Implementing Custom Higher-Order JavaScript Functions

1. map: It calls the specified function for every array element and returns the new array. This method doesn't change the original array.
2. filter: It filter and extract the element of an array that satisfying the provided condition. It doesn't change the original array.
3. reduce: It reduces the given array into a single value by executing a reducer function. The user implements the reducer function that works on every element present in the array.
4. forEach: It is used to invoke the specified function once for each array element.