Avoid manipulating prototype directly. It is a built-in way to inherit prototype functionality without breaking instanceof. An empty constructor function or one that just delegates to a parent class is unnecessary.
Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug. Being an instance method should indicate that it behaves differently based on properties of the receiver. You can always transpile to your preferred module system. Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent. Having multiple lines that import from the same path can make code harder to maintain.
Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
To encourage more files that only ever export one thing, which is better for readability and maintainability. Since import s are hoisted, keeping them all at the top prevents surprising behavior.
The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas. Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack. Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.
This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. You can also step through each declaration with the debugger, instead of jumping through all of them at once. This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.
Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. If your assignment violates max-len , surround the value in parens. Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.
Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. The logical operator should begin the line. Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining.
Local aliases should be used whenever they improve readability over fully-qualified names. Follow the same rules as goog. Aliases may also be used within functions. Aliases must be const. Non-constant field names static or otherwise are written in lowerCamelCase , with a trailing underscore for private fields. These names are typically nouns or noun phrases. Parameter names are written in lowerCamelCase. Note that this applies even if the parameter expects a constructor.
This exception does not apply to any other identifiers e. Local variable names are written in lowerCamelCase , except for module-local top-level constants, as described above. Constants in function scopes are still named in lowerCamelCase.
Note that lowerCamelCase is used even if the variable holds a constructor. Module-local names that are not exported are implicitly private. They are not marked private and do not end in an underscore.
This applies to classes, functions, variables, constants, enums, and other module-local identifiers. Sometimes there is more than one reasonable way to convert an English phrase into camel case, such as when acronyms or unusual constructs like IPv6 or iOS are present.
To improve predictability, Google Style specifies the following nearly deterministic scheme. Note: Some words are ambiguously hyphenated in the English language: for example nonempty and non-empty are both correct, so the method names checkNonempty and checkNonEmpty are likewise both correct. JSDoc is used on all classes, fields, and methods. Many tools extract metadata from JSDoc comments to perform code validation and optimization.
As such, these comments must be well-formed. Note that tools that automatically extract JSDoc e. JsDossier will often ignore plain text formatting, so if you did this:. Google style allows a subset of JSDoc tags. Most tags must occupy their own line, with the tag at the beginning of the line. Simple tags that do not require any additional data such as private , const , final , export may be combined onto the same line, along with an optional type when appropriate.
Line-wrapped block tags are indented four spaces. Wrapped description text may be lined up with the description on previous lines, but this horizontal alignment is discouraged. A file may have a top-level file overview. A copyright notice , author information, and default visibility level are optional. File overviews are generally recommended whenever a file consists of more than a single class definition.
The top level comment is designed to orient readers unfamiliar with the code to what is in this file. If present, it may provide a description of the file's contents and any dependencies or compatibility information. Wrapped lines are not indented. Classes, interfaces and records must be documented with a description and any template parameters, implemented interfaces, visibility, or other appropriate tags. The class description should provide the reader with enough information to know how and when to use the class, as well as any additional considerations necessary to correctly use the class.
Textual descriptions may be omitted on the constructor. All enums and typedefs must be documented with appropriate JSDoc tags typedef or enum on the preceding line.
Public enums and typedefs must also have a description. Individual enum items may be documented with a JSDoc comment on the preceding line. Typedefs are useful for defining short record types, or aliases for unions, complex functions, or generic types.
Typedefs should be avoided for record types with many fields, since they do not allow documenting individual fields, nor using templates or recursive references. For large record types, prefer record. In methods and named functions, parameter and return types must be documented, except in the case of same-signature override s, where all types are omitted.
The this type should be documented when necessary. Return type may be omitted if the function has no non-empty return statements.
Method descriptions begin with a verb phrase that describes what the method does. This phrase is not an imperative sentence, but instead is written in the third person, as if there is an implied This method If a method overrides a superclass method, it must include an override annotation. Overridden methods inherit all JSDoc annotations from the super class method including visibility annotations and they should be omitted in the overridden method.
However, if any type is refined in type annotations, all param and return annotations must be specified explicitly. If you only need to document the param and return types of a function, you may optionally use inline JSDocs in the function's signature. These inline JSDocs specify the return and param types without tags. If you need descriptions or tags, use a single JSDoc comment above the method. For example, methods which return values need a return tag. In anonymous functions annotations are generally optional.
If the automatic type inference is insufficient or explicit annotation improves readability, then annotate param and return types like this:. Property types must be documented. The description may be omitted for private properties, if name and type provide enough documentation for understanding the code. Type annotations are found on param , return , this , and type tags, and optionally on const , export , and any visibility tags.
Type annotations attached to JSDoc tags must always be enclosed in braces. The type system defines modifiers! These modifiers must precede the type. Nullability modifiers have different requirements for different types, which fall into two broad categories:.
In cases where the compiler doesn't accurately infer the type of an expression, and the assertion functions in goog. Note that the parentheses are required. Always specify template parameters.
