How to Pass Parameter to array.map()?

You're looking for partial application. It can be done with bind for example (or a number of helper functions that come with functional libraries, like in Underscore):

arr.map(multiplyBy.bind(null, 4))

however a simple arrow function would be easier:

arr.map(x => multiplyBy(4, x))

But you can also get the partial application for free if you curry your multiplyBy function, taking the multiplier and returning a new function:

function multiplyBy(multiplier) {
  return function(num) {
    return num * multiplier;
  };
}
// ES6 arrow functions:
const multiplyBy = multiplier => num => num * multiplier;

arr.map(multiplyBy(4));

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

You can use Function.prototype.bind to create a new function with bound arguments. For example

var arr = [1, 2, 3, 4, 5];

function multiplyBy(multiplyBy, num) {
  // note the "num" argument must come last if it is to represent the argument from "map"
  return num * multiplyBy;
}

console.log( arr.map(multiplyBy.bind(null, 4)) ); // null is for the "this" argument