TypeScript's "satisfies" keyword

satisfies is a pretty cool keyword that came with TypeScript 5! There aren't many places it can be used, but for config objects and default exports, it's especially good.

You might already know that as has its limitations. It's not very good because we're telling the TypeScript compiler that we know better than it does. It turns off some checks and just takes what we say at face value:

type Person = {
  name: string;
  age: number;
};

const person = {
  name: "Sindre",
} as Person;
// No error because `name` is a valid field in Person,
// and when we use `as`, TS says "okay, fine" even though
// `age` is missing in the `person` object.

Instead, we can use satisfies to ask TypeScript to check if we've included everything we need:

type Person = {
  name: string;
  age: number;
};

const person = {
  name: "Sindre",
} satisfies Person; // Error: `age` is missing.

In this case, it would have been the same to write const person: Person = { ... }, but we can't type with : everywhere - that's where "satisfies" comes into play.

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