syntax

• 如果不清楚什么时候用interface/type，能用 interface 实现，就用 interface , 如果不能就用 type
• [not-null assertion operator]
  • postfix !(exclamation mark) after a variable
  • It removes null and undefined from the type of an expression.

Declaration Merging

“declaration merging” means that the compiler merges two separate declarations declared with the same name into a single definition. This merged definition has the features of both of the original declarations. Any number of declarations can be merged;