Explain Map, Reduce and Filter functions.

Map, Reduce and Filter are three functions which operate on a given array of values.

• Map function works like a projection, where we can project each value from the input set into some transformed output.
• It takes a callback function which has two parameters - the current value and the index of the current value and expects a return value from the callback.

let arr = [1, 2, 3, 4, 5];
let result2 = arr.map(function (current, index) {
    return current % 2;
});
// output:
[ 1, 0, 1, 0, 1 ]

• Reduce function works like an aggregate function, where we can create an aggregation from a given input set.
• It takes a callback function which has three parameters - the previous value, the current value and the current index and expects a return value from the callback, which is applied onto the next array element iteration.

let result1 = arr.reduce(function (prev, current) {
    console.log(`${prev}, ${current}`);
    return prev + current;
});
// output:
1, 2
3, 3
6, 4
10, 5
result1: 15

• Filter function is an extraction function where the input array is filtered based on a condition.
• It takes a callback function which has two parameters - the current value and the current index and expects a boolean return value which represents whether the current element satisfies the condition or not.

let result3 = arr.filter(function (current, index) {
    return current % 2 === 0;
});
// output:
[ 2, 4 ]