Knockout is a popular JavaScript framework that offers convenient data binding functionality without the complexity of similar frameworks such as Angular. It's a great choice for prototyping small applications or introducing data binding into a legacy codebase. This article captures a number of common Knockout pitfalls I've run into over the past few years deploying Knockout code in production. Hopefully it'll save you a headache or two!
Note: All the code in this article can be found on GitHub. Some modifications have been made to examples that contain HTML files so that you can run them in your browser, but the important parts are all the same. If you notice errors in any of the code, please ping me on twitter @fivethirtyam or make a pull request to the GitHub repo.
Unlike other UI frameworks, Knockout does not come with a built in way to modularize your application. It's possible to write an entire Knockout app in a single HTML file:
<html>
<head>
<title>My Bad App</title>
<script type="text/javascript" src="knockout.js"></script>
</head>
<body>
<span data-bind="text: message"></span>
<script type="text/javascript">
function ViewModel() {
this.message = ko.observable("This is bad!");
}
ko.applyBindings(new ViewModel());
</script>
</body>
</html>
Get expert markup helpWhile this runs, it definitely won't scale to applications of any significant complexity. The official Knockout docs suggest using RequireJS for Asynchronous Module Definition (AMD), and I wholeheartedly second this recommendation. Using RequireJS and the domReady module, the above code refactors into three separate files.
Good.html:
<html>
<head>
<title>My Good App</title>
<script type="text/javascript" data-main="main.js" src="require.js"></script>
</head>
<body>
<span data-bind="text: message"></span>
</body>
</html>
Get expert markup helpmain.js:
require(['knockout', 'viewModel', 'domReady!'], function(ko, viewModel) {
ko.applyBindings(new viewModel());
});
Get expert javascript helpviewModel.js:
define(['knockout'], function(ko) {
return function viewModel() {
this.message = ko.observable("This is good!");
}
});
Get expert javascript helpvar self = this;Keeping track of this in Knockout can be tough. Setting var self = this; at the top of every view model makes it easy.
For example, if var self = this; is not set then this must be passed into every computed observable:
function viewModel() {
this.cat = ko.observable("cat");
this.pants = ko.observable("pants");
this.catpants = ko.computed(function() {
return this.cat() + this.pants();
}, this);
}
Get expert javascript helpForgetting to pass in this is easy to do and can result in hard to track down bugs. If var self = this; is set then passing this is not necessary:
function viewModel() {
var self = this;
self.cat = ko.observable("cat");
self.pants = ko.observable("pants");
self.catpants = ko.computed(function() {
return self.cat() + self.pants();
});
}
Get expert javascript helpvar self = this; can also help avoid bugs when one binding context refers to another. Consider the following code that displays a list of names. When a user clicks on an name in the list, that name is displayed in the <span> below.
HTML:
<ul data-bind="foreach: people">
<li data-bind="click: $parent.selectPerson, text: name"></li>
</ul>
Get expert markup helpJavaScript:
function viewModel() {
var self = this;
self.people = [
{
name: "Mike"
},
{
name: "Sara"
},
...
];
self.selectedPerson = ko.observable();
self.selectPerson = function() {
Get expert javascript helpWhen selectPerson is called its scope is set to the object in person corresponding to the name clicked. However, because var self = this; is set the view model object is still accessible within the event callback. This would not be the case otherwise.
In production code I almost always define var self = this;. However, for the sake of brevity, I won't use it in some examples in this article.
Knockout makes it easy to write complicated logic in HTML templates. Avoid doing this at all costs! It results in difficult to test and maintain code. Instead of using logic in templates...
<span data-bind="text: firstName() + ' ' + lastName()"></span>
Get expert markup help function viewModel() {
this.firstName = ko.observable("Mike");
this.lastName = ko.observable("Mellenthin");
};
Get expert javascript help...push all logic into computed observables:
<span data-bind="text: fullName"></span>
Get expert markup help function viewModel() {
var self = this;
self.firstName = ko.observable("Mike");
self.lastName = ko.observable("Mellenthin");
self.fullName = ko.computed(function() {
return self.firstName() + ' ' + self.lastName();
});
};
Get expert javascript helpSpeaking of testing, by default Knockout offers no tools to help you. This does not mean you shouldn't write tests! There are ample JS testing frameworks, and you should be able to pick up and drop any of them into your Knockout application. As with anything, the earlier you start writing tests the better. I usually don't start a new Knockout project without also including a testing framework.
ko.mapping is a plugin for Knockout that makes working with data fetched from REST endpoints much more enjoyable.
Without ko.mapping, a view model that consumes objects from a REST API might look like this:
function viewModel() {
Get expert javascript helpWith ko.mapping the view model is much simpler:
function viewModel() {
var person = getPersonFromServer();
this.person = ko.mapping.fromJS(person);
}
Get expert javascript helpWant to convert part of the view model back to plain JSON so that you can POST it back to the API? That's a one-liner too:
var json = ko.mapping.toJS(this.person);
Get expert javascript helpWhile ko.mapping is to some degree officially part of Knockout (it's in the official documentation), it is a separate project that you'll have to download and include.
It's also worth nothing that some people prefer the functionally analogous ko.viewModel plugin to ko.mapping. ko.mapping has always worked fine for me, but it only seemed fair to link to both. Use one or the other, but don't use neither!
