js

JS reference

Map.prototype.entries() VS Object.entries()

Object.entries() The Object.entries() static method returns an array of a given object's own enumerable string-keyed property key-value pairs.

The entries() method of Map instances returns a new map iterator object that contains the [key, value] pairs for each element in this map in insertion order.

Methods of creating Array

1. By using constructor function

Create a new Array object.

let colors = new Array();

when there is only a single number parameter provided, an array is created with its length property set to that number.

const arrayOfOne = new Array("2"); // Not the number 2 but the string "2"

Any constructor with multiple parameters, a new Array with the given elements is created.

2. By using array literal

let colors = ["red", "blue"]

let values = [1,2] // [1,2]

let values = [1] // [1]

3. By using Array.from()

The Array.from() static method creates a new, shallow-copied Array instance from an iterable (e.g.,Map and Set) or array-like (objects with a length property and indexed elements) object.

Array.from(arrayLike)
Array.from(arrayLike, mapFn)
Array.from(arrayLike, mapFn, thisArg)

The mapFn function is called with the following arguments:

element The current element being processed in the array.
index The index of the current element being processed in the array.

Expamples

• Array from a String

  Array.from("foo");
  // [ "f", "o", "o" ]

• Array from a Set

  const m = new Set() .add(1)
                      .add(2)
                      .add(3)
                      .add(4);
  console.log(Array.from(m)); // [1, 2, 3, 4]

• Array from a Map

  const s = new Map() .set(1, 2)
                      .set(3, 4);
  console.log(Array.from(s)); // [[1, 2], [3, 4]]

• Array from an array-liked object

  arguments object

  function getArgsArray() {
      return Array.from(arguments);
  }
  console.log(getArgsArray(1, 2, 3, 4)); // [1, 2, 3, 4]

  customized object

  const arrayLikeObject = {
      0: 1,
      1: 2,
      2: 3,
      3: 4,
      length: 4
  };
  console.log(Array.from(arrayLikeObject)); // [1, 2, 3, 4]