Iteration Problems

Question 1

Create a custom forEach function that takes an array and call back as inputs

Answer 1

class Foo {
constructor(prefix) {
this.prefix = prefix;
}

showItem(item) {
console.log(this.prefix, item);
}
}

let foo = new Foo(“Item: “);

function forEach(array, callback, thisArg) {
for (let index = 0; index < array.length; index += 1) {
callback.call(thisArg, array[index]);
}
}

Question 2

Foreach callback functions first parameter is for the array elements.
Second is for the index, and third is for the array itself.
Rewrite forEach to account for this

Answer 2

function forEach(array, callback, thisArg) {
for (let index = 0; index < array.length; index += 1) {
callback.call(thisArg, array[index], index, array);
}
}

forEach([“a”, “b”, “c”], function(value, index, arr) {
console.log(After ${value} comes ${arr[index + 1]});
});

Question 3

Write a function that acts like the built-in Array.prototype.filter method. For this problem, you only need to emulate the most basic behavior: filtering elements of an array by examining the array values. You don’t have to support multiple arguments to the callback function, but feel free to add them if you like. Your function should work like this:

Note that the function should not mutate the input array.

let numbers = [1, 2, 3, 4, 5];
console.log(filter(numbers, number => number > 3)); // => [ 4, 5 ]
console.log(filter(numbers, number => number < 0)); // => []
console.log(filter(numbers, () => true)); // => [ 1, 2, 3, 4, 5 ]

let values = [1, “abc”, null, true, undefined, “xyz”];
console.log(filter(values, value => typeof value === “string”));
// => [ ‘abc’, ‘xyz’ ]

Answer 3

function filter(array, callback) {
let filteredItems = [];
for (let index = 0; index < array.length; index += 1) {
let value = array[index];
if (callback(value)) {
filteredItems.push(value);
}
}

return filteredItems;
}

OR

let numbers = [1, 2, 3, 4, 5];
console.log(map(numbers, number => number * 3)); // => [ 3, 6, 9, 12, 15 ]
console.log(map(numbers, number => number + 1)); // => [ 2, 3, 4, 5, 6 ]
console.log(map(numbers, () => false));
// => [ false, false, false, false, false ]

let values = [1, “abc”, null, true, undefined, “xyz”];
console.log(map(values, value => String(value)));
// => [ ‘1’, ‘abc’, ‘null’, ‘true’, ‘undefined’, ‘xyz’ ]

Question 4

Write a function that acts like the built-in Array.prototype.reduce method. For this problem, you only need to emulate the most basic behavior: reducing the elements of an array down to a single value based on the original array values. The result may be a primitive value, an object, or another array. You don’t have to include the thisArg argument or support multiple arguments to the callback function, but feel free to add them if you like. Your function should work like this:

Note that the function should not mutate the input array. Don’t forget to account for the initialValue argument!

let numbers = [1, 2, 3, 4, 5];
console.log(reduce(numbers, (accum, number) => accum + number)); // => 15
console.log(reduce(numbers, (prod, number) => prod * number)); // => 120
console.log(reduce(numbers, (prod, number) => prod * number, 3)); // => 360
console.log(reduce([], (accum, number) => accum + number, 10)); // => 10
console.log(reduce([], (accum, number) => accum + number));
// => undefined

let stooges = [“Mo”, “Larry”, “Curly”];
console.log(reduce(stooges, (reversedStooges, stooge) => {
reversedStooges.unshift(stooge);
return reversedStooges;
}, []));
// => [“Curly”, “Larry”, “Mo”]

Answer 4

function reduce(array, callback, initialValue) {
let accumulator = initialValue;
let index = 0;

if (accumulator === undefined) {
accumulator = array[0];
index = 1;
}

while (index < array.length) {
accumulator = callback(accumulator, array[index]);
index += 1;
}

return accumulator;
}

Question 5

Array.prototype.reduce can be an incredibly useful function. You’re not limited to simple accumulation-style processing, but can perform a wide variety of different tasks with it. For instance, you can emulate many of the standard Array methods, including filter, map, and more.

Let’s try it. Write a function that works like the filter function from problem 1. This time, though, you should use Array.prototype.reduce to filter the input array.

Answer 5

function filter(array, callback) {
return array.reduce((filteredItems, value) => {
if (callback(value)) {
filteredItems.push(value);
}
return filteredItems;
}, []);
}

Question 6

Let’s put reduce to work with emulating map as well. Write a function that works like the map function from problem 2. This time, though, use Array.prototype.reduce to transform the input array.

Answer 6

function map(array, callback) {
return array.reduce((transformedItems, value) => {
transformedItems.push(callback(value));
return transformedItems;
}, []);
}

Question 7

Write a forEach method that operates on objects

Answer 7

function objForEach(object, callback) {
for (let property in object) {
if (object.hasOwnProperty(property)) {
callback(property, object[property]);
}
}
}

let obj = { foo: 1, bar: 2, qux: 3 };
objForEach(obj, (property, value) => {
console.log(the value of ${property} is ${value});
});

Question 8

Put a filter method on the todo list
Try not to resassign the array
Make it return an object

https://launchschool.com/lessons/896d0d67/assignments/38ab06c7

Answer 8

class TodoList {
// omitted code

filter(callback) {
let newList = new TodoList(this.title);
this.forEach(todo => {
if (callback(todo)) {
newList.add(todo);
}
});

return newList;   } }

// omitted code

console.log(list.filter(todo => todo.isDone()).first());
// => Todo { title: ‘Buy milk’, done: true }

Question 9

Go through the todo list and practice visualizing passing callbacks and funtion objects around
https://launchschool.com/lessons/896d0d67/assignments/b8f796db

Answer 9

https://launchschool.com/lessons/896d0d67/assignments/b8f796db