Version 24 (modified by 8 years ago) ( diff ) | ,
---|
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: https://bugs.webkit.org/show_bug.cgi?id=125045)
Tokens, spacing, indentation, syntax
- No trailing whitespace.
- Indent with 4 spaces.
- Double quoted strings; use template strings if a bunch of interpolation is required.
- 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}
. When key and variable names coincide, use the syntax {expression} rather than {expression: expression}. - Add new lines before and after different tasks performed in the same function.
- Else-blocks should share a line with leading } or }).
- Long promise chains should place
.then()
blocks on a new line. - Calling a constructor with no arguments should have no parenthesis
'()'
. eg.var map = new Map;
- Put anonymous functions inline if they are not used as a subroutine.
- Use arrow functions when possible, unless it makes code less readable. See below for examples.
- For default parameters, add a space around the default assignment:
function foo(isGood = false)
- Trivial public getters can be made a single line and moved to the top of the list of getters in a class.
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 extendingWebInspector.Object
(unless they are a represented object), and deep hierarchies such asDebuggerSidebarPanel
<NavigationSidebarPanel
<SidebarPanel
<Object
. - Spell out
identifier
instead ofid
if not doing so would result in a name ending with capitalizedId
. For example, justthis.id
is fine, butthis.breakpointId
should bethis.breakpointIdentifier
. - An object's events live on the
Event
property of the constructor. Event names are properties on theEvent
object, and property values duplicate the event name, but are lowercased, hyphenated, and prefixed with the constructor name. See the skeleton example below. - When serializing a function to be evaluated in a different execution context, such as from inspector to inspected page or layout test to inspector, make it obvious where the function is going to be evaluated. For example, if a function will be sent from Inspector context to inspected page context, the name
inspectedPage_node_getFlowInfo()
signifies that the function will be evaluated in the inspected page, withthis
bound to a node, and it performs the actiongetFlowInfo
.
API preferences
- Use `Map` and `Set` collections instead of plain objects if the key values are unknown or not monotonic (i.e., frequently added then removed).
- Use `hsla()' over hex or RGB colors in CSS.
- Use
for..of
syntax when performing actions on each element. UseforEach
when chaining methods in a functional style. Use a classical for loop when doing index math. - When using
forEach
ormap
, supply thethis
-object as the optional second parameter rather than binding it. - In promise chains, use arrow functions for lexical
this
, rather than assigningconst instance = this;' or
.binding every function's
this`-argument. - Use destructuring assignment when digging values out of a JSON object or "args" object.
- Use default parameters when it makes sense.
- Use
super
to make calls to base class (possibly overridden) methods.
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.)
- In the backend, 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 Error instance or by throwing an exception
A promise that is eiher fulfilled or rejected is said to be settled. A promise that has not settled is said to be pending.
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 change.org 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()
withmap()
to process an array of asynchronous work in parallel. UsePromise.all()
withreduce()
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. Most promise chains should have a catch block to avoid dropping errors. - To reject a promise, throw an
Error
instance or call thereject
callback with anError
instance. - A
.catch()
block is considered resolved if it does not re-throw anError
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 toObject.addEventListener
, as it will leak the promise if the event never fires. Instead, use a single-fireWebInspector.EventListener
object defined outside of the promise chain and connect it inside a.then()
body. Inside the.catch
block, disconnect theEventListener
if necessary. - For APIs that return promises, document what the fulfilled value will be, if any. Example:
createSession() // --> (sessionId)
Arrow Functions
Arrow functions simplify a common use of anonymous functions by providing a shorter syntax, lexical binding of this
and arguments
, and implicit return. While this new syntax enables new levels of terse code, we must take care to keep our code readable.
Implicit return
Arrow functions with one expression have an implicit return. All of these are equivalent (modulo this
binding, arguments, constructor usage, etc.):
1 let foo = val => val; 2 let foo = (val) => val 3 let foo = (val) => val; 4 let foo = (val) => { return value++; } 5 let foo = function doStuff(val) { return value++; }
Never use option (1), because it is a special case that only applies when the function has one argument, reducing predictability.
In cases where the return value is used and the expression is a constant ("foo"), a variable (foo), or a member (this.foo), use option (2). Never use braces though, because implicit return only works if there are no braces around the single expression.
In cases where the expression computes a value (a + 42) or performs a side effect (++a), prefer option (4). In some sense, curly braces are a signpost to the effect of "careful, we do actual work here".
GOOD:
setTimeout(() => { testRunner.notifyDone(); }, 0)
BAD:
setTimeout(() => { testRunner.notifyDone() }, 0); // return value not implicitly returned
setTimeout(() => testRunner.notifyDone(), 0); // implicit return value not used
When not to arrow
When assigning a function to a subclass prototype (in the old way of setting up classes), always use the normal function syntax, to avoid breaking subclasses who use a different 'this' binding. Note that arrow functions are NOT acceptable for assigning functions to singleton objects like WebInspector, since the captured lexical this
is typically the global object.
GOOD:
Base.prototype.compute = function(a, b, c) { ... } Foo.prototype.compute = function(a, b, c) { Base.prototype.compute.call(this, a, b, c); } WebInspector.UIString = function(format, args) { ... }
BAD:
Base.prototype.compute = (a, b, c) => { ... } Foo.prototype.compute = (a, b, c) => { Base.prototype.compute.call(this, a, b, c); } WebInspector.UIString = (format, args) => { ... } // this will be window.
Also use the normal function syntax when naming an anonymous function improves readability of the code. In this case, use Function.prototype.bind or assign the arrow function into a local variable first.
GOOD:
Promise.resolve() .then(function resolved(value) { ... }, function rejected(value) { ... });
BAD:
Promise.resolve() .then((value) => { ... }, (value) => { ... })
New class skeleton
New Inspector object classes use ES6 class syntax and should have the following format:
WebInspector.NewObjectType = class NewObjectType extends WebInspector.Object { constructor(type, param) { super(); console.assert(param instanceof WebInspector.ExpectedType); this._type = type; this._propertyName = param; } // Static static computeBestWidth(things) { .... return 3.14159; } // Public get type() { return this._type; } 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 */ } }; WebInspector.NewObjectType.Event = { PropertyWasChanged: "new-object-type-property-was-changed" };
CSS
z-index
Z-index variables are defined in Variables.css. Usage example:
.popover { z-index: var(--z-index-popover); }
Read more about the rationale in Bug 151978.