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04_arrays-iterables.md

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Arrays and Iterables

  • Iterable: is an object that implements the iterable protocol and have a @@iterator method (i.e. Symbol.iterator)

    • Iterables: Array, NodeList, String, Map, Set

  • Array-like Object: is an object with a length property and use an index to access items

    • Array-like objects: NodeList, String

Creating Arrays

const arr1 = [1, 2];

const arr2 = Array.from('Hi') // ['H', 'i'] -> transforms an iterable into an array

// rarely used
const arr3 = new Array(1, 2) // notice: new Array(5) -> empty array with length 5

const arr4 = Array.of(1, 2);

Useful Methods of Arrays

arr.push(); // adds item at the end
arr.pop(); // removes item at the end

arr.unshift(); // adds item at the beginning and moves all items one index higher
arr.shift(); // removes item at the beginning and moves all items one index lower

arr.splice(START_INDEX, NUMBER_OF_ITEMS_TO_DELETE?, ...ITEMS_TO_INSERT?); // returns removed items

arr.slice(START_INDEX, END_INDEX?) // returns new shallow copy of array, end index not included

arr.concat(ARRAY[]) // merges two or more arrays; does NOT change existing arrays, but instead returns a new array

arr.indexOf(ITEM_VALUE, START_INDEX?) // returns first index where the passed value is found in an array
arr.lastIndexOf(ITEM_VALUE) // same as above, but starts searching in array from the end

arr.find((item, index?, fullArray?) => item === 'test'); // returns first matching item (i.e. return is true)
arr.findIndex(); // same as above, but returns found index

arr.includes(ITEM_VALUE) // returns boolean value

const newArr = arr.map((item, index?, fullArray?) => {
  // ... logic
  return newItem;
}) // transforms items of array, returns new array, does NOT mutate original array

const sortedArr = [10, 1, 5].sort((a, b) => {
  if(a > b) return 1; // a must be after b (-> a +1 position)
  if (a < b) return -1; // a must be before b (-> a -1 position)
  return 0; // no change
})

const filteredArr = [1, 2].filter((item, index?, fullArray?) => item === 1); // return true means, that item is kept (-> filtered out in new array), false means opposite

const total = [1, 5, 10].reduce((accumulator, currentValue, currentIndex?, fullArray?) => accumulator + currentIndex, 0); // reduces values of array items to another new value or wished construct

const data = ['test;10']
const splitData = data.split(";"); // ['test', '10']

const arr = ['hello', 'world'];
const joinedString = arr.join(' '); // 'hello world'