TypeScript can infer variable types from initial values and return statements. Even still, we may not know exactly what type inference to expect when dealing with arrays. For example:
What is the type of examAnswers? It seems it could equally well be boolean[] or [boolean, boolean, boolean]. In reality, it is always the first of these, since this is the less restrictive type. This enables us to expand the array:
Since tuples have fixed lengths, we wouldn’t be able to add additional boolean elements to a tuple:
We also get the same kind of type inference when we use the .concat() method:
In the code above, TypeScript infers the variable concatResult as an array of numbers, not a tuple.
The takeaway here is that type inference returns arrays. When we want tuples, we need to use explicit type annotations.
Instructions
Let’s do a tricky example. We have defined the tuple dogTup in the code editor. Your challenge is to define a variable myArr of the type string[].
Not so fast! The hard part is that you are not allowed to use the word Array, or the characters [ and ]. Use the .concat() method and your wits to solve the problem.
You figured it out! Now demonstrate that myArr is an array rather than a tuple by adding the code myArr[50] = 'not a dog' and running tsc. If myArr were a tuple, accessing element 50 would be impossible!