Question 281095 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – Canvas is already in use. Chart with ID '0'

I’m trying to refresh a function without refreshing the page but i’m getting this error.

HTML

<canvas id="js-chartjs-bars"></canvas>

JavaScript

  var 1 = "22";
  var 2 = "13";
  var 3 = "41";
  var 4 = "60";

  !function diagram() {
    const data = [1, 2, 3, 4];
    const backgroundcolor = [];
    for(i = 0; i < data.length; i++){
      if(data[i] <= 29) { backgroundcolor.push('rgba(194, 23, 0, 1)')}
      if(data[i] >= 30 && data[i] <= 54) { backgroundcolor.push('rgba(194, 149, 0, 1)')}
      if(data[i] >= 55) { backgroundcolor.push('rgba(10, 194, 0,1)')}
    }
    class t {
        static initChartsChartJS() {
            Chart.defaults.color = "#6a6f73", Chart.defaults.font.weight = "600", Chart.defaults.scale.grid.color = "rgba(0, 0, 0, .05)", Chart.defaults.scale.grid.zeroLineColor = "rgba(0, 0, 0, .1)", Chart.defaults.scale.beginAtZero = !0, Chart.defaults.elements.line.borderWidth = 2, Chart.defaults.elements.point.radius = 4, Chart.defaults.elements.point.hoverRadius = 6, Chart.defaults.plugins.tooltip.radius = 3, Chart.defaults.plugins.legend.labels.boxWidth = 15;
            let t, a, i = document.getElementById("js-chartjs-lines"),
                d = document.getElementById("js-chartjs-bars");
            i = {
                labels: ["1", "2", "3", "4"],
                datasets: [{
                    label: "Charts",
                    fill: !0,
                    data: data,
                    backgroundColor: backgroundcolor
                },]
            }, null !== d && (a = new Chart(d, {
                type: "bar",
                data: i,
                options: {
                    responsive: !0,
                    maintainAspectRatio: !1,
                    plugins: {
                      datalabels: {
                        labels: {
                          title: {
                            color: 'white',
                            font:{
                              size: '18px'
                            }
                          }
                        }
                      }
                    }
                },
                plugins: [
                  ChartDataLabels
                ]
                
            }))
        }
        static init() {
            this.initChartsChartJS()
        }
    }
    One.onLoad((() => t.init()));
    setInterval( diagram, 10000 );
  }();

ERROR:

Uncaught Error: Canvas is already in use. Chart with ID '0' must be destroyed before the canvas with ID 'js-chartjs-bars' can be reused

Is it possible to refresh the function itself without refreshing the page? or how I could handle it?

The Problem is what I found with google, i’ve to destroy the chart. When I try to destroy it, i do alway get, this is not defined.
I’m new to javascript, any advice would be amazing.

Tags:

2

Answers


  1. 0

    Yes, it’s possible to refresh the function that updates the chart without refreshing the page. The error message you’re seeing indicates that you’re trying to create a new chart on a canvas element where a chart already exists. To resolve this, you should destroy the existing chart before creating a new one.

    Here’s a revised approach that includes destroying the existing chart:

    // Assuming Chart.js v3 or newer
var chartInstance; // This will hold the instance of the chart.

function updateChart() {
  // Your data variables should be named properly; starting with a letter, $ or _
  var val1 = "22";
  var val2 = "13";
  var val3 = "41";
  var val4 = "60";
  
  const data = [val1, val2, val3, val4];
  const backgroundColor = [];
  
  for(let i = 0; i < data.length; i++){
    if(data[i] <= 29) { backgroundColor.push('rgba(194, 23, 0, 1)'); }
    else if(data[i] >= 30 && data[i] <= 54) { backgroundColor.push('rgba(194, 149, 0, 1)'); }
    else if(data[i] >= 55) { backgroundColor.push('rgba(10, 194, 0,1)'); }
  }

