How Hooks Are Built

When we build a custom hook with complex logic in React, we either return an array or an object that has state and logic. The following shows the process:

1. Returning an object

The first option is to return an object from the custom hook, like this:

const customHook = (configs) => {
  const isLoading = true //logic
  const reload    = () => {/*logic*/}
  return { isLoading, reload }
}

The usage would look like this:

const { isLoading, reload } = customHook(configs)

Where:

• It is developer-friendly, where you don’t need to remember the order of the props
• The IntelliSense will help you know the params

But when we use multiples of that hook, it doesn’t look nice. It will look like this:

const { 
  isLoading: isLoadingWidget1, 
  reload: reloadWidget1 
} = customHook(widget1Config)
const { 
  isLoading: isLoadingWidget2, 
  reload: reloadWidget2 
} = customHook(widget2Config)
const { 
  isLoading: isLoadingWidget3, 
  reload: reloadWidget3 
} = customHook(widget3Config)

Which is not really nice. We have to rename each key to another one so they don’t clash.

2. Returning an array

The other option is to return an array from the custom hook, like the following:

const customHook = (configs) => {
  const isLoading = true //logic
  const reload    = () => {/*logic*/}
  return [isLoading, reload]
}

The usage would look like this:

const [isLoading, reload] = customHook(configs)

Where:

It solves the multiple usages problem. You don’t have to rename each key

But:

You have to maintain the order of props Doesn’t have the same IntelliSense as in Objects

Can We Combine the Options

Can we combine these patterns together? In a sense, if you want to use it as an array or as an object for the same hook.

Yes, that’s possible. Look at this implementation:

const customHook = () => {  
  const isLoading = true //logic
  const reload    = () => {/*logic*/}
  const result = [isLoading, reload]
  result.isLoading = isLoading
  result.reload    = reload
  return result
}

result is an array, yet we add isLoading and reload keys to it.

That implementation will give us the ability to use the hook’s return value as:

1. An object

const { isLoading, reload } = customHook(configs)

1. An array

const [isLoading, reload] = customHook(configs)

Both of these options will work. But why…

Why Does That Work

Why is it possible to use both the Array and Object return types? Why is it possible to attach keys to an array?

“The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name, and has members for performing common array operations.” — MDN

As mentioned, “The Array object.” That reveals the fact that an array is actually just an object, with a special constructor, and a set of methods performing common array operations.

Proof that Array is actually an Object

Proof 1

typeof [] // 'object'

Yes, typeof will result in an 'object'

1. Proof 2

const {0: first, 1: second} = [1, 2, 3, 4] // first: 1, second: 2

We can destruct the array object using the indices because the keys in any array are just the indices and values of the object, and they are what you pass between the brackets []. That’s why we’re able to destruct it and rename keys that way.

1. Proof 3

const { length } = [1, 2, 3, 4]

An array is an object that also has length key in it.

1. Proof 4

Object.keys([1, 2, 3, 4])   // ['0', '1', '2', '3']
Object.values([1, 2, 3, 4]) // [1, 2, 3, 4]

Object.keys and Object.values only work on Objects, and Array is an object (a special object).

Conclusion

We have just seen a pattern that allows us to make hooks’ return value to be destructed as an array or as an object. We’ve seen how that is helpful. And we’ve seen why that is possible because it’s all about the fact that Arrays are just Objects with a cap.

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