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:

let examAnswers= [true, false, false];

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:

examAnswers[3] = true; // No type error.

Since tuples have fixed lengths, we wouldn’t be able add additional boolean elements to a tuple:

let tupleOfExamAnswers: [boolean, boolean, boolean] = [true, false, false]; tupleOfExamAnswers[3] = true; // Type error! The tuple only has 3 elements.

We also get the same kind of type inference when we use the .concat() method:

let tup: [number, number, number] = [1,2,3]; let concatResult = tup.concat([4,5,6]); // concatResult has the value [1,2,3,4,5,6].

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.



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!

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