Type Parameters

The problem generics solve. You want one function that works with arrays of anything. Without generics you have two bad options: return any and lose the type, or write the function once per type.

A type parameter is a placeholder the caller fills in, usually without noticing.

The type flows through. The last line is rejected because first was given numbers, so it cannot produce a string. Hover over a and b in your editor to see the types it worked out.

You rarely pass the type explicitly. Inference works it out from the arguments, which is why generics are far more common in code you read than in code you write.

T is a convention rather than a rule. Name it for what it holds when that helps: <Item>, <Row>, <Payload>.