Explicit Type Annotations

Inference works great for variables with an initial value, but it can't read your mind about function parameters — TypeScript has no way to know what a function is supposed to receive unless you tell it. That's where explicit annotations stop being optional and become essential.

function multiply(a: number, b: number): number {
  return a * b;
}

multiply(3, "4"); // Error: Argument of type 'string' is not assignable to parameter of type 'number'.

Without those annotations, a and b would silently fall back to the any type, meaning TypeScript would allow absolutely anything to be passed in — multiply(3, "4") wouldn't be flagged at all, and the bug would only show up once the code actually ran.

let names: string[] = ["Kofi", "Lena", "Ines"];

Arrays get their own annotation syntax: write the element type followed by square brackets. number[] means an array containing only numbers.

let scores: number[] = [92, 85, 78];
scores.push(100);   // fine — 100 is a number
scores.push("A+");  // Error: Argument of type 'string' is not assignable to parameter of type 'number'.

To annotate a function parameter explicitly, you write the parameter name, a colon, then its ____.

How do you write the type annotation for an array that should only ever contain numbers?

Function parameters are automatically inferred by TypeScript the same way variables with initial values are.