tekul · January 20, 2022 21:01
diff --git a/arrayFilter.js b/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]);
    }
 }
	// 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]);
	}
	}