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Johan du Toit
Johan du Toit

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TypeScript: Tuples are just fixed length arrays‽ 🤔

Tewple or Tupple? Well both actually, keep this in mind during discussions.

Table of contents

📚 What is a Tuple
🧬 Usage
The "tread with caution"

📚 What is a Tuple

Okay, so what is a Tuple? Simply put, it's a data structure, that consists of a fixed length array where each item may or may not be of the same type.

// Using a type
type MyTupleType = [boolean, string];

// Using an interface
interface MyTupleInterface
 0: boolean
 1: string,

The above definitions are identical and lets us choose which is best for our situation.

🧬 Usage

Using our definitions above, let's create our first Tuple!

const myTuple: MyTupleType = [false, "stringValue"] 

Great, so what can we do with our Tuple?

// Accessing our data
const myBoolean = myTuple[0]
const myString = myTuple[1]

const [myBoolean, myString] = myTuple; 

// Setting our data
myTuple[0] = true;
myTuple[1] = "Another String";

// Looping our data... remember, it's an Array.
myTuple.forEach(m => console.log(m));

// Useful for functions, this pattern is widely used with React Hooks
function validate(value: any): MyTupleType {
  // Some validation here
  return [false, "Invalid value provided"]

The "tread with caution"

Because a Tuple is an array, we're allowed to push in new data 👀.

// Continuing from above
myTuple.push(true, "why is this possible");
myTuple.push("why is this possible", true);

Both the cases above are valid, as the compiler is unioning our types. In our example this produces the type of string | boolean.

This may not immediately come across as a bad thing, however try accessing any of your newly pushed data and experience the wrath of the compiler!

const [a1, a2, b1, b2] = myTuple;
// const b2: undefined
// Tuple type 'MyTupleType' of length '2' has no element at index '2'.

const myValue = myTuple[3];
// Tuple type 'MyTupleType' of length '2' has no element at index '3'

Proof read & edited by my beautiful wife ❤!

Remember to #KeepHavingFun

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