CYF filter and forEach discussion

arrayFilter.js

// We start with an array of pet objects
let pets = [
    {
        name: "Hector",
        type: "dog",
        likes: "chicken"
    },
    {
        name: "Mackie",
        type: "cat",
        likes: "fish"
    },
    {
        name: "Ramona",
        type: "cat",
        likes: "cheese"
    },
    {
        name: "Charlie",
        type: "dog",
        likes: "catfood"
    },
];
console.log("All pets:");
console.log(pets);

// Each element of the array is a pet object, so the element pets[0]
// is the dog called Hector.

console.log("Hector:");
console.log(pets[0]);

// We can use the map method as before to convert the array into an
// array of just the pet names, by passing a function to map which
// returns the name of each pet.
function petName(pet) {
    return pet.name;
}

let petNames = pets.map(petName);

// The new array is an array of strings (just the names) instead of objects
// As always when using map, it has the same length as the original array.
console.log(petNames);

// We can use filter to create a new array, which only contains the pets who
// are cats (based on the "type" property of each object).
// The function passed to filter should return true if we want to keep the
// pet in the new array, or false to leave it out (i.e. ignore it).
function isCat(pet) {
    return pet.type === "cat";
}

let cats = pets.filter(isCat);

console.log("Just the cats:");
console.log(cats);

// We can filter based on other properties too
function likesFish(pet) {
    return (pet.likes === "fish");
}

// We can chain calls together. Here we filter first to get an array of cats
// (still an array of pet objects) then filter that again to get the cats who like fish.
cats = pets.filter(isCat).filter(likesFish);

console.log("Cats that like fish:");
console.log(cats);

// forEach is different in that it just takes a function which does something for
// each element in the array. It doesn't return a new array and the function doesn't
// need to return anything either.
function sayHello(pet) {
    if(pet.type === "cat") {
        console.log("Meow!");
    } else {
        console.log("Woof!");
    }
}

console.log("Each pet says Hello:");
pets.forEach(sayHello);

// More advanced stuff below
// =========================

// As with map, we can implement our own version of filter as a simple function
// which takes a function "keepElement" and an array as arguments
// As with map it creates a new array to store the elements we want to keep,
// it iterates through the provided array and adds the elements for which the
// function returns true.to the new array 
function myFilter(keepElement, arr) {
    let newArray = [];

    for(i=0; i < arr.length; i++) {
        if(keepElement(arr[i])) {
            newArray.push(arr[i]);
        }
    }
    return newArray;
}

// We can then check that we get the same result when filtering for cats.
// So for our keepElement function we pass the function isCat which we
// used above with filter.
cats = myFilter(isCat, pets);

console.log("Filter for cats again using our implementation:");
console.log(cats);

// Implementing forEach is simple by comparison
// It just calls the function with every element in the array
function myForEach(f, arr) {
    for(i=0; i < arr.length; i++) {
        f(arr[i]);
    }
}