I'd like to start by saying that I am doing this series to learn and understand better Typescript, so feel free to correct me or contact me.
Let's create a type for the JavaScript Array.unshift function. Which takes pushes an element into the array in the last position.
Unshift<[1, 2], 0> // [0, 1, 2,]
Unshift<['1', 2, '3'], boolean> \\ [boolean, '1', 2, '3']
So we know we have an array and our type should return the new value + array.
type Unshift<TArray extends unknown[], U> = [U, ...TArray]
TArray extends readonly unknown[]
We first check our TArray is of type array.
Then we pass the value and we spread the array into an array [U, ...TArray]
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