function f(...[a,b]: string[] | number[]) { if (typeof a === 'string') { return a + ':' + b; // b is number | string } else { return a + b; // error both are number | string } }
About this solution you asked: type string[] | number[] doesn't fully fit this function. Can you spot why, and what type would be better?
string[] | number[]
Let me try: is all about the length of the array? We need exactly two elements, so a tuple should be better, right?
tuple
Yes exactly, if we have static number of elements then we should use tuple type. Using number[] means we allow any number of arguments of type number.
number[]
number
function f(...[a, b]: number[]) { } function g(...[a, b]: [number, number]) { } f(1, 2, 3) // allowed g(1,2,3) // only two allowed so error
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About this solution you asked: type
string[] | number[]doesn't fully fit this function. Can you spot why, and what type would be better?Let me try: is all about the length of the array? We need exactly two elements, so a
tupleshould be better, right?Yes exactly, if we have static number of elements then we should use tuple type. Using
number[]means we allow any number of arguments of typenumber.