如何在页面加载时自动下拉选择器标签的选项？

Question

我一直在使用getusermedia来提取多个摄像头，我用一个单独的.js文件调用它，它工作正常，但我需要选择器标签在页面加载时显示数组中可用的所有选项。我试过onload，但没有工作，如果有任何问题，请告诉我。

<div id="container">

    <label for="videoSource">Video source: </label><select id="videoSource"></select>

</div>

外部js文件

var videoSelect = document.querySelector('select#videoSource');
var selectors = [videoSelect];

function gotDevices(deviceInfos) {
  // Handles being called several times to update labels. Preserve values.
  var values = selectors.map(function(select) {
    return select.value;

  });
  selectors.forEach(function(select) {
    while (select.firstChild) {
      select.removeChild(select.firstChild);
    }
  });
  for (var i = 0; i !== deviceInfos.length; ++i) {
    var deviceInfo = deviceInfos[i];
    var option = document.createElement('option');
    option.value = deviceInfo.deviceId;
   if (deviceInfo.kind === 'videoinput') {
      option.text = deviceInfo.label || 'camera ' + (videoSelect.length + 1);
      videoSelect.appendChild(option);

    } else {
      console.log('Some other kind of source/device: ', deviceInfo);
    }
  }
  selectors.forEach(function(select, selectorIndex) {
    if (Array.prototype.slice.call(select.childNodes).some(function(n) {
      return n.value === values[selectorIndex];
    })) {
      select.value = values[selectorIndex];
    }
  });
}

navigator.mediaDevices.enumerateDevices().then(gotDevices).catch(handleError);

// Attach audio output device to video element using device/sink ID.
function attachSinkId(element, sinkId) {
  if (typeof element.sinkId !== 'undefined') {
    element.setSinkId(sinkId)
    .then(function() {
      console.log('Success, audio output device attached: ' + sinkId);
    })
    .catch(function(error) {
      var errorMessage = error;
      if (error.name === 'SecurityError') {
        errorMessage = 'You need to use HTTPS for selecting audio output ' +
            'device: ' + error;
      }
      console.error(errorMessage);
      // Jump back to first output device in the list as it's the default.
   //   audioOutputSelect.selectedIndex = 0;
    });
  } else {
    console.warn('Browser does not support output device selection.');
  }
}


function gotStream(stream) {
  window.stream = stream; // make stream available to console
  videoElement.srcObject = stream;
  // Refresh button list in case labels have become available
  return navigator.mediaDevices.enumerateDevices();
}

function start() {
  if (window.stream) {
    window.stream.getTracks().forEach(function(track) {
      track.stop();
    });
  }

  var videoSource = videoSelect.value;
  var constraints = {
    video: {deviceId: videoSource ? {exact: videoSource} : undefined}
  };
  navigator.mediaDevices.getUserMedia(constraints).
      then(gotStream).then(gotDevices).catch(handleError);
}


videoSelect.onchange = start;

start();

我试过的方法，不工作..

function show(){
    document.getElementById("videoSource");

 }
 $(document).ready(function(){   
   show();
});