Thread by @swapnakpanda

There may be a "Hole" in your Array

🟪🟪🟪🕳🟪🟪

A JavaScript 🧵

➊ What is a hole?

⬒ Holes are indices “inside” an Array that have no associated element.

⬔ An array with at least a single hole in it is called a "Sparse Array".

⬓ An array without any hole is called a "Dense Array".

➋ How to detect a hole?

Check if

⬙ 0 ≤ index < Array's length
⬘ index is not in the array

Example:

To check if there is a hole at index "2" in "arr" whose length is 5

⇥ !(2 in arr)

➌ How to create a hole?

There are at least 4 ways.

⬖ Omitting values in array literals
⬘ Assigning a value to an index > length of array
⬗ Increasing value of length property
⬙ Delete an element from existing index

➍ What value will you get from a hole?

undefined

⇥ let arr = [10,,20]
⇥ console.log(arr[1])

➎ How holes are handled?

They are handled in many different ways. So be cautious.

⮑ Ignores holes
⮑ Ignores holes, but preserves in output
⮑ Preserves holes
⮑ Treats holes as undefined
⮑ Doesn't ignore holes

➎.➊ Ignores Holes

⮑ for...in loop
⮑ Array.prototypes's
⮑ forEach()
⮑ indexOf()
⮑ lastIndexOf()
⮑ filter()
⮑ every()
⮑ some()
⮑ reduce()
⮑ reduceRight()

➎.➋ Ignores holes, but preserves in output

⮑ Array.prototype's map()

➎.➌ Preserve Holes

Array.prototype's
⮑ concat()
⮑ copyWithin()
⮑ push()
⮑ unshift()
⮑ slice()
⮑ sort()

➎.➍ Treats holes as undefined

⮑ for...of loop
⮑ Array.from()
⮑ Spread (...)
⮑ Array.prototype's
⮑ entries(), keys(), values()
⮑ find(), findIndex()
⮑ pop(), shift()
⮑ join(), toString()*
⮑ Function.prototype.apply()

* They convert undefined to ''

➎.➎ Doesn't ignore holes

⮑ Array.prototype's fill()

➏ Final Words

❑ JavaScript allows to assign values to a non existent index which may create holes.

❒ While handling an array with holes (sparse array), always pay attention.

With this we come to the end of this 🧵 where we discussed about "Holes in JavaScript Arrays".

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Till we meet again, stay safe and stay sanitised. 👋