Types you did not write

The first assignment fixes a variable's type: let x = 3 makes x a number. A const gets the exact literal, so const y = 3 has type 3.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Most TypeScript has few annotations because inference does the work. Knowing what it infers explains why x = 'hi' fails and why a const string is narrower than string.

How to think about it

Hover, or reason: let widens to the general type, const keeps the literal. Annotate only when inference would be wrong or too wide, such as an empty array that will hold numbers.

Worked example

let x = 3;

Inferred as number from the first assignment.

x = 7;

Fine.

x = "hi";

Error: string is not assignable to number.

const y = 3;

A const can never change, so its type is the literal 3, not number.

Your turn

Reassign a string variable.

let city = "Leeds"; city = ;

The trap

let items = [] infers never[] or any[] depending on settings. Annotate an empty array: let items: number[] = [].

Practise inference on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.