CYF array.map discussion

arrayMap.js

// We want to apply a function to each element of an array and get
// back a new array of the same length, with a function applied to
// each element.

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

console.log(arr);

// Then function takes one parameter (same type as the elements in
// the array) and has to return something (or we end up with an
// array of "undefined" values).
function addOne(n) {
    return n+1;
}

// We can do it using a for loop
let arr2 = [];

for(i=0; i < arr.length; i++) {
    let newElement = addOne(arr[i]);
    arr2.push(newElement);
}

console.log(arr2);

// But arr.map gives us the same result and is much simpler and
// easier to read
let arr3 = arr.map(addOne);

console.log(arr3);

// It works with strings too
let names = ["Virginie", "Andy", "Tara"];

function upperCase(s) {
    return s.toUpperCase();;
}

console.log(names.map(upperCase));

// What's going on inside map is the same as our original for loop
// version
function myMap(f, someArray) {
    let arr2 = [];

    for(i=0; i < someArray.length; i++) {
        let newElement = f(someArray[i]);
        arr2.push(newElement);
    }
    return arr2;
}

let arr4 = myMap(addOne, arr);

console.log(arr4);

// The tricky thing is that we are passing a function as a parameter
// to the function, rather than calling the function ourselves. map calls
// the function for us.
function doSomethingToANumber(f, n) {
    return f(n);
}

console.log(doSomethingToANumber(addOne, 2));