CODEWITHSUNDEEP
javascriptknockout.js
Jimmy Breck-McKye
Jimmy Breck-McKye

Reputation: 3034

Knockout: can observable extenders be called without parameters?

In Knockout.js, the typical syntax for invoking an observable extender is as follows:

this.myObservable = ko.observable('foo').extend({myExtenderKey: myExtenderOption});

However, what if I have an extender that doesn't take a parameter? A simple logger, for instance, might not need any arguments. Nor would an auto-updating 'hasChanged' flag.

Most examples in the wild just pass an unused boolean:

this.myObservable = ko.observable('foo').extend({logsChanges : true});

This looks nice because a naive reader will guess that a logsChanges behaviour is being activated. But the boolean is misleading, because the following will also work:

this.myObservable = ko.observable('foo').extend({logsChanges : false});

We could also pass a null value, but it seems a little redundant:

this.myObservable = ko.observable('foo').extend({logsChanges : null});

It also implies that a parameter might be sent to the logsChanges extender, even when it should never be.

Is there a way of calling an extender that doesn't include the extra parameter?

Upvotes: 3

Views: 923

Answers (1)

Sean Vieira
Sean Vieira

Reputation: 159855

Looking at the source for extend it does not appear to be the case right now. Baring a pull request, that's just how things work.

However, what you could do is update your extender to actually honor the boolean:

function logsChanges(target, shouldLog) {
  if (!shouldLog) return target;
  // Alternatively, you could return `false`
  // but returning `target` is more explicit
  return function loggingHandler() {
    // Log changes and update here.
  };
}

Upvotes: 1

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