CHAPTER 3Array Functional Methods
Some methods of the Array class are higher-order, because they receive functions as parameters:
map(fn) | Return an new array with each element transformed by fn |
filter(fn) | Return a new array with only element for which fn returns true |
find(fn) | Return the first element for which fn returns true |
findIndex(fn) | Return the first element index for which fn return true |
reduce(fn) | Compute a total value using fn to accumulate the result |
forEach(fn) | Execute fn on every element |
sort(fn) | Sort the array in place using fn to compare adjacent elements |
some(fn) | Determine if any element make fn return true (existential) |
every(fn) | Determine if all elements make fn return true (universal) |
map
const A = [1, 2, 3, 4, 5];
A.map(n => 2*n) // [2, 4, 6, 8, 10]
A.map(String) // ["1", "2", "3", "4", "5"]
const stars = n => "*".repeat(n);
A.map(stars) // ["*", "**", "***", "****", "*****"]
filter
const A = [1, "hi", null, undefined, "", 10, "ho"];
A.filter(Boolean) // [1, "hi", 10, "ho"]
const isNum = x => typeof x === "number";
const is = (type) => typeof x === type;
A.filter(isNum) // [1, 10]
A.filter(is('string')) // ["hi, "", "ho"]
A.filter((_, i) => i % 2 === 0) // [1, null, "", "ho"]
find, findIndex
const A = [0, -1, 2, -3, 4, -5, 6, -7];
A.find(n => n < 0); // -1
A.find(n => n > 5); // 6
A.findIndex(n => n === -7); // 7
A.findIndex(n => n < 0); // 1
reduce
const A = [1, 2, 3, 4, 5];
A.reduce((a, b) => a + b) // 15
A.reduce((a, b) => a * b) // 120
A.reduce((a, b) => Math.max(a, b)) // 5
A.reduce((a, b) => Math.min(a, b)) // 1
const B = ["a", "b", "c", "d", "e"];
B.reduce((a, b) => `${a};${b}`); // a;b;c;d;e
const C = [["a", 1], ["b", true], ["c", "hi"]];
C.reduce((a, v) => Object.assign(a, { [v[0]]: v[1] }), {})
// { a: 1, b: true, c: "hi" }
forEach
const A = ["to", "be", "or", "not", "to", "be"];
A.forEach(word => console.log(word));
sort
const A = [4, 1, 2, 0, 5, 3];
A.sort((a, b) => a - b) // [0, 1, 2, 3, 4, 5]
A.sort((a, b) => b - a) // [5, 4, 3, 2, 1, 0]
const B = [
{ name: "Per", points: 5 },
{ name: "Ava", points: 9 },
{ name: "Tom", points: 1 },
];
B.sort((a, b) => b.points - a.points).map(x => x.name);
// ["Ava", "Per", "Tom"]
some, every
const A = [-1, -2, 0, 1, -3, 5, -9];
A.some(x => x < 0) // true
A.some(x => x === 0) // true
A.every(x => x < 0) // false
A.every(x => typeof x === 'number') // true