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I'm learning TypeScript.

Playing with types I got stuck trying to resolve the following error:

Element implicitly has an 'any' type because expression of type '0' can't be used to index type 'number | (number | number[])[]'. Property '0' does not exist on type 'number | (number | number[])[]'.

Here is my code

let arr = [2, [22, [222]], 2222];

console.log(arr[1][0]); // 22

I searched around and found examples for objects but I don't know how to do it with a nested array.

How to fix this?

Thanks.

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    You have to check if it's an array before indexing into it. Commented Sep 16, 2022 at 20:43

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The error makes a lot of sense since the typesystem is not certain that the index arr[1] you are accessing is definitely an array. It could equally be likely the arr[1] is actually a number.

This is the exact advantage typesript gives you. In plain javascript you could write arr[0][1] and you would have no compilation error. You would, however, get a nasty error at runtime since arr[0] is not an array at all.

You should only be able to use arr[1] as an array type after ensuring arr[1] is actually an array. So you must do something like:

if (Array.isArray(arr[1])) {
  console.log(arr[1][0]); // 22 - No typescript error
}
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This seem to work for me.

Before Error:

const slug = props.data[0].attributes.slug;

After adding check "Array.isArray":

const slug = Array.isArray(props.data) ? props.data[0].attributes.slug : null;
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