Here is a code snippet for calculating the sum:
//Function to calculate the sum of an array of numbers
let sum =
([head, ...tail]: number[]) => head ? head + sum(tail) : 0
let result: string = sum([1, 2, 3]);
alert(result);
Can anyone explain why TypeScript infers the return type of sum as any? Interestingly, Flow reports an error for this code snippet, which seems correct.
A matter was raised (Regarding recursive functions being inferred to have a return type of any) on June 2nd, 2015, and it was resolved as "by design" stating:
We had outlined a specification that detailed how this could potentially function theoretically, but it never made it to the implementation phase.
The current guideline states that any function encountering itself during the determination of its return type defaults to type 'any'. This approach seems sufficient in practice, as one can always include the necessary type annotation, and most functions do not exhibit recursion like this due to tail call optimizations not yet being part of the ES spec.
In essence, just specify the return type:
let sum =
([head, ...tail]: number[]): number => head ? head + sum(tail) : 0
let x: string = sum([1, 2, 3]); // Error: Type 'number' is not assignable to type 'string'
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