在多选中实现独占选项:选择时清除其他条目,选择其他时清除

这里也有类似的线索, Mutual exclusion for <option>s in a <select>? 。我们需要跟踪 现在的 选择数组并用 最近的 选择数组。这将告诉我们 只是 已选择。

解决方案:

// Global Var:
// To implement an exclusive option in a MultiSelect dropdown,
// we need to keep track of the current selection (a global var here) 
// and diff this array with the latest selection array in change() event.
var currSel = $('#educationLevel').val();

// Change Event Definition
$('#educationLevel').on('change', function() {
    var newSel = $(this).val();
    // Get diff from currSel; a 1-element array expected.
    // Diff in arrays: see: https://stackoverflow.com/a/33034768/1005607
    var diff = newSel.filter(x => !currSel.includes(x));
    // If exclusive option just got selected, delete all others
    if (diff.length) {
        if (diff[0] === '405') { // '405' is my exclusive option
           // If exclusive option just got selected ("405"), deselect all others 
           // in modified selection array
           newSel = ['405'];
        } else {
           // If non-exclusive option just got selected, ensure that the exclusive option 
           // is not in the modified selection array
           newSel = newSel.filter(el => el !== '405');
        }
        // Set dropdown to the modified new selection array
        $('#educationLevel').val(newSel);
    }
    // Clone latest selection array into current selection array
    currSel = [...newSel];
}