async & await

【await】

　　The await operator is used to wait for a Promise. It can only be used inside an async function.

　　Returns the resolved value of the promise, or the value itself if it's not a Promise.

　　The await expression causes async function execution to pause, to wait for the Promise's resolution, and to resume the async function execution when the value is resolved. It then returns the resolved value. If the value is not a Promise, it's converted to a resolved Promise.

　　If the Promise is rejected, the await expression throws the rejected value.

async function f3() {
  try {
    var z = await Promise.reject(30);
  } catch(e) {
    console.log(e); // 30
  }
}
f3();

【async】

　　When an async function is called, it returns a Promise. When the async function returns a value, the Promise will be resolved with the returned value.  When the async function throws an exception or some value, the Promise will be rejected with the thrown value.

參考：

一、https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await

二、https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function