It might be easiest to start of the discussion of widening and narrowing with an example:

const welcomeString = "Hello There";
let replyString = "Hey";

// Aside from the text differences of the strings, welcomeString is a const (which means the value will never change) and replyString is a let (which means it can change). If you hover over both variables, you get very different type information from TypeScript: const welcomeString: "Hello There" let replyString: string TypeScript has inferred the type of welcomeString to be the literal string "Hello There", whereas replyString is general string. This is because a let needs to have a wider type, you could set replyString to be any other string - which means it has a wider set of possibilities.

replyString = "Hi :wave:";

// If replyString had the string literal type "Hey" - then you could never change the value because it could only change to "Hey" again. Widening and Narrowing types is about expanding and reducing the possibilities which a type could represent. An example of type narrowing is working with unions, the example on code flow analysis is almost entirely based on narrowing: example:code-flow Type narrowing is what powers the strict mode of TypeScript via the nullability checks. With strict mode turned off, markers for nullability like undefined and null are ignored in a union.

declare const quantumString: string | undefined;
// This will fail in strict mode only
quantumString.length;

// In strict mode the onus is on the code author to ensure that the type has been narrowed to the non-null type. Usually this is as simple as an if check:

if (quantumString) {
  quantumString.length;
}

// In strict mode the type quantumString has two representations. Inside the if, the type was narrowed to just string. You can see more examples of narrowing in: example:union-and-intersection-types example:discriminate-types And even more resources on the web: https://mariusschulz.com/blog/literal-type-widening-in-typescript https://sandersn.github.io/manual/Widening-and-Narrowing-in-Typescript.html