I need to narrow down the type of a variable that is a union of different array types in order to run specific code for each type. I attempted to use Array.every along with a custom type guard, but encountered an error in TypeScript stating "This expression is not callable," with a confusing explanation.
Here is a simplified example:
const isNumber = (val: unknown): val is number => typeof val === 'number';
const unionArr: string[] | number[] = Math.random() > 0.5 ? [1, 2, 3, 4, 5] : ['1', '2', '3', '4', '5'];
if (unionArr.every(isNumber)) { // <- Error
unionArr;
}
The error message reads:
This expression is not callable.
Each member of the union type
'{
<S extends string>(predicate: (value: string, index: number, array: string[]) => value is S, thisArg?: any): this is S[];
(predicate: (value: string, index: number, array: string[]) => unknown, thisArg?: any): boolean;
} | {
...;
}'
has signatures, but none of those signatures are compatible with each other.
To resolve this issue, I found that using a type assertion to convert my array to unknown[] before narrowing its type, or changing the array from string[] | number[] to (string | number)[] helped eliminate the error without compromising type safety.
However, even after these adjustments, further type assertions were needed to correctly narrow the array type:
const isNumber = (val: unknown): val is number => typeof val === 'number';
const unionArr: string[] | number[] = Math.random() > 0.5 ? [1, 2, 3, 4, 5] : ['1', '2', '3', '4', '5'];
if ((unionArr as unknown[]).every(isNumber)) { // <- No error
unionArr; // <- Incorrectly typed as string[] | number[]
}
if ((unionArr as (string | number)[]).every(isNumber)) { // <- No error
unionArr; // <- Type incorrectly remains as string[] | number[]
}
A similar comparison with a non-array union did not yield any errors and successfully narrowed the type:
const isNumber = (val: unknown): val is number => typeof val === 'number';
const union: string | number = Math.random() > 0.5 ? 1 : '1';
if (isNumber(union)) {
union; // <- Correctly typed as number
}
While the workaround works fine, the root cause of the error still remains unclear. It seems to be related to how TypeScript interprets the typings of Array.every, and despite trying various approaches, the type assertion as unknown[] seems to be the most suitable solution. If there are better alternatives or if something could be done differently, please feel free to share your insights!