Knockout - write a value to a ko.computed

Question

I've built a very data/number heavy app in Knockout. I'm currently getting the error:

Uncaught Error: Cannot write a value to a ko.computed unless you specify a 'write' option. If you wish to read the current value, don't pass any parameters.

This is happening when my custom bindingHandler (which formats the numbers into 'large' form, ie. 123,345,678,987) tries to write back to the original input which displays the value of a computed function.

Value displayed in an input element:

self.value = ko.computed(function(){
    return self.chosenAge().population; // 'fetched' from an array.
});

Binding Handler:

ko.bindingHandlers.largeNumber = {
    init: function(element, valueAccessor) {
        numberInit(element);
        var value = valueAccessor();
        var interceptor = ko.computed({
            read: function() {
                // inject number formatting
                return numeral(ko.unwrap(value)).format('0,0');
            },
            write: function(newValue) {
                // remove formatting when writing a new value
                value(numeral().unformat(newValue));
            }
        });
        // display new value in target element
        if(element.tagName.toLowerCase() == 'input' ) {
            ko.applyBindingsToNode(element, {
                value: interceptor
            });
        }
        else {
            ko.applyBindingsToNode(element, {
                text: interceptor
            }); 
        }
    }
};

Solution

You need to specify a 'write' option in your ko.computed function. Please see the documentation on computed observables. Your binding handler has nothing to do with your value failing to update. Your computed should look something like this:

self.value = ko.computed({
    read: function () {
        return self.chosenAge().population; // 'fetched' from an array.
    },
    write: function (value) {
        //update your self.chosenAge().population value here
    },
    owner: self
});

Hope this helps.

Comments

[leaksterrr]

Yeah I tried this originally, the issue I'm having is with writing it back? Not sure how to go about it

[Zack Huber]

Without some more information about your chosenAge object I can't really give you a good answer, could you update your post with more information on it?

[Hp93]

I think your code is incorrect, inside ko.computed should be an object, not a function. Also the link is outdated, the new link is knockoutjs.com/documentation/computed-writable.html

[Julio Marins]

this is not even proper javascript