the result must be a number. Perhaps you are returning a value to a function as a string. sorry for my english

I meant, arrow function doesn't need return. Read this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

No.

const flip = (d, a) => {...}

is the same as:

function flip(d, a) {...}

d and a are arguments.

the method is not in first question here, instead compare the difference of data type used for the return: [] is an array type, and 0 is a number. (the function should return a number)

it means you returned [] but the test expected the result 0