Array

Arrays technically aren't a type in Javascript.

Since everything in Javascript is an object, Array doesn't follow the same limitations as an actual array (Private), such as being of fixed length.

• for example, the length property returns the size of the internal storage area for indexed items of the array.

in JavaScript, array indexes are coerced to strings by an implicit toString() call.

When to use Arrays for lists of data

If you have a list of data that you are going to be transforming somehow, or mapping over it somehow, you should try to structure that data as an array

• expl: if you had an object of keys, and each key was a list item, then to operate on that might get a little messy and imperative. Imagine if we stored taxes in an object like this:

const taxes = {
    GST: 0.05,
    PST: 0.07
}

We can loop over this with Object.entries without problem, but now imagine that we want to map over each entry, and display it in the UI in React. That might look something like this:

Object.entries(taxes).map(tax =>
    <>
        <p>Tax type: {tax[0]}</p>
        <p>Amount: {tax[1]}</p>
    </>
)

Alternatively, imagine we structured taxes like this:

const taxes = [
    {
        jurisdiction: 'PST',
        percentage: 0.05,
    },
    {
        jurisdiction: 'GST',
        percentage: 0.05,
    },
]

Although this is less succinct, there is a benefit to doing it this way. This is how we would handle that React task:

taxes.map(tax => {
    <>
        <p>Tax type: {tax.jurisdiction}</p>
        <p>Amount: {tax.percentage}</p>
    </>
})

This is yet another example of why you should go from a data-consumption perspective (in other words, consider how it will be used; think of the end-user of that code.)

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Extra commas in Array literals

const fish = ["Lion", , "Angel"];
console.log(fish);
// [ 'Lion', <1 empty item>, 'Angel' ]

• note that the second item is "empty", which is not exactly the same as the actual undefined value. When using array-traversing methods like .map, empty slots are skipped. However, index-accessing fish[1] still returns undefined.
• naturally, if the final element is followed by a comma, that gets removed.

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Children

1. Cook
2. Method
3. Set Theory

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Backlinks

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