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CSS

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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

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HTML 

              
                <div class="container mt-3">
  <header class="clearfix">
    <h1 class='d-inline-block'>Fetch Example</h1>
    <a href="https://time2hack.com/2017/11/goodbye-xmlhttprequest-ajax-with-fetch-api-demo/" target="_blank" class="btn btn-info float-right">Read full post</a>
  </header>
  <hr/>
  <div class="row">
  <div class="col-sm-8">
    <div class="row">
      <div class="col-sm-12"  data-param="false">
        <div class="form-group">
          <label for="endpoint">Endpoint</label>
          <input type="text"  value="https://jsonplaceholder.typicode.com/posts" class="form-control" id="endpoint" >
        </div>
      </div>        
      <div data-param="true" class="col-sm-12 d-none">
        <div class="row">
          <div class="col-sm-9">
            <div class="form-group">
              <label for="endpoint">Endpoint</label>
              <input type="text" readonly value="https://jsonplaceholder.typicode.com/posts" class="form-control" id="endpoint" >
            </div>
          </div>
          <div class="col-sm-3">
            <div class="form-group">
              <label for="param">Param (postId)</label>
              <input type="text" readonly value="10" class="form-control" id="param" >
            </div>
          </div>        
        </div>
      </div>
    </div>
  </div>
  <div class="col-sm-4">
    <div class="form-group">
      <label for="method">Method</label>
      <select class="form-control" id="method">
        <option value="GET">GET</option>
        <option value="POST" data-body selected>POST</option>
        <option value="PUT" data-body data-param>PUT</option>
        <option value="DELETE" data-param>DELETE</option>
      </select>
    </div>  
  </div>
  </div>
  <div class="row">
    <div class="col-md-6"></div>
    <div class="col-md-6"></div>
  </div>
  <div class="form-group fields-group">
    <label >Fields: <button type="button" id="add" class="btn btn-info btn-sm">Add Field</button></label>
    <table id="fields" class="table table-bordered table-sm">
      <thead>
        <tr>
          <th>Key</th>
          <th>Value</th>
          <th>Remove</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td><input type="text" class="form-control key" placeholder="Key"></td>
          <td><input type="text" class="form-control value" placeholder="Value" /></td>
          <td>
            <button type="button" class="btn btn-outline-danger remove">Remove</button>
          </td>
        </tr>
      </tbody>
    </table>
    <small class="form-text text-muted">Empty keys won't be sent</small>
  </div>
  <div>
    <button type="button" class="btn btn-outline-primary btn-lg" id="send">Send</button>
    <span class="d-none" id="status">
      <svg width="24" height="24" viewBox="0 0 24 24">
  <path d="M19,8L15,12H18A6,6 0 0,1 12,18C11,18 10.03,17.75 9.2,17.3L7.74,18.76C8.97,19.54 10.43,20 12,20A8,8 0 0,0 20,12H23M6,12A6,6 0 0,1 12,6C13,6 13.97,6.25 14.8,6.7L16.26,5.24C15.03,4.46 13.57,4 12,4A8,8 0 0,0 4,12H1L5,16L9,12"></path>
</svg>
    </span>
  </div>
  <hr/>
  <div class="response">
    <div class="row">
      <div class="col-sm-8">
        <h4>Response:</h4>
        <pre id="result" class="border border-primary rounded"></pre>        
      </div>
      <div class="col-sm-4">
        <h4>Request:</h4>
        <pre id="request" class="border border-primary rounded"></pre>
      </div>
    </div>
  </div>
</div>
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CSS 

              
                #result, #request {
  padding: 10px;
  max-height: 500px;
  overflow: auto;
}
header {
  padding-top: 10px; 
}
#status svg{
    animation: spin infinite 2s linear;
}
@keyframes spin {
  from {
    transform: rotate(360deg);
  }
  to {
    transform: rotate(0deg);
  }
}
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JS 

              
                const input = name => `<input type="text"
  class="form-control ${name.toLowerCase()}" placeholder="${name}" />`;

const fieldTpl = data => `
<tr>
  <td>${input("Key")}</td>
  <td>${input("Value")}</td>
  <td><button type="button" class="btn btn-outline-danger remove">Remove</button></td>
</tr>
`;
document.querySelector("#add").addEventListener("click", e => {
  $append(fieldTpl(), document.querySelector("#fields tbody"));
});

document.querySelector("#method").addEventListener("change", e => {
  const val = document.querySelector("#method").value;
  const option = document.querySelector(`#method option[value="${val}"]`);
  const param = option.dataset.param !== undefined;
  
  document
    .querySelector(".fields-group")
    .classList[option.dataset.body === undefined ? "add" : "remove"]("d-none");
  console.log(param, document.querySelector(`[data-param='${param}']`));
  document.querySelector(`[data-param='${param}']`).classList.remove('d-none')
  document.querySelector(`[data-param='${!param}']`).classList.add('d-none')
});

document.querySelector("#fields").addEventListener("click", e => {
  if (e.target.classList.contains("remove")) {
    e.target.parentElement.parentElement.remove();
  }
});

const getKeyValues = () => {
  const keys = document.querySelectorAll('.key');
  const values = document.querySelectorAll('.value');
  let obj = {};
  keys.forEach((key, index) => {
    if(key.value !== '') {
      obj[key.value] = values[index].value;
    }
  });
  return obj;
}

document.querySelector("#send").addEventListener("click", e => {
  document.querySelector("#send").classList.add('d-none')
  document.querySelector("#status").classList.remove('d-none')
  const suffix = {
    GET: '',
    POST: '',
    PUT: '/10',
    DELETE: '/10',
  };
  const method = document.querySelector("#method").value;
  const url = `https://jsonplaceholder.typicode.com/posts${suffix[method]}`;
  let reqConf = {method};
  if(method === 'POST' || method === 'PUT') {
    const headers = new Headers();
    headers.append("Content-Type", "application/json");
    let body = getKeyValues();
    
    reqConf = Object.assign({}, {
      headers, 
      body: JSON.stringify(body),
    }, reqConf);
  }
  const request = new Request(url, reqConf)
  document.querySelector('#request').innerHTML = JSON.stringify(request, replacer, 2);
  fetch(request)
    .then(response => response.json())
    .then(data => {
      document.querySelector("#send").classList.remove('d-none')
      document.querySelector("#status").classList.add('d-none')
      document.querySelector('#result').innerHTML = JSON.stringify(data, replacer, 2);
  });
});

const $append = (markup, parent) => {
  const temp_container = document.createElement('tbody');
  temp_container.innerHTML = markup;
  while (temp_container.firstChild) {
    parent.appendChild(temp_container.firstChild);
  }
};

function replacer(key, value) {
  // Filtering out properties
  if (value instanceof Request) {
    return ['method', 'url', 'headers', 'referrer', 'mode', 'body'].reduce((acc, key) => {
      acc[key] = replacer(key, value[key]);
      return acc;
    }, {});
  }
  if (value instanceof Headers) {
    const i = value.entries();
    const ret = {};
    let v = i.next();
    while(!v.done) {
      ret[v.value[0]] = v.value[1]
      v = i.next();
    }
    return ret;
  }
  return value;
}
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