array.js

// Reduce loops through an array, giving you access to each item. It'll also give access to the current "result".
// You can specify an initial value which reduce will use to initialize the "result" variable.

const myArray = [
    1,
    2,
    3
];

const initialValue = [];


const myNewValue = myArray.reduce((result, currentArrayItem) => {
    // For the first iteration, result is "initialValue". So result is an empty array. currentArrayItem is "1" because
    // that's the first item in myArray.

    // Whatever you return here, is the "result" variable for the next iteration. You could return "1" and then for the
    // next iteration, "result" would be "1". In this case, we want to return our current result but also add something
    // to it.
    //
    // We push a new entry into our result array with value currentArrayItem + 2. For our first iteration that would be
    // 1 + 2 = 3. Result is now an array with one item with value 3.
    //
    // The next iteration now has the "result" variable which is an array with one entry with value "3" and "currentArrayItem"
    // is now "2".
    //
    // This goes on for every item in "myArray".
    result.push(currentArrayItem + 2);

    return result;
}, initialValue);

console.log(myNewValue); // [ 3, 4, 5 ]

buildObject.js

// Of course, initialValue can be anything. You could make it an object and then build it from an array like so:

const myArray = [
    {
        id: 1,
        name: "Jordy"
    },
    {
        id: 2,
        name: "Mandy"
    }
];

// Now let's turn that array into an object that is keyed by the id and has values containing our names.
// {1: "Jordy", 2: "Mandy"}

const initialValue = {}; // We start with an empty object

const myNewObject = myArray.reduce((result, currentArrayItem) => {
    // Assign the current array item's name to the key in our new object:
    result[currentArrayItem.id] = currentArrayItem.name;

    // Make sure to return the object so we can use it in the next iteration.
    return result;
}, initialValue);

console.log(myNewObject); // { '1': 'Jordy', '2': 'Mandy' }

forLoop.js

// You could also use a regular 'ole for loop to achieve the same thing (but with different scoping)

const myArray = [
    {
        id: 1,
        name: "Jordy"
    },
    {
        id: 2,
        name: "Mandy"
    }
];

const myNewObject = {}; // This is also our initial value now.

for(const arrayItem of myArray) {
    myNewObject[arrayItem.id] = arrayItem.name;
}

console.log(myNewObject); // { '1': 'Jordy', '2': 'Mandy' }