 

This works because the scope of the anonymous function that is returned is within that of the factory outer function. So, whenever we produce a new function, it will retain the value of scale that was given for its production.

Edit: The last part of @Bergi 's answer is the same as mine. The concept is apparently called currying. Thanks @Bergi ! The Factory pattern is more often applied to the production of objects, as Bergi noted, but it was the only thing I could think of at the time, and javascript sort of treats functions like objects. In this specific case, they are effectively similar. Here is a good reference for currying in JavaScript