How do I change an HTML selected option using JavaScript?

Question

Change

document.getElementById('personlist').getElementsByTagName('option')[11].selected = 'selected'

to

document.getElementById('personlist').value=Person_ID;

Tools as pure JavaScript code for handling Selectbox:

Graphical Understanding:

Image - A

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Image - B

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Image - C

Updated - 25-June-2019 | Fiddler DEMO

JavaScript Code:

/**
 * Empty Select Box
 * @param eid Element ID
 * @param value text
 * @param text text
 * @author Neeraj.Singh
 */
function emptySelectBoxById(eid, value, text) {
    document.getElementById(eid).innerHTML = "<option value='" + value + "'>" + text + "</option>";
}


/**
 * Reset Select Box
 * @param eid Element ID
 */

function resetSelectBoxById(eid) {
    document.getElementById(eid).options[0].selected = 'selected';
}


/**
 * Set Select Box Selection By Index
 * @param eid Element ID
 * @param eindx Element Index
 */

function setSelectBoxByIndex(eid, eindx) {
    document.getElementById(eid).getElementsByTagName('option')[eindx].selected = 'selected';
    //or
    document.getElementById(eid).options[eindx].selected = 'selected';
}


/**
 * Set Select Box Selection By Value
 * @param eid Element ID
 * @param eval Element Index
 */
function setSelectBoxByValue(eid, eval) {
    document.getElementById(eid).value = eval;
}


/**
 * Set Select Box Selection By Text
 * @param eid Element ID
 * @param eval Element Index
 */
function setSelectBoxByText(eid, etxt) {
    var eid = document.getElementById(eid);
    for (var i = 0; i < eid.options.length; ++i) {
        if (eid.options[i].text === etxt)
            eid.options[i].selected = true;
    }
}


/**
 * Get Select Box Text By ID
 * @param eid Element ID
 * @return string
 */

function getSelectBoxText(eid) {
    return document.getElementById(eid).options[document.getElementById(eid).selectedIndex].text;
}


/**
 * Get Select Box Value By ID
 * @param eid Element ID
 * @return string
 */

function getSelectBoxValue(id) {
    return document.getElementById(id).options[document.getElementById(id).selectedIndex].value;
}

I believe that the blog post JavaScript Beginners – Select a dropdown option by value might help you.

<a href="javascript:void(0);" onclick="selectItemByValue(document.getElementById('personlist'),11)">change</a>

function selectItemByValue(elmnt, value){

  for(var i=0; i < elmnt.options.length; i++)
  {
    if(elmnt.options[i].value === value) {
      elmnt.selectedIndex = i;
      break;
    }
  }
}

mySelect.value = myValue;

Where mySelect is your selection box, and myValue is the value you want to change it to.

You could also change select.options.selectedIndex DOM attribute like this:

function selectOption(index){ 
  document.getElementById("select_id").options.selectedIndex = index;
}

<p>
<select id="select_id">
  <option selected>first option</option>
  <option>second option</option>
  <option>third option</option>
</select>
</p>
<p>
  <button onclick="selectOption(0);">Select first option</button>
  <button onclick="selectOption(1);">Select second option</button>
  <button onclick="selectOption(2);">Select third option</button>
</p>

Your own answer technically wasn't incorrect, but you got the index wrong since indexes start at 0, not 1. That's why you got the wrong selection.

document.getElementById('personlist').getElementsByTagName('option')[**10**].selected = 'selected';

Also, your answer is actually a good one for cases where the tags aren't entirely English or numeric.

If they use, for example, Asian characters, the other solutions telling you to use .value() may not always function and will just not do anything. Selecting by tag is a good way to ignore the actual text and select by the element itself.