Typed Arrays

lesson 2-1 · ~9 min · 4/26

Quiz

Warm-up from lesson 1-2: what is the type of let price = 4.99 with no annotation?

An array of exactly one thing

In the JavaScript courses your arrays could hold anything: [1, "two", true]. Convenient, until a .toUpperCase() meets a number. TypeScript arrays declare their element type:

const scores: number[] = [88, 92, 79];
const names: string[] = ["Ada", "Grace"];

Read number[] as "array of numbers". Now the compiler guards every entry point:

scores.push("95") is an error, only numbers get in
scores[0].toUpperCase() is an error, elements are numbers
Inside for (const s of scores), the loop variable s is already known to be a number

Inference works here too: const scores = [88, 92] is inferred as number[] automatically.

Code exercise · typescript

Run the typed array. Then try scores.push("oops") and run again, the compiler will reject it.

Code exercise · typescript

Your turn. Create names: string[] with "Ada", "Grace", "Alan". Push "Linus". Then loop over the array and print each name uppercased with .toUpperCase().