These are JavaScript coding styles used in the [https://trac.webkit.org/browser/trunk/Source/WebInspectorUI/ Source/WebInspectorUI/UserInterface] folder.

(Note: Few if any of these guidelines are checked by `check-webkit-style`. There's a tracking bug for that: [https://bugs.webkit.org/show_bug.cgi?id=125045])

== Tokens, spacing, indentation, syntax

* No trailing whitespace
* Indent with 4 spaces
* The opening bracket `'{'` after a named, non-inlined function goes on the next line. Anywhere else, the opening bracket `'{'` stays on the same line.
* Style for object literals is: `{key1: value1, key2: value2}`.
* Add new lines before and after different tasks performed in the same function.
* Else-blocks and Promise `.then()` blocks should share a line with leading } or }).
* Calling a constructor with no arguments should have no parenthesis `'()'`. eg. `var map = new Map;`
* Inline anonymous functions, especially if they don't need a bound `this`-object (using `.bind()`). Example:
{{{
    this.requestRootDOMNode(function(rootNode) {
        ...
    });
}}}

== Naming things

* Avoid using the "on" prefix where possible. The `_onFoo` methods can just be `_foo` or `_handleFoo`.
* New class names should use the name of the base class as a suffix. (ex: `TimelinesContentView` < `ContentView`). Exceptions: classes extending `WebInspector.Object` (unless they are a represented object), and deep hierarchies such as `DebuggerSidebarPanel` < `NavigationSidebarPanel` < `SidebarPanel` < `Object`.
* Spell out `identifier` instead of `id` if not doing so would result in a name ending with capitalized `Id`. For example, just `this.id` is fine, but `this.breakpointId` should be `this.breakpointIdentifier`.
* An object's events live on the `Event` property of the constructor. Event names are properties on the `Event` object, and property values duplicate the event name, but are lowercased, hyphenated, and prefixed with the constructor name. See the skeleton example below.

== API preferences

* Consider using [http://people.mozilla.org/~jorendorff/es6-draft.html#sec-keyed-collection `Map` and `Set` collections] instead of plain objects.
* Consider using `rgb()` over hex colors in CSS
* Use `for..of` syntax when performing small actions on each element. Use `forEach` when the function body is longer. Use a classical for loop when doing index math.
* When using `forEach` or `map`, supply the `this`-object as the optional second parameter rather than binding it.


== Layering and abstractions

* Firewall the protocol inside the Manager classes. JSON objects received from the protocol are called "payload" in the code. The payload is usually deconstructed at the Managers level and passes down as smart objects inheriting from `WebInspector.Object`.
* Avoid accessing *View classes from *Manager or *Object classes. This is a layering violation that prevents writing tests for models.
* Avoid storing DOM elements in *Manager or *Object classes. (see above.)
* Avoid using Inspector TypeBuilders outside of InspectorAgent classes. We want to isolate protocol considerations from other functionality in JavaScriptCore and WebCore.

== Understanding and Using Promises

[http://blog.parse.com/2013/01/29/whats-so-great-about-javascript-promises/ What's so great about Promises?] [http://domenic.me/2012/10/14/youre-missing-the-point-of-promises/ The point of promises is to give us back functional composition] and error bubbling in the async world. They do this by saying that your functions should return a promise, which can do one of two things:

1. Become __fulfilled__ by a **value**
2. Become __rejected__ with an **exception**

And, if you have a correctly implemented `then()` function, then fulfillment and rejection will compose just like their synchronous counterparts, with fulfillments flowing up a compositional chain, but being interrupted at any time by a rejection that is only handled by someone who declares they are ready to handle it.

=== Promise Gotchas

(Summarized from [http://making.change.org/post/69613524472/promises-and-error-handling change.org Blog] and [http://taoofcode.net/promise-anti-patterns/ The Art of Code Blog])

* Don't nest promises to perform multiple async operations; instead, chain them or use `Promise.all()`.
* Beware of storing or returning promise values that are not from the end of a chain. Each `.then()` returns a new promise value, so return the last promise.
* Use `Promise.all()` with `map()` to process an array of asynchronous work in parallel. Use `Promise.all()` with `reduce()` to sequence an array asynchronous work.
* If a result may be a promise or an actual value, wrap the value in a promise, e.g., `Promise.resolve(val)`
* Use `.catch()` at the end of a chain to perform error handling.
* To reject a promise, throw an `Error` instance or call the `reject` callback with an `Error` instance.
* A `.catch()` block is considered resolved if it does not re-throw an `Error` instance. Re-throw if you want to log an error message and allow other parts of a chain (i.e, an API client) to handle an error condition.
* Don't directly pass a promise's `resolve` function to `Object.addEventListener`, as it will leak the promise if the event never fires. Instead, use a single-fire `WebInspector.EventListener` object defined outside of the promise chain and connect it inside a `.then()` body. Inside the `.catch` block, disconnect the `EventListener` if necessary.

== New class skeleton

The new Inspector object classes should have the following format:

{{{
WebInspector.NewObjectType = function()
{
    WebInspector.Object.call(this);

    this._propertyName = ...;
}

WebInspector.NewObjectType.Event = {
    PropertyWasChanged: "new-object-type-property-was-changed"
};

WebInspector.NewObjectType.prototype = {
    constructor: WebInspector.NewObjectType,
    __proto__: WebInspector.Object.prototype,

    // Public

    get propertyName()
    {
        return this._propertyName;
    },

    set propertyName(value)
    {
        this._propertyName = value;
        this.dispatchEventToListeners(WebInspector.NewObjectType.Event.PropertyWasChanged);
    },

    publicMethod: function()
    {
        /* public methods called outside the class */
    }

    // Protected

    handleEvent: function(event)
    {
        /* delegate methods, event handlers, and overrides. */
    },

    // Private

    _privateMethod: function()
    {
        /* private methods are underscore prefixed */
    }
};
}}}