フォームが送信されないようにするにはどうすればよいですか？

const element = document.querySelector('form');
element.addEventListener('submit', event => {
  event.preventDefault();
  // actual logic, e.g. validate the form
  console.log('Form submission cancelled.');
});

<form>
  <button type="submit">Submit</button>
</form>

$(function() {
    var $form = $('#my-form');
    $form.removeAttr('onsubmit');
    $form.submit(function(ev) {
        // quick validation example...
        $form.children('input[type="text"]').each(function(){
            if($(this).val().length == 0) {
                alert('You are missing a field');
                ev.preventDefault();
            }
        });
    });
});

label {
    display: block;
}

#my-form > input[type="text"] {
    background: cyan;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form id="my-form" action="http://google.com" method="GET" onsubmit="return false;">
    <label>Your first name</label>
    <input type="text" name="first-name"/>
    <label>Your last name</label>
    <input type="text" name="last-name" /> <br />
    <input type="submit" />
</form>

const formToJSON = elements => [].reduce.call(elements, (data, element) => {
  data[element.name] = element.value;
  return data;

}, {});

const handleFormSubmit = event => {
    event.preventDefault();
    const data = formToJSON(form.elements);
    console.log(data);
  //  const odata = JSON.stringify(data, null, "  ");
  const jdata = JSON.stringify(data);
    console.log(jdata);

    (async () => {
      const rawResponse = await fetch('/', {
        method: 'POST',
        headers: {
          'Accept': 'application/json',
          'Content-Type': 'application/json'
        },
        body: jdata
      });
      const content = await rawResponse.json();

      console.log(content);
    })();
};

const form = document.forms['myForm']; 
form.addEventListener('submit', handleFormSubmit);

<form id="myForm" action="/" method="post" accept-charset="utf-8">
    <label>Checkbox:
        <input type="checkbox" name="checkbox" value="on">
    </label><br /><br />

    <label>Number:
        <input name="number" type="number" value="123" />
    </label><br /><br />

    <label>Password:
        <input name="password" type="password" />
    </label>
    <br /><br />

    <label for="radio">Type:
        <label for="a">A
            <input type="radio" name="radio" id="a" value="a" />
        </label>
        <label for="b">B
            <input type="radio" name="radio" id="b" value="b" checked />
        </label>
        <label for="c">C
            <input type="radio" name="radio" id="c" value="c" />
        </label>
    </label>
    <br /><br />

    <label>Textarea:
        <textarea name="text_area" rows="10" cols="50">Write something here.</textarea>
    </label>
    <br /><br />

    <label>Select:
        <select name="select">
            <option value="a">Value A</option>
            <option value="b" selected>Value B</option>
            <option value="c">Value C</option>
        </select>
    </label>
    <br /><br />

    <label>Submit:
        <input type="submit" value="Login">
    </label>
    <br /><br />


</form>