[Chartjs]-How to integrate charts with js and jQuery?

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You should assign the chart to a global variable and update its data each time a different state is selected from the drop-down. After that, you need to invoke chart.update().

For further information, please consult Updating Charts from the Chart.js documentation.

Please take a look at your amended code and see how it works.

$(document).ready(function() {
  $('select').on('change', function() {
    const state = this.value.toLowerCase();
    fetch('https://api.covidtracking.com/v1/states/' + state + '/daily.json')
      .then(response => response.json())
      .then(json => {
        myChart.data.labels = json.map((x) => x.date);
        myChart.data.datasets[0].data = json.map((x) => x.positive);        
        myChart.update();
      });
  });
});

const myChart = new Chart('myChart', {
  type: 'line',
  data: {
    labels: [],
    datasets: [{
      label: 'Positive',
      backgroundColor: 'blue',
      borderColor: 'rgb(255, 99, 132)',
      data: [],
      fill: false
    }]
  },
  options: {
    tooltips: {
      mode: 'index'
    }
  }
});
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-BmbxuPwQa2lc/FVzBcNJ7UAyJxM6wuqIj61tLrc4wSX0szH/Ev+nYRRuWlolflfl" crossorigin="anonymous">
  <link rel="stylesheet" href="/styles.css" />
  <title>Covid Data</title>
</head>

<body>
  <div class="card text-center m-5">
    <div class="card-header">
      <h2>Covid Data</h2>
      <select>
        <option>Select State...</option>
        <option value="AL">Alabama</option>
        <option value="AK">Alaska</option>
        <option value="AZ">Arizona</option>
        <option value="AR">Arkansas</option>
        <option value="CA">California</option>
        <option value="CO">Colorado</option>
        <option value="CT">Connecticut</option>
        <option value="DE">Delaware</option>
        <option value="DC">District Of Columbia</option>
        <option value="FL">Florida</option>
        <option value="GA">Georgia</option>
        <option value="HI">Hawaii</option>
        <option value="ID">Idaho</option>
        <option value="IL">Illinois</option>
        <option value="IN">Indiana</option>
        <option value="IA">Iowa</option>
        <option value="KS">Kansas</option>
        <option value="KY">Kentucky</option>
        <option value="LA">Louisiana</option>
        <option value="ME">Maine</option>
        <option value="MD">Maryland</option>
        <option value="MA">Massachusetts</option>
        <option value="MI">Michigan</option>
        <option value="MN">Minnesota</option>
        <option value="MS">Mississippi</option>
        <option value="MO">Missouri</option>
        <option value="MT">Montana</option>
        <option value="NE">Nebraska</option>
        <option value="NV">Nevada</option>
        <option value="NH">New Hampshire</option>
        <option value="NJ">New Jersey</option>
        <option value="NM">New Mexico</option>
        <option value="NY">New York</option>
        <option value="NC">North Carolina</option>
        <option value="ND">North Dakota</option>
        <option value="OH">Ohio</option>
        <option value="OK">Oklahoma</option>
        <option value="OR">Oregon</option>
        <option value="PA">Pennsylvania</option>
        <option value="RI">Rhode Island</option>
        <option value="SC">South Carolina</option>
        <option value="SD">South Dakota</option>
        <option value="TN">Tennessee</option>
        <option value="TX">Texas</option>
        <option value="UT">Utah</option>
        <option value="VT">Vermont</option>
        <option value="VA">Virginia</option>
        <option value="WA">Washington</option>
        <option value="WV">West Virginia</option>
        <option value="WI">Wisconsin</option>
        <option value="WY">Wyoming</option>
      </select>
    </div>
    <canvas id="myChart"></canvas>
  </div>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.6.2/chart.min.js"></script>
  <script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>
  <script src="app.js"></script>
</body>

</html>

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