关于javascript:记几个前端面试问输出问题

this 指向

1.

function foo() {console.log( this.a);
}
function doFoo() {foo();
}
var obj = {
    a: 1,
    doFoo: doFoo
};
var a = 2; 
obj.doFoo()   // 2

2.

var inner = 'window'
function say() {console.log(inner)
    console.log(this.inner)
}
let obj = (function f() {
    var inner = 'inner-1'
    return {
        inner: 'inner-2',
        say:  function(){console.log(inner);
            console.log(this.inner);
        }
    }
})()
say()  // window, window
obj.say()      // inner-1 inner-2
obj.say = say
obj.say()    // window inner-2

事件循环

1.

setTimeout(() => {console.log('1');
  Promise.resolve().then(() => {console.log('2');
  });
}, 0);
new Promise((resolve) => {console.log('3');
  resolve();}).then(() => {console.log('4');
  setTimeout(() => {console.log('5');
  }, 0);
}).then(() => {console.log('6');
});
console.log('7');

// 3 7 4 6 1 2 5

2.

     async function async1() {console.log('async1 start');
        await async2();
        console.log('async1 end');
      }
      async function async2() {console.log('async2');
      }
      console.log('script start');
      setTimeout(function() {console.log('setTimeout');
      }, 0);
      async1();
      new Promise(function(resolve) {console.log('promise1');
        resolve();}).then(function() {console.log('promise2');
      });
      console.log('script end');

      // script start, async1 start, async2, promise1, script end,  async1 end, promise2, setTimeout