javascript – Chart.js – 无法读取null的属性’getContext’

我的main.js文件中有以下 Javascript：

//array object of API stuff

function createChartWinLoss(wins,losses) {

  var pieData = [
    {
      value: losses,color: "#F7464A",highlight: "#FF5A5E",label: "Losses"
    },{
      value: wins,color: "#46BFBD",highlight: "#5AD3D1",label: "Wins"
    }
  ];

  var pieOptions = {
    segmentShowstroke : false,animateScale : true
  }


  var winLossChart = document.getElementById('winLossChart').getContext('2d');
  new Chart(winLossChart).Pie(pieData,pieOptions);
}

//creates the chart with test data
createChartWinLoss();

function summonerLookUp() {
  var SUMMONER_ID = "";
  var API_KEY = "keyhere";
  var sumID = $("#theKey").val();
  var div = document.getElementById('stuff');
  var combine = "";
  var array = [sumID];
  var wins;
  var losses;

  div.innerHTML = div.innerHTML + "<br />array count: " + array.length + "<br />";
  for (var i = 0; i < array.length; i++) {
    combine = "";
    SUMMONER_ID = array[i];
    getStuff(SUMMONER_ID,combine,API_KEY,div,i);
  }
}

function getStuff(SUMMONER_ID,count) {
  var Topuser = SUMMONER_ID;
  $.ajax({
    url: 'https://euw.api.pvp.net/api/lol/euw/v2.5/league/by-summoner/' + SUMMONER_ID + '/entry?api_key=' + API_KEY,type: 'GET',dataType: 'json',async: false,data: {},success: function (json) {
      var user = Topuser;
      if (typeof json[user][0].queue != "undefined") {
        if (json[user][0].queue == "RANKED_SOLO_5x5") {
          combine = json[user][0].tier + " " + json[user][0].entries[0].division  + " - " + json[user][0].entries[0].wins + " Wins " + json[user][0].entries[0].losses + " losses";
          div.innerHTML = div.innerHTML + "<br />" + count + ": " + user + " " + combine;
          var wins = json[user][0].entries[0].wins;
          var losses = json[user][0].entries[0].losses;
          //populates chart with wins losses from api
          createChartWinLoss(wins,losses);
        }
      }
    },error: function (XMLHttpRequest,textStatus,errorThrown) {
      var user = Topuser;
      console.log(errorThrown);
      if (errorThrown === "Not Found") {
        div.innerHTML = div.innerHTML + "<br />" + count + ": " + user + " " + "UNRANKED";
      }
    }
  });
}

HTML如下：

<div class="container">
  <h2>Wins/Losses</h2>
  <canvas id="winLossChart" width="400" height="200"></canvas>
</div>

正如标题所示,我得到了Uncaught TypeError：无法读取null的属性’getContext’,我不完全确定问题是什么.如果我不得不猜测我会说它试图引用那些不存在的东西,但我不是百分之百确定我是否正确以及如何解决它.任何建议都会很棒.

解决方法

抛出错误的行是

var winLossChart = document.getElementById('winLossChart').getContext('2d');

这是说document.getElementById(‘winLossChart’)不存在.

这是因为在DOM中创建元素之前,您的脚本正在被解释.

您可以在window.onload函数中启动脚本：

window.onload = function() {
   createChartWinLoss();
}

或者您可以将脚本标记本身作为html文件的body元素中的最后一个元素.

<body>
  <div class="container">
    <h2>Wins/Losses</h2>
    <canvas id="winLossChart" width="400" height="200"></canvas>
  </div>
  <script src="myscript.js"></script>
</body>

这两种解决方案都意味着代码的主要入口点(createChartWinLoss)只会在页面上的其他元素(包括画布)创建之后发生.

作为解决这类问题的一般过程,当您在Javascript控制台中看到异常时,您应该能够打开堆栈跟踪,这会导致错误源于行var winLossChart = ……,这会让你更有可能发现问题的根源.

javascript – Chart.js – 无法读取null的属性’getContext’

解决方法

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