Implementing document.getElementById in javascript

Question

I'm trying to implement native document.getElementById in javascript. I've implemented document.getElementsByClassName in javascript.

function getElementsByClassName (className) {
  var nodeList = [];
  function test(node) {
      if (node.classList && node.classList.contains(className)) {
        nodeList.push(node);
      }
      
      for (var index = 0; index < node.childNodes.length; index++) {
        test(node.childNodes[index]);
      }
      
      return nodeList;
  }
  
    test(document.body);
    
  return nodeList;
};

// Fails here.
function getElementById(className) {
    const result = [];
    
    function getEachIDNode(node) {
        if(node.contains(className)) {
            return node;
        }

        for(let i=0; i<node.childNodes.length; i++) {
            getEachIDNode(node.childNodes[i]);
        }

    }

    getEachIDNode(document.body);
}

console.log(getElementsByClassName('winner'));
console.log(getElementById('test'));

  <table>      
        <tr id="test">
            <td>#</td>
            <td class="winner">aa</td>
            <td>bb</td>
            <td>cc</td>
            <td>dd</td>
        </tr>
   </table>

   <table>      
        <tr>
            <td>#</td>
            <td class="winner">aa</td>
            <td>bb</td>
            <td>cc</td>
            <td>dd</td>
        </tr>
   </table>

   <table>      
        <tr>
            <td>#</td>
            <td class="winner">dd</td>
            <td>cc</td>
            <td>bb</td>
            <td>aa</td>
        </tr>
   </table>

I'm trying to understand how I can check if a node has an attribute ID.

Can someone enlighten me?

Answer 1

Check the id property of the node against the passed argument (probably better to use id as an argument rather than className):

function getElementById(id) {
    const result = [];

    function getEachIDNode(node) {
        if(node.id === id) {
            result.push(node);
        }
        for(let i=0; i<node.childNodes.length; i++) {
            getEachIDNode(node.childNodes[i]);
        }
    }
    getEachIDNode(document.body);
    return result;
}
console.log(getElementById('subchild')[0].innerHTML);

<div id="parent">
  <div id="child1">
  </div>
  <div id="child2">
    <div id="subchild">
      subchild!
    </div>
  </div>
</div>

But if you actually want to replicate getElementById, don't try to return an array, return a single element :

function getElementById(id) {
  let match = null;
  const doFind = node => {
    if (!match && node.id === id) match = node;
    if (!match) return [...node.childNodes].find(doFind);
  }
  doFind(document.body);
  return match;
}
console.log(getElementById('subchild').innerHTML);

<div id="parent">
  <div id="child1">
  </div>
  <div id="child2">
    <div id="subchild">
      subchild!
    </div>
  </div>
</div>