What is the meaning of "narrowing it down" in TypeScript

• In the context of TypeScript, "narrowing it down" refers to the process of refining the type of a variable or expression to a more specific and constrained type within a certain code block or conditional statement. It involves using type guards or specific type-checking techniques to eliminate certain types and work with a subset of possibilities.

For example, consider the following TypeScript code snippet:

    function printLength(value: string | string[] | null) {

        if (typeof value === 'string') {

            // Here, `value` is narrowed down to the `string` type

            console.log(value.length);

        } else if (Array.isArray(value)) {

            // Here, `value` is narrowed down to the `string[]` type

            console.log(value.length);

        } else {

            // Here, `value` is narrowed down to the `null` type

            console.log('Value is null');

        }

    }

• In this example, the `printLength` function takes an argument `value` that can be of type `string`, `string[]` (an array of strings), or `null`. Within the function, we narrow down the type of `value` using the `typeof` check and `Array.isArray` check.

• By narrowing down the type, we gain more specific type information within each conditional branch, allowing us to safely access properties or perform operations specific to that particular type. This enables us to write type-safe code and leverage the benefits of TypeScript's static type checking.