JuniorTheoryCommon

What happens when this program is compiled and run (lines separated by spaces)?

TypeScript

type Handler = (value: number, index: number) => void;
function each(nums: number[], fn: Handler) {
    for (let i = 0; i < nums.length; i++) {
        fn(nums[i], i);
    }
}
each([1, 2], (v) => console.log(v));

tsc is silent, and the program prints 1 2

Expecting an error about the missing index: a function with fewer parameters fits a function type with more, because it may leave the extra arguments unread.
Expecting v to be implicitly any: an arrow passed where a Handler is expected takes its parameter types from Handler.

What does tsc report if the callback is (v, i, extra) => console.log(v)?
Why does TypeScript allow a callback with fewer parameters but not one with more?

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JuniorTheoryCommon

What happens when this program is compiled and run (lines separated by spaces)?

TypeScript

type Handler = (value: number, index: number) => void;
function each(nums: number[], fn: Handler) {
    for (let i = 0; i < nums.length; i++) {
        fn(nums[i], i);
    }
}
each([1, 2], (v) => console.log(v));

tsc is silent, and the program prints 1 2

Expecting an error about the missing index: a function with fewer parameters fits a function type with more, because it may leave the extra arguments unread.
Expecting v to be implicitly any: an arrow passed where a Handler is expected takes its parameter types from Handler.

What does tsc report if the callback is (v, i, extra) => console.log(v)?
Why does TypeScript allow a callback with fewer parameters but not one with more?

Back to questions Open topic in roadmap →Answer it yourself — Quiz