typescript-esnext
Typescript 3.6
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-6.html
stricter-generators
see yield, function* first
Typescript 3.7
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html
// 可选链操作符
a?.b
// 空值合并运算符
const foo = null ?? 'default string';