Type Inference and Literal Types

In TypeScript, variables are "born" with their types.

// when using "let", TS will infer a more general type
// allows for value reassignment
let age = 6;
// let age: number

// when using "const", TS will infer a more specific type
// can NOT be reassigned and "number" is an immutable value type
const age = 6;
// const age: 6  <- literal type

The type 6 is called a literal type. It's not just any number type, it's a number that's also a 6.

Inference is not so specific as to get in the way of common behavior.

For example, the let variable declaration above could have assumed age to be of type 6, but this would have interfered with our ability to set this re-assignable variable to 7 or 8.

Objects

This is why object properties will be non-literal types when inferred...b/c object properties are mutable.

If you want them to be literal types, then finish with as const:

const obj = {
  name: 'Joe',
  age: 30,
};
// const obj: {
//   name: string;
//   age: number;
// }

const obj = {
  name: 'Joe',
  age: 30,
} as const;
// const obj: {
//   readonly name: "Joe";
//   readonly age: 30;
// }

From the TypeScript Fundamentals, v3↗ course on FEM↗ taught by Mike North↗.