Javascript data structure, map, array reduce, array some()

 Javascript data structure :

https://adrianmejia.com/data-structures-time-complexity-for-beginners-arrays-hashmaps-linked-lists-stacks-queues-tutorial/

Javascript maps :

https://www.geeksforgeeks.org/map-in-javascript/

https://www.w3schools.com/js/js_object_maps.asp

A Map holds key-value pairs where the keys can be any datatype.

A Map remembers the original insertion order of the keys.

A Map has a property that represents the size of the map.

// Create a Map
const fruits = new Map();

// Set Map Values
fruits.set("apples", 500);
fruits.set("bananas", 300);
fruits.set("oranges", 200);

Or 

const fruits = new Map([

  ["apples", 500],

  ["bananas", 300],

  ["oranges", 200]

]);

fruits.get("apples");    // Returns 500

Javascript array Reduce :

https://www.w3schools.com/jsref/jsref_reduce.asp

Subtract all numbers in an array:

const numbers = [175, 50, 25];

document.getElementById("demo").innerHTML = numbers.reduce(myFunc);

function myFunc(total, num) {
  return total - num;
}

// result 100

Round all the numbers and display the sum:

const numbers = [15.5, 2.3, 1.1, 4.7];
document.getElementById("demo").innerHTML = numbers.reduce(getSum, 0);

function getSum(total, num) {
  return total + Math.round(num);
}

// 24

Parameter	Description
function()	Required. A function to be run for each element in the array.
Reducer function parameters: total Required. The initialValue, or the previously returned value of the function. currentValue Required. The value of the current element. currentIndex Optional. The index of the current element. arr Optional. The array the current element belongs to.

Javascript : some can be used together with filter

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some

The some() method tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn't modify the array.

const array = [1, 2, 3, 4, 5];

// checks whether an element is even

const even = (element) => element % 2 === 0;

console.log(array.some(even));

// expected output: true