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

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 x%3A [string%2C number]%3B %2F%2F When accessing an element outside the set of known indices%2C a union type is used instead%3A %2F%2F Actually，this code will throw error x[3] %3D “world”%3B %2F%2F OK%2C ‘string’ can be assigned to ‘string | number’

Related Issues: NO

[weswigham]

Aye, the docs are out of date - I think tuples became fixed length around TS 2.5.

[xuniaoji]

should fix that obvious mistake