On Typescript

A few notes on Typescript for my future self.

First Principles

• Let your types be flexible
• Let your constraints be rigid

TL;DR Rules

• Use unknown instead of any, then use type narrowing to get the correct type.
• Use type over interface, unless you actually need to reach for an interface or need to express objects/class inheritance.
• Avoid using as to assert types, most of the time you actually want to narrow the type with checks (if/else).
• Use array.at(index) instead of array[index] unless array is a tuple (fixed size array).
• NEVER use TS specifics (enum, private in constructor, etc.).

Recommandations

• Use satisfies to check if an object fits a type but not erase the type.
• Use as const whenever possible. (Immutable data, enum-like objects, etc.)
• Define (and export) types where they are consumed, and import them from other files if needed.

Explanations

Narrowing over using as

Suppose you have a function that takes a number

function double(a: number) {
    return a * 2;
}

And you have a variable that could be a number or a string

function getNumberOrString(): number | string {
    return Math.random() > 0.5 ? 1 : "1";
}
const a: number | string = getNumberOrString();

Typescript will allow you to use as to assert the variable to a number (this is one of the ways that TypeScript is not sound)

const result: number = double(a as number);

But this not correct/sound at runtime!

The correct way to do this is to narrow the type with a check (if/else/early return).

if (typeof a === "number") {
    const result = double(a);
}