Variables & Basic Types

Use const for values that never get reassigned and let for values that change. Avoid var — it has confusing scoping rules.

The basic (primitive) types are string, number (integers and decimals), boolean, null and undefined. You can annotate a variable with : type, but TypeScript usually infers the type from the value, so annotations are mostly needed for function parameters and empty values.

Arrays are written number[] or Array<number>. A tuple like [string, number] is a fixed-length array where each position has its own type.

any switches type checking off — avoid it. When you truly don't know a value's type (for example, parsed JSON), use unknown, which forces you to check the type before using it.

types.tsTypeScript

const school = "Azania Secondary"; // inferred as string
let students = 420;                 // inferred as number
students = students + 15;           // OK: let can change

const isOpen: boolean = true;
const scores: number[] = [78, 64, 91];
const subjects: string[] = ["Maths", "Physics"];
const pair: [string, number] = ["Biology", 85]; // tuple

let nickname: string | undefined; // no value yet
nickname = "Doli";

const raw: unknown = JSON.parse('{"marks": 72}');
if (typeof raw === "object" && raw !== null && "marks" in raw) {
  console.log("Parsed:", raw.marks);
}

console.log(school, students, isOpen, scores, subjects, pair, nickname);

Key points

Prefer const; use let only when a value must change.
Let TypeScript infer types where it can; annotate parameters and empty values.
Use unknown instead of any, then narrow it with checks like typeof.

Exercise

Create an array of five test scores, then compute and print the total and average. Store a student's name and average together in a tuple [string, number].

Show solution

Try the exercise yourself first — then compare your approach with this one.

Use reduce (or a loop) to add the scores, divide by the array's length for the average, and store the result in a tuple whose two positions have different types.

scores.tsTypeScript

const scores: number[] = [78, 64, 91, 55, 82];

let total = 0;
for (const s of scores) total += s;
const average = total / scores.length;

const record: [string, number] = ["Amina", average];

console.log("Total:", total);                     // Total: 370
console.log("Average:", average.toFixed(1));     // Average: 74.0
console.log(record[0] + " averaged " + record[1]); // Amina averaged 74

Key points

Exercise

Check your understanding