JS: Array.prototype.with

(new in ECMAScript 2023)

xArray.with(n, newval)

• return a new copy of array, with element at index n changed to newval
• n can be negative, meaning counting from right.

this is similar to xArray[n] = newval but does not change the original.

For Sparse Array, empty items get value of undefined

🟢 TIP: this is good for functional programing style.

console.log([1, 2, 3].with(0, "a"));
// [ "a", 2, 3 ]

console.log([1, 2, 3].with(-1, "a"));
// [ 1, 2, "a" ]

/* on sparse array. all empty index gets undefined */

console.log(Array(4));
// [ <4 empty items> ]

console.log(Array(4).with(0, "a"));
// [ "a", undefined, undefined, undefined ]

// what happens if no arg
console.log([1, 2, 3, 4].with());
// [ undefined, 2, 3, 4 ]