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Style guide {#style-guide}

Here are the coding rules and principles used throughout the book.

This chapter is by nature subjective and opinionated. Feel free to make your own choices.


Naming things right goes a long way into making code cleaner and easier to understand. Some general naming rules are presented below.

Choose meaningful names

The most important rule is to give each element (variable, function, class, etc) a specific name that reflects its role. A variable holding the value of a circle radius should be named radius rather than num or myVal.

Brevity should be limited to short-lived elements, like loop counters.

Don't use reserved words

Each JavaScript keyword is a reserved name. They should not be used as variable names. Here's the list of reserved words in JavaScript.

Follow a naming convention

It can take several words to describe precisely the role of certain elements. This book adopts the popular camelCase naming convention, based on two main principles:

  • All names begin with a lowercase letter.
  • If a name consists of several words, the first letter of each word (except the first word) is uppercase.

In addition, this book uses the following naming rules:

  • Functions and method names include an action verb: computeTotal(), findFirstParent(), attackTarget(), etc.
  • To be consistent with other programming languages, class names start with an uppercase letter: User instead of user.
  • Since they may contain multiple elements, arrays are named plurally or suffixed with List: movies or movieList, but not movie.
  • To distinguish them from other variables, DOM elements are suffixed with Element (or Elements for array-like variables): divElement rather than simply div.

W> Like many other languages, JavaScript is case sensitive. For example, myVariable and myvariable are two different variable names. Be careful!

Code formatting

This is a subject of many debates in the JavaScript community: using spaces or tabulations for indenting, omitting semicolons, simple vs double quotes for strings, and so on.

A simple and efficient solution is to rely on a tool to automate the low-level task of formatting code, so that you can concentrate on more high-level work. This book uses Prettier with default configuration (double quotes and semicolons).

Code quality

Since JavaScript is a dynamically typed language, a number of errors don't show up until execution: misnaming a function, loading a nonexistent module, etc. In addition, many other mistakes like declaring a variable without ever using it won't affect execution outcome, but make your code harder to read and lower its overall quality.

Fortunately, specialized tools called linters can check your code against rules during edition and warn about potential defects. By allowing to fix many bugs before they happen, linters greatly enhance developer productivity.

This book uses ESLint for linting code. ESLint is a very flexible tool and you can tailor it to your specific needs. Different set of ESLint rules have emerged, notably one based on the popular AirBnb Style Guide.

This opinionated style guide is well worth a read.

This book's ESLint configuration extends the AirBnb and Prettier rules (Prettier getting the precedence), with a few minor deviations.

Here is the content of the book's .eslintrc configuration file.

  "extends": ["airbnb", "prettier"],
  "env": {
    "browser": true
  "plugins": ["prettier"],
  "rules": {
    "no-console": "off",
    "no-alert": "off",
    "no-plusplus": "off",
    "default-case": "off",
    "no-param-reassign": [
        "props": false
    "arrow-body-style": [
      { "requireReturnForObjectLiteral": true }

The deviations from predefined rules are explained below.

  • "no-console" and "no-alert": to enable console.XXX() and alert() calls.
  • "no-plusplus": to enable unary operators like ++, commonly used and generally harmless.
  • "default-case": to enable switch statements without a default case, which are common.
  • "no-param-reassign": to enable updating properties of an object passed as a parameter.
  • "arrow-body-style": to use the more explicit return syntax for arrow functions that return an object literal.