"Repeatedly encountering 'undefined' when subscribing to the selected option of a select list in Knockout.js

In my dropdown list, I have prepopulated a selection of strings. My select list is bound as follows:

<select id="industryDropDown"
    data-bind="options: $root.orgIndustrySuggestions, value: $root.selectedIndustryTag, optionsCaption: ''"></select>

Within my viewmodel, I have the following code snippet:

self.selectedIndustryTag.subscribe(function (newValue) {
    if (newValue !== '') {
        if (newValue !== 'undefined') {
            alert(self.selectedIndustryTag());
            self.selectedIndustryTag('');
        }
    }
}, self);

The goal is to retrieve the selected value from the drop-down list, perform an action at the current location of the alert(), and then reset the selected value back to the default empty string option.

Currently, when this code is executed, the initial page load works fine. However, upon selecting an item, it displays the selected value once correctly but then repeats showing 'undefined' multiple times afterward.

Answer №1

Was it really a gazillion, or more like a zillion?

You may be experiencing this issue because you are stuck in an endless loop. Every time there is a change to selectedIndustryTag in the user interface, it triggers a subscription. Within the subscription, you then make another change to selectedIndustryTag, triggering a new subscription, and so on.

To solve this problem, consider having two properties: one that is edited by the UI and subscribed to, and a second property that is used internally (and can be updated by the subscription).

Additionally, you are using the strict non-equality operator !== but comparing it to the string 'undefined'. This will never evaluate correctly, as you should be comparing against undefined without quotes.

Therefore, update this line of code:

if (newValue !== 'undefined')

to:

if (newValue !== undefined)

For a live example, check out: http://jsfiddle.net/2NqFY/

Answer №2

Your approach to the problem is being overly complex. If you provide a code example in a fiddle, we can help identify where things are going wrong. It seems like you may have a recursive loop (subscribing to a value and setting it within the same function) and unnecessary multiple tests

self.selectedIndustryTag.subscribe(function (newValue) {
    if (newValue) {
        alert(self.selectedIndustryTag());
         // The next line will refire the subscription
         //self.selectedIndustryTag('');
    }
}, self);

The condition - if (newValue) checks for null, undefined, false, or an empty string ('')

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