There are a number of text conventions used throughout this book.

Code in text: Indicates code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles. Here is an example: “In the first test, change the word appendChild to replaceChildren.”

Bold: Indicates a new term, an important word, or words that you see onscreen. For instance, words in menus or dialog boxes appear in bold. Here is an example: “The presenter clicks the Start sharing button.”

Tips or important notes

Appear like this.

Code snippet conventions

A block of code is set as follows:

it("renders the customer first name", () => {  const customer = { firstName: "Ashley" };  render(<Appointment customer={customer} />);  expect(document.body.textContent).toContain("Ashley");});

There are two important things to know about the code snippets that appear in this book.

The first is that some code samples show modifications to existing sections of code. When this happens, the changed lines appear in bold, and the other lines are simply there to provide context:

export const Appointment = ({ customer }) => (  <div>{customer.firstName}</div>);

The second is that, often, some code samples will skip lines in order to keep the context clear. When this occurs, you’ll see this marked by a line with three dots:

if (!anyErrors(validationResult)) {
  ...
} else {
  setValidationErrors(validationResult); 
} 

Sometimes, this happens for function parameters too:

if (!anyErrors(validationResult)) {
  setSubmitting(true);
  const result = await window.fetch(...);
  setSubmitting(false); 
  ... 
}

Any command-line input or output is written as follows:

npx relay-compiler

JavaScript conventions

The book almost exclusively uses arrow functions for defining functions. The only exceptions are when we write generator functions, which must use the standard function’s syntax. If you’re not familiar with arrow functions, they look like this, which defines a single-argument function named inc:

const inc = arg => arg + 1;

They can appear on one line or be broken into two:

const inc = arg =>
  arg + 1;

Functions that have more than one argument have the arguments wrapped in brackets:

const add = (a, b) => a + b;

If a function has multiple statements, then the body is wrapped in curly braces:

const dailyTimeSlots = (salonOpensAt, salonClosesAt) => {
  ...
  return timeIncrements(totalSlots, startTime, increment);};

If the function returns an object, then that object must be wrapped in brackets so that the runtime doesn’t think it’s executing a block:

setAppointment(appointment => ({  ...appointment,  [name]: value }); 

This book makes liberal use of destructuring techniques to keep the code base as concise as possible. As an example, object destructuring generally happens for function parameters:

const handleSelectBoxChange = (
  { target: { value, name } }
) => {
  ...
}; 

This is equivalent to saying this:

const handleSelectBoxChange = (event) => {
  const target = event.target;
  const value = target.value;
  const name = target.name;
  ...
}; 

Return values can also be destructured in the same way:

const [customer, setCustomer] = useState({});

This is equivalent to the following:

const customerState = useState({});
const customer = customerState[0];
const setCustomer = customerState[1];