Version 11 (modified by Brian Burg, 9 years ago) (diff)

i promise to document promises

These are JavaScript coding styles used in the Source/WebInspectorUI/UserInterface folder.

(Note: Few if any of these guidelines are checked by check-webkit-style. There's a tracking bug for that:

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 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 `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

What's so great about 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:

Become fulfilled by a value 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 Blog and 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. 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()

    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;

    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 */