This way compiler can do a better job and it makes it easier for readers to understand what code does. Terminology Note : function type expression refers to a type annotation for function types with the keyword function in the annotation see examples below. Where the function definition is given, do not use a function type expression.
Specify parameter and return types with param and return , or with inline annotations see?? This includes anonymous functions and functions defined and assigned to a const where the function jsdoc appears above the whole assignment expression.
Function type expressions are needed, for example, inside typedef , param or return. Use it also for variables or properties of function type, if they are not immediately initialized with the function definition. When using a function type expression, always specify the return type explicitly. Otherwise the default return type is unknown? Within a type annotation, a single space or line break is required after each comma or colon.
Additional line breaks may be inserted to improve readability or avoid exceeding the column limit. These breaks should be chosen and indented following the applicable guidelines e. No other whitespace is allowed in type annotations. Visibility annotations private , package , protected may be specified in a fileoverview block, or on any exported symbol or property.
Do not specify visibility for local variables, whether within a function or at the top level of a module. All private names must end with an underscore. For any style question that isn't settled definitively by this specification, prefer to do what the other code in the same file is already doing.
If that doesn't resolve the question, consider emulating the other files in the same package. Before doing anything, make sure you understand exactly what the warning is telling you. If you're not positive why a warning is appearing, ask for help. Warnings are suppressed at the narrowest reasonable scope, usually that of a single local variable or very small method. Often a variable or method is extracted for that reason alone.
Even a large number of suppressions in a class is still better than blinding the entire class to this type of warning. Mark deprecated methods, classes or interfaces with deprecated annotations. A deprecation comment must include simple, clear directions for people to fix their call sites.
You will occasionally encounter files in your codebase that are not in proper Google Style. These may have come from an acquisition, or may have been written before Google Style took a position on some issue, or may be in non-Google Style for any other reason. Brand new files use Google Style, regardless of the style choices of other files in the same package.
When adding new code to a file that is not in Google Style, reformatting the existing code first is recommended, subject to the advice in?? If this reformatting is not done, then new code should be as consistent as possible with existing code in the same file, but must not violate the style guide.
Teams and projects may adopt additional style rules beyond those in this document, but must accept that cleanup changes may not abide by these additional rules, and must not block such cleanup changes due to violating any additional rules. Beware of excessive rules which serve no purpose. The style guide does not seek to define style in every possible scenario and neither should you.
Source code generated by the build process is not required to be in Google Style. However, any generated identifiers that will be referenced from hand-written source code must follow the naming requirements.
As a special exception, such identifiers are allowed to contain underscores, which may help to avoid conflicts with hand-written identifiers. JSDoc serves multiple purposes in JavaScript. In addition to being used to generate documentation it is also used to control tooling. The best known are the Closure Compiler type annotations. In addition to the JSDoc described in Annotating JavaScript for the Closure Compiler the following tags are common and well supported by various documentation generation tools such as JsDossier for purely documentation purposes.
You may also see other types of JSDoc annotations in third-party code. Documents the author of a file or the owner of a test, generally only used in the fileoverview comment.
The owner tag is used by the unit test dashboard to determine who owns the test results. Multiple bugs should each have their own bug line, to make searching for regression tests as easy as possible.
Indicates that a term in a JSDoc description is code so it may be correctly formatted in generated documentation. Historical note: link tags have also been used to create external links in generated documentation. For external links, always use Markdown's link syntax instead:. Do not use. Here is a collection of lesser-known or commonly misunderstood facts about Google Style for JavaScript. The following are true statements; this is not a list of myths. This program reformats JavaScript source code into Google Style, and also follows a number of non-required but frequently readability-enhancing formatting practices.
The output produced by clang-format is compliant with the style guide. Authors are allowed to change its output, and reviewers are allowed to ask for such changes; disputes are worked out in the usual way. However, subtrees may choose to opt in to such enforcement locally. The JS Conformance Framework is a tool that is part of the Closure Compiler that provides developers a simple means to specify a set of additional checks to be run against their code base above the standard checks.
Conformance checks can, for example, forbid access to a certain property, or calls to a certain function, or missing type information unknowns. These rules are commonly used to enforce critical restrictions such as defining globals, which could break the codebase and security patterns such as using eval or assigning to innerHTML , or more loosely to improve code quality.
For additional information see the official documentation for the JS Conformance Framework. This section describes exceptions and additional rules to be followed when modern ECMAScript 6 syntax is not available to the code authors. Exceptions to the recommended style are required when ECMAScript 6 syntax is not possible and are outlined here:. The following code gives an example:. Even though var declarations are scoped to the beginning of the enclosing function, var declarations should be as close as possible to their first use, for readability purposes.
However, do not put a var declaration inside a block if that variable is referenced outside the block. For global declarations where the const keyword would be used, if it were available, annotate the var declaration with const instead this is optional for local variables. Implementations were inconsistent with each other and with the now-standard ECMAScript 6 behavior for block scoped function declaration.
