Unless stated below, we follow the conventions listed in the Chromium style guide and Google JavaScript style guide.
File names should_look_like_this.html.
Keep to one concept per file, always. In practice, this usually means one component or class per file, but can lead to multiple if they’re small and closely related. If you can, group utility functions into a static class to clarify their relationship, e.g. base/statistics.html.
<!-- tracing/model/point.html --> <script> ‘use strict’; tr.exportTo(‘tr.model’, function() { function Point() {} return { Point: Point }; }); </script>
The exception to this rule is when there are multiple small, related classes or methods. In this case, a file may export multiple symbols:
<!-- tracing/base/dom_helpers.html --> <script> ‘use strict’; tr.exportTo(‘tr.ui.b’, function() { function createSpan() { // … } function createDiv() { // … } function isElementAttached(element) { // … } return { createSpan: createSpan, createDiv: createDiv, isElementAttached: isElementAttached }; }); </script>
Any tests for a file should be in a file with the same name as the implementation file, but with a trailing _test.
touch tracing/model/access_point.html touch tracing/model/access_point_test.html
All symbols that exist in the global namespace should be exported using the exportTo method.
Exported package names show the file’s location relative to the root tracing/ directory. These package names are abbreviated, usually with a 1 or 2 letter abbreviation - just enough to resolve naming conflicts. All files in the same directory should share the same package.
<!-- tracing/extras/chrome/cc/input_latency_async_slice.html → tr.exportTo(‘tr.e.cc’, function() { // ... });
Polymer element names should use the convention hyphenated-package-name-element-name.
<!-- tracing/ui/analysis/counter_sample_sub_view.html --> <dom-module id='tr-ui-a-counter-sample-sub-view'> ... </dom-module>
Classes should expose public fields only if those fields represent a part of the class’s public interface.
All fields should be initialized in the constructor. Fields with no reasonable default value should be initialized to undefined.
Do not set defaults via the prototype chain.
function Line() { // Good this.yIntercept_ = undefined; } Line.prototype = { // Bad xIntercept_: undefined, set slope(slope) { // Bad: this.slope_ wasn’t initialized in the constructor. this.slope_ = slope; }, set yIntercept() { // Good return this.yIntercept_; } };
From the Blocks section of the airbnb style guide: Use braces with all multi-line blocks.
// bad if (test) return false; // good if (test) return false; // good if (test) { return false; } // bad function foo() { return false; } // good function bar() { return false; }
If you‘re using multi-line blocks with if and else, put else on the same line as your if block’s closing brace.
// bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); }
Use const and let instead of var in all new files and functions. Prefer const over let when a variable can only refer to a single value throughout its lifetime.
// bad function() { let hello = ' hello '; return hello.trim(); } // good function() { const hello = ' hello '; return hello.trim(); }
The <script> block for the Polymer element can go either inside or outside of the element’s definition. Generally, the block outside is placed outside when the script is sufficiently complex that having 2 fewer spaces of indentation would make it more readable.
<dom-module id="tr-bar"> <template><div></div></template> <script> // Can go here... </script> </dom-module> <script> 'use strict'; (function(){ // Use this if you need to define constants scoped to that element. Polymer('tr-bar', { // ... or here. }); })(); </script>
Style sheets should be inline rather than in external .css files.
<dom-module id="tr-bar"> <style> #content { display: flex; } </style> <template><div id=”content”></div></template> </dom-module>
undefined and nullPrefer use of undefined over null.
function Line() { // Good this.yIntercept_ = undefined; // Bad this.slope = null; }
UI element tests that make sure that an element is instantiable should have names that start with “instantiate”. These tests should, as a general rule, should not make assertions.
Assertions should specify the actual value before the expected value.
assert.strictEqual(value.get(), 42); assert.isBelow(value.get(), 42); assert.isAbove(value.get(), 42); assert.lengthOf(value.get(), 42);
self.assertEqual(value.Get(), 42) self.assertLess(value.Get(), 42)
Use of ES6 features is prohibited unless explicitly approved in the table below. However, we're currently working to allow them.
| Feature | Status |
|---|---|
| Arrows | Approved |
| Classes | Approved |
| Enhanced object literals | Approved |
| Template strings | Approved |
| Destructuring | Approved |
| Default, rest, and spread | Approved |
let and const | Approved and required for new code |
Iterators and for...of | Approved |
| Generators | Approved |
| Unicode | To be discussed |
| Modules | To be discussed |
| Module loaders | To be discussed |
Map, Set, WeakMap, and WeakSet | Approved |
| Proxies | To be discussed |
| Symbols | To be discussed |
| Subclassable Built-ins | Approved |
| Promises | Approved |
Math, Number, String, Array, and Object APIs | To be discussed |
| Binary and octal literals | To be discussed |
| Reflect API | To be discussed |
Use of ES7 features is prohibited unless explicitly approved in the table below. However, we're currently working to allow them.
| Feature | Status |
|---|---|
| Array.prototype.includes | Approved |
| Exponentiation operator | To be discussed |
Use of ES8 features is prohibited unless explicitly approved in the table below. Generally, ES8 features are still experimental and liable to change and therefore not fit for use in Catapult. However, in a few rare cases, features may be stable enough for use.
| Feature | Status |
|---|---|
| Object.entries and Object.values | Approved |
| async/await | Approved |
Use of an ES6 features shouldn't be considered until that feature is supported in both Chrome stable and our current version of D8.
If you see that Catapult’s version of D8 is behind Chrome stable's, use this script to update it.
Don't silently handle unexpected conditions. When such conditions occur, you should:
If fixing the problem is hard but a simple workaround is possible, then using the workaround is OK so long as:
// TODO linking to that issue.