Map Functions

##What is a map function?

The map() function in JavaScript is used to create a new array with the results of calling a provided function on every element in the original array. It calls the provided function once for each element in the array, in order.

Here's a basic example:

const numbers = [1, 2, 3, 4, 5];
 
const squaredNumbers = numbers.map(function(num) {
  return num * num;
});
 
console.log(squaredNumbers); // Outputs: [1, 4, 9, 16, 25]

In this example, the map() function is called on the numbers array. The function passed to map() takes each number in the numbers array, squares it, and the squared result is added to a new array. This new array is then returned and stored in the squaredNumbers variable.

The map()` function does not change the original array, it creates a new array. This is one of the characteristics of a "pure" function, which is a function that does not cause side effects, like changing input values.

Arrow functions combined with map functions

You can also use arrow functions with map(), like so:

const numbers = [1, 2, 3, 4, 5];
 
const squaredNumbers = numbers.map(num => num * num);
 
console.log(squaredNumbers); // Outputs: [1, 4, 9, 16, 25]

This does exactly the same thing as the previous example, but the function is written as an arrow function, which is a shorter syntax and automatically binds this to the surrounding code's context.

Additional parameters

In addition, map() passes three arguments to the provided function:

The current element being processed. (optional) The index of the current element being processed. (optional) The array that map() was called upon. Here's an example that uses these additional parameters:

const numbers = [1, 2, 3, 4, 5];
 
const moreNumbers = numbers.map((num, index, array) => {
  console.log(`Current value: ${num}`);
  console.log(`Index: ${index}`);
  console.log(`Array: ${array}`);
  return num * num;
});

In this case, the function provided to map() is logging the current value, its index, and the original array to the console before returning the squared number.

Basics Promises