ECMAScript 5 and prior only allow for function declarations in the root statement list of a script or function and explicitly ban them in block scopes in strict mode. To get consistent behavior, instead use a var initialized with a function expression to define a function within a block:. All new files, even in projects using goog. The following rules are for pre-existing goog. The two lists should be separated with an empty line. Similar to import statements in other languages, goog.
All members defined on a class should be in the same file. Only top-level classes should be provided in a file that contains multiple members defined on the same class e.
New files should not use goog. Only one goog. Always place it in the global scope. The opening goog. Most use cases for mixins can be accomplished in better ways via components, higher-order components, or utility modules.
Component Naming : Use the filename as the component name. This repository Search Pull requests Issues Gist. JavaScript Style Guide javascript eslint namingconventions arrowfunctions styleguide stylelinter es6 es linting styleguide. Table of Contents 1. Types 2. References 3. Objects 4. Help to translate the content of this tutorial to your language! That is actually the art of programming — to take a complex task and code it in a way that is both correct and human-readable.
A good code style greatly assists in that. There should also be a space before the opening bracket, like this:. An identifier is a sequence of characters that can be used to identify a variable, a function, or an object. Using Unicode, a letter can be any allowed character, for example, an emoji?. Comments are one of the most important parts of any program, in any programming language.
They are important because they let us annotate the code and add important information that otherwise would not be available to other people or ourselves reading the code. Another type of comment is a multi-line comment. I said optionally, because the JavaScript interpreter is smart enough to introduce semicolons for you. In most cases, you can omit semicolons altogether from your programs without even thinking about it.
This fact is very controversial. Some developers will always use semicolons, some others will never use semicolons, and you'll always find code that uses semicolons and code that does not. A hello string is a value. A number like 12 is a value. The type is the kind of value, its category. We have many different types in JavaScript, and we'll talk about them in detail later on.
Each type has its own characteristics. When we need to have a reference to a value, we assign it to a variable. The variable can have a name, and the value is what's stored in a variable, so we can later access that value through the variable name. A variable is a value assigned to an identifier, so you can reference and use it later in the program. This is because JavaScript is loosely typed , a concept you'll frequently hear about.
This means the reference cannot be changed. You cannot reassign a new value to it. In particular, it does not mean the value cannot change - it means it cannot be reassigned. If the variable points to an object or an array we'll see more about objects and arrays later the content of the object or the array can freely change. My advice is to always use const and only use let when you know you'll need to reassign a value to that variable.
Because the less power our code has, the better. If we know a value cannot be reassigned, it's one less source for bugs. Now that we saw how to work with const and let , I want to mention var. Until , var was the only way we could declare a variable in JavaScript.
Today, a modern codebase will most likely just use const and let. There are some fundamental differences which I detail in this post but if you're just starting out, you might not care about them. Just use const and let. Once you assign a value with some type to a variable, you can later reassign the variable to host a value of any other type without any issues.
In JavaScript we have 2 main kinds of types: primitive types and object types. Any value that's not of a primitive type a string, a number, a boolean, null or undefined is an object.
Object types have properties and also have methods that can act on those properties. An expression is a single unit of JavaScript code that the JavaScript engine can evaluate, and return a value.
Arithmetic expressions are expressions that take a variable and an operator more on operators soon , and result in a number:. Operators allow you to get two simple expressions and combine them to form a more complex expression. We can classify operators based on the operands they work with. Some operators work with 1 operand. Most work with 2 operands. Just one operator works with 3 operands. In this first introduction to operators, we'll introduce the operators you are most likely familiar with: operators with 2 operands.
Let's now introduce another set of binary operators that you're already familiar with from basic math. If you divide by zero, JavaScript does not raise any error but returns the Infinity value or -Infinity if the value is negative.
A remainder by zero is always NaN , a special value that means "Not a Number":. Every complex statement with multiple operators in the same line will introduce precedence problems. Some operations have more precedence than the others.
The precedence rules are listed in this table:. After assignment and math operators, the third set of operators I want to introduce is conditional operators. Comparison operators always return a boolean, a value that's true or false. In addition to those, we have 4 equality operators. They accept two values, and return a boolean:. An if statement is used to make the program take a route, or another, depending on the result of an expression evaluation.
The conditional checks the expression you pass to it for a true or false value. If you pass a number, that always evaluates to true unless it's 0.
If you pass a string, it always evaluates to true unless it's an empty string. Those are general rules of casting types to a boolean. Did you notice the curly braces? That is called a block , and it is used to group a list of different statements. A block can be put wherever you can have a single statement. And if you have a single statement to execute after the conditionals, you can omit the block, and just write the statement:.
The first is using the array literal syntax. The second uses the Array built-in function. Since we can add an array into an array, we can create multi-dimensional arrays, which have very useful applications e. You can initialize a new array with a set of values using this syntax, which first initializes an array of 12 elements, and fills each element with the number 0 :.
Note that you can set the length of the array. If you assign a bigger number than the arrays current capacity, nothing happens. If you assign a smaller number, the array is cut at that position:.
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