Source Code Fundamentals: Comments

Hack has three comment syntaxes.

// A single line comment.

/* A multi line comment.
 *
 */

/**
 * A doc comment starts with two asterisks.
 *
 * It summarises the purpose of a definition, such as a
 * function, class or method.
 */
function foo(): void {}

Multi-line comments start with /* and end with */. Comments starting /** are also used for documentation.

Single-line comments start with // and end with a newline.

# is not a valid comment character, as it is used to represent an Enum Class Label.

A number of special comments are recognized; they are:

• // FALLTHROUGH in switch statements
• // strict and // partial in headers
• /* HH_FIXME[1234] */ or /* HH_IGNORE_ERROR[1234] */, which suppresses typechecker error reporting for error 1234.