Page 123 - JavaScript

var deeplyNested = [4,[5,6,[7,8],9]];

It can be flattened with this magic

console.log(String(deeplyNested).split(',').map(Number);
#=> [4,5,6,7,8,9]

Or

const flatten = deeplyNested.toString().split(',').map(Number)
console.log(flatten);
#=> [4,5,6,7,8,9]

Both of the above methods only work when the array is made up exclusively of numbers. A multi-
dimensional array of objects cannot be flattened by this method.

Insert an item into an array at a specific index

Simple item insertion can be done with Array.prototype.splice method:

arr.splice(index, 0, item);

More advanced variant with multiple arguments and chaining support:

/* Syntax:
array.insert(index, value1, value2, ..., valueN) */

Array.prototype.insert = function(index) {
this.splice.apply(this, [index, 0].concat(
Array.prototype.slice.call(arguments, 1)));
return this;
};

["a", "b", "c", "d"].insert(2, "X", "Y", "Z").slice(1, 6); // ["b", "X", "Y", "Z", "c"]

And with array-type arguments merging and chaining support:

/* Syntax:
array.insert(index, value1, value2, ..., valueN) */

Array.prototype.insert = function(index) {
index = Math.min(index, this.length);
arguments.length > 1
&& this.splice.apply(this, [index, 0].concat([].pop.call(arguments)))
&& this.insert.apply(this, arguments);
return this;
};

["a", "b", "c", "d"].insert(2, "V", ["W", "X", "Y"], "Z").join("-"); // "a-b-V-W-X-Y-Z-c-d"

The entries() method

https://riptutorial.com/ 80