typescript中的async await-不等待等待的函数完全执行

我在typescript文件中有以下场景:

async function a() : Promise<void> {
  try {
    var result = await b(); 
    console.log('result=' + result);
  } catch(err){

  } finally {
    console.log('fin');
  }
}

async function b() : Promise<number> {
    let x: number = 1;
    console.log('start');
    // A long time taking http call - tried making the http request, both with or without await, but no change in result.
    // x got modified to 2 as a result of above call.
    console.log('x=' + x);
    return x;
}
a();

所有这些都发生在一个单独的文件中,几乎超过了框架,我关心的是对于上面的代码,我希望输出 start x=2 result=2 fin . 但我得到的结果是 start result=undefined fin x=2 .

我的tsconfig有以下功能:

"target": "es2015",
"module": "commonjs",
"lib": ["es5","es2015"] 

有人能帮我一下吗?谢谢