Sometimes, we want to fix ‘instanceof’ in TypeScript give me the error "’Foo’ only refers to a type, but is being used as a value here." error.
In this article, we’ll look at how to fix ‘instanceof’ in TypeScript give me the error "’Foo’ only refers to a type, but is being used as a value here." error.
How to fix ‘instanceof’ in TypeScript give me the error "’Foo’ only refers to a type, but is being used as a value here." error?
To fix ‘instanceof’ in TypeScript give me the error "’Foo’ only refers to a type, but is being used as a value here." error, we should only use instanceof with constructors.
For instance, we write
interface Foo {
abcdef: number;
}
let x: Foo | string;
if ('abcdef' in x) {
// ...
}
to create the Foo instance.
And we check if x implements Foo with 'abcdef' in x.
If the abcdef property is in x, then we know x is a Foo object since strings don’t have the abcdef property and x is either a Foo object or a string.
Conclusion
To fix ‘instanceof’ in TypeScript give me the error "’Foo’ only refers to a type, but is being used as a value here." error, we should only use instanceof with constructors.
