What advantages does using a predicate as a return type offer over a simple boolean?

Question

What advantages does using a predicate as a return type offer over a simple boolean?

Recently, I stumbled upon the concept of user-defined typeguards while perusing through this enlightening article: https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards

One intriguing example presented in the aforementioned article showcases the use of pet is Fish as a method return type, acting as a predicate.

Upon further exploration, it came to my attention that instead of using this return type, one could simply opt for boolean. This raised the question in my mind - is the parameter is Type return type merely a form of syntactic sugar, or does it serve a distinct purpose?

angular typescript

Answer 1

Answer №1

When you return a boolean, the function becomes a simple one and not a typeguard. The compiler recognizes the pet is Fish syntax as an indicator that this function will affect the argument's type.

Here is an example :

class Fish { f: boolean }
class Dog { d: boolean; }

declare let x: Fish | Dog;
declare function isFish(p: Fish | Dog): boolean
declare function isFishGuard(p: Fish | Dog): p is Fish;

if (isFishGuard(x)) {
    x.f // x is Fish
}

if (isFish(x)) {
    x.f // error x is still Fish|Dog
}

Playground link

Answer 2

When you return a boolean, the function becomes a simple one and not a typeguard. The compiler recognizes the pet is Fish syntax as an indicator that this function will affect the argument's type.

Here is an example :

class Fish { f: boolean }
class Dog { d: boolean; }

declare let x: Fish | Dog;
declare function isFish(p: Fish | Dog): boolean
declare function isFishGuard(p: Fish | Dog): p is Fish;

if (isFishGuard(x)) {
    x.f // x is Fish
}

if (isFish(x)) {
    x.f // error x is still Fish|Dog
}

Playground link

What advantages does using a predicate as a return type offer over a simple boolean?

Answer №1

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