How can I disable an in a based on its value in JavaScript?

JavaScript, in 2022

You can use querySelectorAll, and forEach off of the resulting NodeList to do this same thing more easily in 2022.

document.querySelectorAll("#foo option").forEach(opt => {
    if (opt.value == "StackOverflow") {
        opt.disabled = true;
    }
});

Do be mindful of string-comparisons, however. ‘StackOverflow’ and ‘stackoverflow’ are not the same string. As such, you can call .toLowerCase() on strings before comparing, or even go with a case-insensitive regular expression comparison like the this:

if ( /^stackoverflow$/i.test(option.value) ) {
  option.disabled = true;
}

Pure Javascript (2010)

With pure Javascript, you’d have to cycle through each option, and check the value of it individually.

// Get all options within <select id='foo'>...</select>
var op = document.getElementById("foo").getElementsByTagName("option");
for (var i = 0; i < op.length; i++) {
  // lowercase comparison for case-insensitivity
  (op[i].value.toLowerCase() == "stackoverflow") 
    ? op[i].disabled = true 
    : op[i].disabled = false ;
}

Without enabling non-targeted elements:

// Get all options within <select id='foo'>...</select>
var op = document.getElementById("foo").getElementsByTagName("option");
for (var i = 0; i < op.length; i++) {
  // lowercase comparison for case-insensitivity
  if (op[i].value.toLowerCase() == "stackoverflow") {
    op[i].disabled = true;
  }
}

###jQuery

With jQuery you can do this with a single line:

$("option[value="stackoverflow"]")
  .attr("disabled", "disabled")
  .siblings().removeAttr("disabled");

Without enabling non-targeted elements:

$("option[value="stackoverflow"]").attr("disabled", "disabled");

​
Note that this is not case insensitive. “StackOverflow” will not equal “stackoverflow”. To get a case-insensitive match, you’d have to cycle through each, converting the value to a lower case, and then check against that:

$("option").each(function(){
  if ($(this).val().toLowerCase() == "stackoverflow") {
    $(this).attr("disabled", "disabled").siblings().removeAttr("disabled");
  }
});

Without enabling non-targeted elements:

$("option").each(function(){
  if ($(this).val().toLowerCase() == "stackoverflow") {
    $(this).attr("disabled", "disabled");
  }
});