This may seem minor, but it's caused me so many headaches that it's worth including. Knockout binds the value of input elements a somewhat strange way. To illustrate, let's say we have an input element and want to update some text on the page in real time as the input's value changes:
<input type="text" data-bind="value: myValue">
Value: <span data-bind="text: myValue"></span>
Get expert markup help function viewModel() {
this.myValue = ko.observable();
}
Get expert javascript helpSurprisingly, the above won't work. The myValue observable (and thus our text in the span) updates whenever the input loses or gains focus, not as it's value changes. Why? For performance reasons this is how Knockout behaves by default.
To fix this in Knockout 3.2 (current at the time of writing) or newer, use the textInput binding instead of the value binding.
<input type="text" data-bind="textInput: myValue">
Get expert markup helpThis will provide immediate updates to myValue with the minor caveat that it won't guarantee that the value attribute of the input is always synced with myValue.
If you are using an older version of Knockout or if you are dead set on using the value binding, then you must use the valueUpdate flag.
<input type="text" data-bind="value: myValue, valueUpdate='afterkeydown'">
Get expert markup helpIn Knockout 3.2 there are 4 possible values for valueUpdate, each with its own idiosyncrasies:
input - Updates when the value of the element changes. Doesn't work in IE8-.keyup - Updates when a keyup event is firedkeypress - Updates when a keypress event is fired.afterkeydown - Updates as soon as the user starts typing a character. Does not work on all mobile browsers -- notably Safari on IOS7.Using the textInput binding is strongly recommended over using the valueUpdate flag.
Pushing multiple times into observable arrays can cause significant performance issues with your application:
function viewModel() {
var arr = [0, 1, ..., 999];
this.numbers = ko.observableArray();
for (var i = 0; i < arr.length; i++) {
this.numbers.push(arr[i]);
}
}
Get expert javascript helpRunning the for loop above will cause Knockout to redraw the page 1000 times -- one for each push. To avoid this, simply overwrite the old value of our entire array:
function viewModel() {
var arr = [0, 1, ..., 999];
this.numbers = ko.observableArray(arr);
}
Get expert javascript helpTo transform data before putting it into our new array, use ko.utils.arrayMap.
function Number(number) {
this.number = number;
}
function viewModel() {
var arr = [0,1, ..., 999];
Get expert javascript helpTo append many items to an existing observable array, exploit the fact that push accepts a variable number of arguments and use apply to push them in all at once.
function viewModel() {
var arr = [500, 501, ... 999];
this.numbers = ko.observableArray([0, 1, ..., 499]);
this.numbers.push.apply(self.numbers, arr);
}
Get expert javascript helpObservable arrays track changes to which objects are in the array. They do not track changes to the state of those objects.
<ul data-bind="foreach: people">
<li data-bind="text: name"></li>
</ul>
<button data-bind="click: makeDerekMike">Make Derek Mike</button>
Get expert markup help function viewModel() {
var self = this;
self.people = ko.observableArray([
{
name: 'Derek',
},
{
name: 'Sara'
},
...
]);
self.makeDerekMike = function() {
for (var i = 0; i < self.people().length; i++) {
if (self.people()[i].name === 'Derek') {
self.people()[i].name = 'Mike';
}
}
};
}
Get expert javascript helpClicking the button in the first example will not trigger a page refresh despite the fact that it will modify an object in the array underlying people. This is because the same objects still belong topeople. To trigger a page refresh, we need to explicitly make the fields of the elements in people observable:
function viewModel() {
var self = this;
self.people = ko.observableArray([
{
name: ko.observable('Derek')
},
{
name: ko.observable('Sara')
},
...
]);
self.makeDerekMike = function() {
for (var i = 0; i < self.people().length; i++) {
if (self.people()[i].name() === 'Derek') {
self.people()[i].name('Mike');
}
}
};
}
Get expert javascript helpBecause Knockout makes building interactive widgets so simple, it's easy to write similar code in different parts of your application. Use Knockout components to abstract common UI widgets and promote code reuse.
Using components, we can build a simple reusable list builder as follows:
<span>This is a list with no initial values</span>
<div data-bind="component: 'list-builder'"></div>
<span>This is a list with some initial values</span>
<div data-bind="component: {
name: 'list-builder',
params: { list: ['item', 'another item'] }
}"></div>
Get expert markup help function Item(text) {
this.text = text;
}
ko.components.register('list-builder', {
viewModel: function(params) {
var self = this;
self.list = ko.observableArray([]);
if (params && params.list) {
self.list(ko.utils.arrayMap(params.list, function(item) {
return new Item(text);
}));
}
self.newText = ko.observable();
self.add = function() {
self.list.push(new Item(text));
self.newText('');
};
},
template:
'<ul data-bind="foreach: list">' +
'<li data-bind="text: text"></li>' +
'</ul>' +
'<input type="text" data-bind="textInput: newText" />' +
'<button data-bind="click: add">Add</button>'
});
Get expert javascript helpNote that you probably don't want to inline the viewModel and template as above in any production code code. Fortunately, this can be solved using require.js (see #1). For a description of how to do this check out the official components documentation.
This sounds like a cop out given that I just wrote a Knockout guide of sorts, but if you're new to Knockout the official tutorials are great! They offer a interactive sandbox to play around with the framework in a guided fashion. They helped me a ton when I was first starting out and hopefully they'll help you too. And if you're still confused, well, there's always AirPair.
Read more at http://www.airpair.com/knockout/posts/top-10-mistakes-knockoutjs#c3KVHg1kRSrz4dbX.99
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