Typescript: is out-of-bounds in tuple of length，but the introduction in the document can be out of bounds

[WormGirl]

TypeScript Version: 3.2.0
Code
This is the official demo

// Declare a tuple type
let x: [string, number];
// Actually，this code will throw error
x[3] = 'world'; // OK, 'string' can be assigned to 'string | number'

Expected behavior:
I want to know if this is a bug or this is document error

Actual behavior:
Tuple out of bounds will trow error, but document say When accessing an element outside the set of known indices, a union type is used instead

Playground Link:
https://www.tslang.cn/play/index.html#src=let%20x%3A%20%5Bstring%2C%20number%5D%3B%0D%0A%2F%2F%20When%20accessing%20an%20element%20outside%20the%20set%20of%20known%20indices%2C%20a%20union%20type%20is%20used%20instead%3A%0D%0A%2F%2F%20Actually%EF%BC%8Cthis%20code%20will%20throw%20error%0D%0Ax%5B3%5D%20%3D%20%22world%22%3B%20%2F%2F%20OK%2C%20'string'%20can%20be%20assigned%20to%20'string%20%7C%20number'

Related Issues: NO

[WormGirl]

@weswigham

[weswigham]

Tuples are considered fixed length - you may not assign to a numeric field outside it's range.

[cpplearner]

I guess it's a issue of https://github.com/Microsoft/TypeScript-Handbook. The quoted part is in https://github.com/Microsoft/TypeScript-Handbook/blob/master/pages/Basic%20Types.md#tuple

[weswigham]

Aye, the docs are out of date - I think tuples became fixed length around TS 2.5.

[WormGirl]

Thank you

[xiaojingzhao]

Why there is nobody to fix this obvious misleading error on Doc?

[RyanCavanaugh]

Why there is nobody to fix this obvious misleading error on Doc?

Sadly, the project still employs a finite number of engineers.

Be the change you want to see in the world.

[hateonion]

https://github.com/Microsoft/TypeScript-Handbook/pull/998

I've created this pr, hope can fix it.

[leinov]

why

[xuniaoji]

should fix that obvious mistake