freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/es6/write-higher-order-arrow-functions.english.md

id	title	challengeType
587d7b88367417b2b2512b45	Write Higher-Order Arrow Functions	1

Description

It's time to look at higher-order functions and their common pair, arrow functions. Arrow functions work really well when combined with higher-order functions, such as map(), filter(), and reduce().
But what are these functions? Lets look at the simplest example forEach(), and run it on the following array of sample Facebook posts.

let FBPosts = [
{thumbnail: "someIcon", likes:432, shares: 600},
{thumbnail: "Another icon", likes:300, shares: 501},
{thumbnail: "Yet another", likes:40, shares: 550},
{thumbnail: null, likes: 101, shares:0},
]

Of the two forEach() versions below, both perform the exact same log function, and each takes an anonymous callback with a parameter post. The difference is the syntax. One uses an arrow function and the other does not.

ES5
FBpost.forEach(function(post) {
console.log(post) // log each post here
});
ES6
FBpost.forEach((post) => {
console.log(post) // log each post here
});

filter() is very similar. Below it will iterate over the FBPosts array, perform the logic to filter out the items that do not meet the requirements, and return a new array, results.

let results = arr1.filter((post) => { return post.thumbnail !== null && post.likes > 100 && post.shares > 500 });

console.log(results); // [{thumbnail: "someIcon", likes: 432, shares: 600}, {thumbnail: "Another icon", likes: 300, shares: 501}]

Instructions

Use arrow function syntax to compute the square of only the positive integers (decimal numbers are not integers) in the array realNumberArray and store the new array in the variable squaredIntegers.

Tests

tests:
  - text: <code>squareList</code> should be a <code>function</code>.
    testString: assert.typeOf(squareList, 'function'), '<code>squareList</code> should be a <code>function</code>';
  - text: <code>squareList</code> should be a constant variable (by using  <code>const</code>).
    testString: getUserInput => assert(getUserInput('index').match(/const\s+squaredIntegers/g), '<code>squaredIntegers</code> should be a constant variable (by using <code>const</code>).');
  - text: <code>function</code> keyword was not used.
    testString: getUserInput => assert(!getUserInput('index').match(/function/g), '<code>function</code> keyword was not used.');
  - text: loop should not be used
    testString: getUserInput => assert(!getUserInput('index').match(/(for)|(while)/g), 'loop should not be used');
  - text: <code>map</code>, <code>filter</code>, or <code>reduce</code> should be used
    testString: getUserInput => assert(getUserInput('index').match(/map|filter|reduce/g), '<code>map</code>, <code>filter</code>, or <code>reduce</code> should be used');
  - text: The function should return an <code>array</code> called <code>squaredIntegers</code>
    testString: assert(Array.isArray(squaredIntegers), '<code>squaredIntegers</code> should be an <code>array</code>');
  - text: <code>squaredIntegers</code> should be <code>[16, 1764, 36]</code>
    testString: assert.deepStrictEqual(squaredIntegers, [16, 1764, 36], '<code>squaredIntegers</code> should be <code>[16, 1764, 36]</code>');

Challenge Seed

const realNumberArray = [4, 5.6, -9.8, 3.14, 42, 6, 8.34, -2];
const squareList = (arr) => {
  "use strict";
  const positiveIntegers = arr.filter((num) => {
     // add code here
  });
  const squaredIntegers = positiveIntegers.map((num) => {
       // add code here
  });

  return squaredIntegers;
};
// test your code
const squaredIntegers = squareList(realNumberArray);
console.log(squaredIntegers);

Solution

const realNumberArray = [4, 5.6, -9.8, 3.14, 42, 6, 8.34, -2];
const squareList = (arr) => {
  "use strict";
  const positiveIntegers = arr.filter((num) => {
    return num >= 0 && Number.isInteger(num);
      // add code here
  });
  const squaredIntegers = positiveIntegers.map((num) => {
    // add code here
    return num ** 2;
  });
  // add code here
  return squaredIntegers;
};
// test your code
const squaredIntegers = squareList(realNumberArray);