JAVASCIPRT Array에 대해 알아보자

const arr = []
arr.length = 10
console.log(arr) // [empty x 10];
arr[15] = "a"
console.log(arr) // [empty x 15, 'a']
arr.length = 8
console.log(arr) // [empty x 8]

Array.from("douglas")
// ["d", "o", "u", "g", "l", "a", "s"]

const s = new Set(["dowoo", "kim"])
Array.from(s)
// ['dowoo', 'kim']

const m = new Map([
  [1, 2],
  [2, 4],
  [4, 8],
])
Array.from(m)
// [[1, 2], [2, 4], [4, 8]]

const mapper = new Map([
  ["1", "a"],
  ["2", "b"],
])
Array.from(mapper.values())
// ['a', 'b'];

Array.from(mapper.keys())
// ['1', '2'];

function f() {
  return Array.from(arguments)
}

f(1, 2, 3)
// [1, 2, 3]

const range = (start, stop, step) =>
  Array.from({ length: (stop - start) / step + 1 }, (_, i) => start + i * step)

// Generate numbers range 0..4
range(0, 4, 1)
// [0, 1, 2, 3, 4]

// Generate numbers range 1..10 with step of 2
range(1, 10, 2)
// [1, 3, 5, 7, 9]

// Generate the alphabet using Array.from making use of it being ordered as a sequence
range("A".charCodeAt(0), "Z".charCodeAt(0), 1).map(x => String.fromCharCode(x))
// ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"]

// Production steps of ECMA-262, Edition 6, 22.1.2.1
if (!Array.from) {
  Array.from = (function () {
    var toStr = Object.prototype.toString;
    var isCallable = function (fn) {
      return typeof fn === 'function' || toStr.call(fn) === '[object Function]';
    };
    var toInteger = function (value) {
      var number = Number(value);
      if (isNaN(number)) { return 0; }
      if (number === 0 || !isFinite(number)) { return number; }
      return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
    };
    var maxSafeInteger = Math.pow(2, 53) - 1;
    var toLength = function (value) {
      var len = toInteger(value);
      return Math.min(Math.max(len, 0), maxSafeInteger);
    };

    // The length property of the from method is 1.
    return function from(arrayLike/*, mapFn, thisArg */) {
      // 1. Let C be the this value.
      var C = this;

      // 2. Let items be ToObject(arrayLike).
      var items = Object(arrayLike);

      // 3. ReturnIfAbrupt(items).
      if (arrayLike == null) {
        throw new TypeError('Array.from requires an array-like object - not null or undefined');
      }

      // 4. If mapfn is undefined, then let mapping be false.
      var mapFn = arguments.length > 1 ? arguments[1] : void undefined;
      var T;
      if (typeof mapFn !== 'undefined') {
        // 5. else
        // 5. a If IsCallable(mapfn) is false, throw a TypeError exception.
        if (!isCallable(mapFn)) {
          throw new TypeError('Array.from: when provided, the second argument must be a function');
        }

        // 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined.
        if (arguments.length > 2) {
          T = arguments[2];
        }
      }

      // 10. Let lenValue be Get(items, "length").
      // 11. Let len be ToLength(lenValue).
      var len = toLength(items.length);

      // 13. If IsConstructor(C) is true, then
      // 13. a. Let A be the result of calling the [[Construct]] internal method
      // of C with an argument list containing the single item len.
      // 14. a. Else, Let A be ArrayCreate(len).
      var A = isCallable(C) ? Object(new C(len)) : new Array(len);

      // 16. Let k be 0.
      var k = 0;
      // 17. Repeat, while k < len… (also steps a - h)
      var kValue;
      while (k < len) {
        kValue = items[k];
        if (mapFn) {
          A[k] = typeof T === 'undefined' ? mapFn(kValue, k) : mapFn.call(T, kValue, k);
        } else {
          A[k] = kValue;
        }
        k += 1;
      }
      // 18. Let putStatus be Put(A, "length", len, true).
      A.length = len;
      // 20. Return A.
      return A;
    };
  }());
}

갑자기 문뜩 든 생각이, array를 deep copy할 때 어떤게 효율이 가장 좋을까 생각을 해 보았습니다.

테스트 코드는 다음과 같습니다.

```js
const arr = Array(10000).fill(Array(10000).fill(0));

console.time('from');
const w = Array.from(arr, arr => Array.from(arr));
console.timeEnd('from')

console.time('json');
const q = JSON.parse(JSON.stringify(arr));
console.timeEnd('json');

console.time('spread');
const s = arr.map(v => [...v]);
console.timeEnd('spread');

console.time('map');
const e = arr.map(v => {v.map(t => t)});
console.timeEnd('map');

const arr2 = [];
console.time('forpush');
for (let i=0; i<10000; i++) {
    arr.push([]);
    for (let j =0; j < 10000; j++) {
        arr[i].push(0);
    }
}
console.timeEnd('forpush')

// from: 1141.065ms
// json: 5399.943ms
// spread: 1373.498ms
// map: 1466.990ms
// forpush: 2653.858ms

if (!Array.isArray) {
  Array.isArray = function(arg) {
    return Object.prototype.toString.call(arg) === "[object Array]"
  }
}

Array.of(1) // [1]
Array.of(1, 2, 3) // [1, 2, 3]
Array.of(undefined) // [undefined]

if (!Array.of) {
  Array.of = function() {
    return Array.prototype.slice.call(arguments)
  }
}