Are your custom guards failing to function properly now?

Question

Are your custom guards failing to function properly now?

Previously, the code below was functioning properly until typescript 2.0:

class Aluno{
  escola: string;
  constructor(public nome: string){
    this.escola = "";
  }
}
class Pessoa{
  morada: string;
  constructor(public nome: string){
    this.morada = "";
  }
}
function ePessoa(p: Pessoa | Aluno): p is Pessoa{
  return (<Pessoa>p).morada !== undefined;
}
function eAluno(p: Pessoa | Aluno): p is Aluno{
  return (<Aluno>p).escola !== undefined; 
}
let a: Pessoa | Aluno = new Pessoa("Luis");
console.log( a.nome ); //works fine
a = new Aluno("Rui");
console.log( a.nome ); //works fine as well
//a.escola = "teste";//error occurs here
if(eAluno(a)){
  console.log("Student at school:" + a.escola);
}
else{
  console.log("Person residing in: " + a.morada); //morada does not exist
}

The compiler is not recognizing 'a' as a Person in the else branch. Am I overlooking something?

typescript typescript2.0

Answer 1

Answer №1

The error message states:

Property 'morada' does not exist on type 'never'

This error occurs because the compiler recognizes that the else clause is never accessed.

In your code snippet, you have:

let a: Pessoa | Aluno = new Pessoa("Luis");
...
a = new Aluno("Rui");

Therefore, at the if statement, a will always be of type Aluno, never Pessoa.

You can learn more about the never type here.

Edit

The purpose of TypeScript is to prevent developers from making mistakes.
While some JavaScript developers argue that types are unnecessary, you are still utilizing TypeScript.

The error you encountered is akin to an Unreachable code error:

function fn(): boolean {
    return false;

    let a = 3; // Error: Unreachable code detected
}

(Try it in the playground)

Why include code that will never run?
It adds unnecessary complexity and maintenance burden, without any benefit.

2nd edit

The compiler only checks for errors within the current scope, like with the Unreachable code error:

function fn(): boolean {
    throw new Error("error");
}

let a = fn();
if (a) { // no error
    console.log("all good");
}

However:

let a = fn();
throw new Error("error");
if (a) { // Error: Unreachable code detected
    console.log("all good");
}

Answer 2

The error message states:

Property 'morada' does not exist on type 'never'

This error occurs because the compiler recognizes that the else clause is never accessed.

In your code snippet, you have:

let a: Pessoa | Aluno = new Pessoa("Luis");
...
a = new Aluno("Rui");

Therefore, at the if statement, a will always be of type Aluno, never Pessoa.

You can learn more about the never type here.

Edit

The purpose of TypeScript is to prevent developers from making mistakes.
While some JavaScript developers argue that types are unnecessary, you are still utilizing TypeScript.

The error you encountered is akin to an Unreachable code error:

function fn(): boolean {
    return false;

    let a = 3; // Error: Unreachable code detected
}

(Try it in the playground)

Why include code that will never run?
It adds unnecessary complexity and maintenance burden, without any benefit.

2nd edit

The compiler only checks for errors within the current scope, like with the Unreachable code error:

function fn(): boolean {
    throw new Error("error");
}

let a = fn();
if (a) { // no error
    console.log("all good");
}

However:

let a = fn();
throw new Error("error");
if (a) { // Error: Unreachable code detected
    console.log("all good");
}

Are your custom guards failing to function properly now?

Answer №1

Edit

2nd edit

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