I am encountering an issue with a type predicate function that is designed to assign a type to a JSON object
interface Duck {
canQuack: true
}
function isDuck(duck: unknown): duck is Duck {
if (typeof duck !== "object" || ! duck) return false
return duck.canQuack === true
}
Unfortunately, TypeScript is indicating that canQuack does not exist on object.
Is there a way to perform a type check on an object with the unknown type?
When referring to the typescript documentation, it suggests casting to the type and then verifying the value to avoid any potential runtime errors. Ensuring the cast is safe, as shown in the provided example, is crucial to prevent unexpected issues. In this scenario, the recommended solution is to:
interface Duck {
canQuack: true
}
function isDuck(duck: unknown): duck is Duck {
// Check that the candidate is an object
return duck != null && (typeof duck === 'object' || typeof duck === 'function')
// check that it has canQuack
&& (duck as Duck).canQuack === true;
}
Update: The object check has been corrected for accuracy, taking inspiration from a similar check in lodash library https://github.com/lodash/lodash/blob/4.17.15/lodash.js#L11745
In order to ensure proper type checking, you will require an additional type guard.
interface Duck {
canQuack: true;
}
function isDuck(duck: unknown): duck is Duck {
if (typeof duck !== "object" || !duck) return false;
const canQuackGuard = (o: object): o is { canQuack: unknown } =>
"canQuack" in o;
return (
canQuackGuard(duck) &&
typeof duck.canQuack === "boolean"
);
}
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