  // If a chart instance exists, destroy it before creating a new one
  if (chartInstance) {
    chartInstance.destroy();
  }

  const ctx = document.getElementById("js-chartjs-bars").getContext("2d");
  chartInstance = new Chart(ctx, {
    type: "bar",
    data: {
      labels: ["1", "2", "3", "4"],
      datasets: [{
        label: "Charts",
        fill: true,
        data: data,
        backgroundColor: backgroundColor
      }]
    },
    options: {
      responsive: true,
      maintainAspectRatio: false,
      plugins: {
        datalabels: {
          labels: {
            title: {
              color: 'white',
              font: {
                size: '18px'
              }
            }
          }
        }
      }
    },
    plugins: [ChartDataLabels]
  });
}

// Initial call to set up the chart
updateChart();

// Set interval to update the chart every 10 seconds
setInterval(updateChart, 10000);

    This script does the following:

    • It defines a global variable chartInstance that holds the instance of the chart.
    • The updateChart function checks if chartInstance is not undefined, which would mean a chart has already been created. If so, it destroys the existing chart instance with chartInstance.destroy().
    • It then creates a new chart as before.
    • Instead of calling your diagram function immediately, we now call updateChart initially to set up the chart.
    • We also call updateChart within setInterval to refresh the chart every 10 seconds.

    Note that you need to ensure that the ChartDataLabels plugin and any necessary Chart.js libraries are correctly included in your page for this to work. Also, variable names in JavaScript cannot start with a digit; they should begin with a letter, an underscore (_), or a dollar sign ($). Therefore, I’ve renamed your variables val1, val2, val3, and val4 accordingly.

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  2. 0

    The error message "Canvas is already in use" indicates that a chart instance is already attached to the canvas element with the ID ‘js-chartjs-bars’. Before creating a new chart instance on the same canvas, the existing chart instance must be destroyed.

    To resolve this, you should keep a reference to the chart object and destroy it before creating a new one. Here’s how you can modify your code to include this:

    Firstly, ensure that your variable names are valid identifiers. Variable names like 1, 2, 3, 4 are not valid in JavaScript. So, let’s rename them to var1, var2, var3, var4.

    Then, modify the JavaScript code as follows:

    // Define the chart reference outside of your diagram function
let myChart;

function diagram() {
  const data = [var1, var2, var3, var4]; // use the variables you have defined
  const backgroundColor = [];

  for (let i = 0; i < data.length; i++) {
    if (data[i] <= 29) { backgroundColor.push('rgba(194, 23, 0, 1)'); }
    else if (data[i] <= 54) { backgroundColor.push('rgba(194, 149, 0, 1)'); }
    else if (data[i] >= 55) { backgroundColor.push('rgba(10, 194, 0, 1)'); }
  }

  const chartData = {
    labels: ["1", "2", "3", "4"],
    datasets: [{
      label: "Charts",
      fill: true,
      data: data,
      backgroundColor: backgroundColor
    }]
  };

  const chartOptions = {
    responsive: true,
    maintainAspectRatio: false,
    plugins: {
      datalabels: {
        color: 'white',
        font: {
          size: '18px'
        }
      }
    }
  };

  // Get the canvas element
  const ctx = document.getElementById('js-chartjs-bars');

  // If the chart instance already exists, destroy it
  if (myChart) {
    myChart.destroy();
  }

  // Create a new chart instance
  myChart = new Chart(ctx, {
    type: 'bar',
    data: chartData,
    options: chartOptions
  });
}

// Call the diagram function initially
diagram();

// Refresh the chart every 10 seconds
setInterval(diagram, 10000);

    Here are the key changes:

    • Declared a let myChart; variable outside of the diagram function to maintain the chart instance.
    • Before creating a new chart, check if myChart exists and if so, call myChart.destroy() to remove the previous instance.
    • Use let and const for variable declarations to avoid scope issues.
    • Renamed the invalid variable names to valid identifiers (var1, var2, var3, var4).

    Now, your function should refresh the chart every 10 seconds without needing to refresh the entire page. Remember to replace var1, var2, var3, var4 with the actual variables you have defined elsewhere in your code.

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