TypeScript Tuples
Typed Arrays
A tuple is a typed array with a pre-defined length and types for each index.
Tuples are great because they allow each element in the array to be a known type of value.
To define a tuple, specify the type of each element in the array:
Example
// define our tuple
let ourTuple: [number, boolean, string];
// initialize correctly
ourTuple = [5, false, 'Coding God was here'];
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As you can see, we have a number, boolean and a string. But what happens if we try to set them in the wrong order:
Example
// define our tuple
let ourTuple: [number, boolean, string];
// initialized incorrectly which throws an error
ourTuple = [false, 'Coding God was mistaken', 5];
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Even though we have a boolean, string, and number the order matters in our tuple and will throw an error.
Readonly Tuple
A good practice is to make your tuple readonly.
Tuples only have strongly defined types for the initial values:
Example
// define our tuple
let ourTuple: [number, boolean, string];
// initialize correctly
ourTuple = [5, false, 'Coding God was here'];
// We have no type safety in our tuple for indexes 3+
ourTuple.push('Something new and wrong');
console.log(ourTuple);
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You see the new valueTuples only have strongly defined types for the initial values:
Example
// define our readonly tuple
const ourReadonlyTuple: readonly [number, boolean, string] = [5, true, 'The Real Coding God'];
// throws error as it is readonly.
ourReadonlyTuple.push('Coding God took a day off');
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To learn more about access modifiers like readonly go to our section on them here: TypeScript Classes.
If you have ever used React before, you have worked with tuples more than likely.
useState returns a tuple of the value and a setter function.
const [firstName, setFirstName] = useState('Dylan') is a common example.
Because of the structure we know our first value in our list will be a certain value type in this case a string and the second value a function.
Named Tuples
Named tuples allow us to provide context for our values at each index.
Example
const graph: [x: number, y: number] = [55.2, 41.3];
Named tuples provide more context for what our index values represent.
Destructuring Tuples
Since tuples are arrays, we can also destructure them.
Example
const graph: [number, number] = [55.2, 41.3];
const [x, y] = graph;
To review destructuring, check it out here.
