Review Basic Types for language changes

[RyanCavanaugh]

Update this file so that it is up-to-date with regard to new language features, especially around union types and ES6 features

[tulushev]

When accessing an element outside the set of known indices, a union type is used instead:

If I understand correctly this line in Basic Types > Tuple is no longer valid.

[xiaojingzhao]

Hi: As what handbook said:

However, when using the --strictNullChecks flag, null and undefined are only assignable to void and their respective types.

I tested this locally and it didn't work as what I expected. null cannot assigned to void ?

Example:

let nullValue: void = null;