JS: Array Constructor

• new Array(…) → same as Array(…).
• Array() → create a empty array. Same as [].
• Array(n) → if n is 0, 1, 2, 3, etc. It creates a Sparse Array with n slots. Same as y=[];y.length=n. If n is negative, it's a RangeError.
• Array(x) → x is not a integer. Result is a array with 1 element x.
• Array(a1, a2 …) → same as [a1,a2…].

warning: Array(9) does not actually create any element in the array at all.

// 「new Array(2)」 does not create any elements, only sets length
const r1 = new Array(2);
console.log(r1.length); // 2
console.log(Object.getOwnPropertyNames(r1)); // [ 'length' ]

// compare to,
// here's array of 2 items, both are undefined
const r2 = [undefined, undefined];
console.log(r2.length); // 2
console.log(Object.getOwnPropertyNames(r2)); // [ '0', '1', 'length' ]

If you create a array by Array(4), then use the array method map to fill in, it doesn't work.

// creating array by map this way doesn't work

const a1 = new Array(3);

const a2 = a1.map( (() => "v"));

console.log(a2); // [ <3 empty items> ]

// expected ['v','v','v',]

console.log ( a2[0] === undefined); // true

Use Fill

Use Array.prototype.fill to fill sparse array. Example:

Array(4).fill(0);

Range Function for Creating Array

Range Function for Array