What is the purpose of this code in below?

Question

const fullNameMaxLength = 10;

class Employee {
private _fullName: string;

set fullName(newName: string) {
    if (newName && newName.length > fullNameMaxLength) {
        throw new Error("fullName has a max length of " + fullNameMaxLength);
    }

    this._fullName = newName;
}
}

if (newName && newName.length > fullNameMaxLength)

I can understand to check that newName is truthy or not in vanillaJS but, in Typescript what is the purpose of that? Typescript already guarantees that newName is string and it has .length property.

full code is here: https://www.typescriptlang.org/docs/handbook/classes.html

TypeScript doesn't exist at runtime, it's still useful to have basic runtime checks to avoid errors. — jonrsharpe, Apr 16 '20 at 19:40
@jonrsharpe But all of the code is Typescript and it is compiled to check for misuse. — mustafa kemal tuna, Apr 16 '20 at 19:43
TypeScript does not guarantee that `newName` is a string and has a `length` property. It guarantees that any code *it knows about* will set that property to a value that is a `string` — Heretic Monkey, Apr 16 '20 at 19:43
@HereticMonkey Monkey I think we say same thing but different ways. — mustafa kemal tuna, Apr 16 '20 at 19:57

score 1 · Accepted Answer · answered Apr 16 '20 at 21:45

Assuming all code is typescript, and compiled by typescript, and no value involved is any, then this existence check serves no purpose.

However, as other commenters point out, if any code is plain javascript, or exposed as a library to be used on unknown codebases or environments, then type safety cannot be guaranteed. So it's up to the developer to decide to do double checks.

But, here's some ways that it might still be good to write it that way:

non-typescript caller

// foo.js
employee.fullName = null // not typescript so no error.

a type of any might be passed in

// foo.ts
const jsonData: any = { employees: [{ fullName: null }] } // data from a remote api

// no error because `any` be cast to any other type without error.
employee.fullName = jsonData.employees[0].fullName is any.

The existence check would guard against an error being thrown by both of these cases. However, the very next line would assign the bad value anyway...

I would argue that, as an example in the typescript docs, the existence check probably should not be there, as it is a bit confusing.

I am new at Typescript but I think 'any' type is dark magic. And we do not use it. there is a good practice to handle remote api data: https://stackoverflow.com/questions/38688822/how-to-parse-json-string-in-typescript/38688861 — mustafa kemal tuna, Apr 16 '20 at 22:14

What is the purpose of this code in below?

1 Answers1