Question

所以我有以下用于phantomjs：

  var time = 1490543999999999;
  var endTime = time - 518400000000 * 10;
  var mintime = time - 518399999999;
  console.log("mintime after assignment is"+mintime);
  var loopnumber = 6;

  page.open('https://myactivity.google.com/item?min=' + mintime + '&max=' + time + '&product=5,6,7,10,11,12,13,15,19,20,25,27,28', callback);

  function callback() {
    console.log("inside callback and loopnumber is "+loopnumber+"and time is "+time+", and mintime is "+mintime+'. URL: https://myactivity.google.com/item?min=' + mintime + '&max=' + time + '&product=5,6,7,10,11,12,13,15,19,20,25,27,28');
    page.render('/Users/jMac-NEW/Documents/FILEMAKER OCLC/login_test33.7_brkpoint' + loopnumber + '.png');
    fs.write('/Users/jMac-NEW/Documents/FILEMAKER OCLC/login_test33.7_brkpoint' + loopnumber + 'html.html', page.content, 'w');
    time -= 518400000000;
    loopnumber++;
    if (time > endTime) {
      var mintime = time - 518399999999;
      page.open('https://myactivity.google.com/item?min=' + mintime + '&max=' + time + '&product=5,6,7,10,11,12,13,15,19,20,25,27,28', callback);
    }
    else {
      phantom.exit();
    }
  }

这是第一个console.log：

的输出

mintime after assignment is1490025600000000

但是当谈到callback（）中的console.log时，这就是输出：

inside callback and loopnumber is 6and time is 1490543999999999, and mintime is undefined. URL: https://myactivity.google.com/item?min=undefined&max=1490543999999999&product=5,6,7,10,11,12,13,15,19,20,25,27,28

为什么mintime = undefined，其他变量（loopnumber和time）不是？

Answer 1

Javascript中有一个名为 hoisting 的概念 - 如果在之后使用声明变量，则脚本解释器会自动移动它＆＃39;在内部声明到当前范围的顶部。

所以你的脚本实际上读到这样的js：

function callback() { var mintime; // <-- declaration moved here // mintime is not yet defined console.log("and time is "+time+", and mintime is "+mintime); // ... if (time > endTime) { mintime = time - 518399999999; // it's been declared already and now it's defined } else { phantom.exit(); } }

所以解决方案很简单：在回调中将var从赋值中删除到mintime，这种方式变量将被分配给外部（窗口）范围mintime，就像你想要的那样。

在Javascript for PhantomJS中的回调函数中仍然未定义赋值后的变量

1 个答案: