How to document return in JavaScript

Question

I'm writing my own library for the project at work for a browser application and I am having the same old problem deciding how to comment the code.

I'm trying to follow the JsDoc syntax, but will probably continue the Google Closure Compiler way. I may end up using two @return and @returns tags in the documentation, just for portability sake (when I setup the auto-generation of the documentation).

Now, the question, how do you document the return of a custom anonymous object from a function? For example:

return {
    username: 'username',
    password: 'password',
    enabled:  true
};

JsDoc has an example of how a @param can be documented to expect object with certain fields, but not the @returns tag. Similarly, the Google Closure Compiler documentation of a Record Type is vague and has no example to work it out.

Answer 1

The Closure-compiler uses a subset of the JSDoc annotations (and adds a few of its own). See the annotation reference for the compiler for the complete set. A JSDoc annotation is similar to a JavaDoc annotation and is a comment block that begins with /** (two stars). While each line of the comment often begins with it's own *, that is a convention that is not required. Only one JSDoc tag is allowed per line, but the arguments for a tag can span multiple lines.

The annotation typically applies to the following statement. Here are some examples:

Variable

/** @type {string} */ var a;

Type Cast

var b = /** @type {string} */ (window['foo']);

note the extra parenthesis

Named Function

/**
 * @param {string} bar
 * @return {boolean}
 */
function foo(bar) { return true; }

Function Expressions

/** @type {function(string):boolean} */
var foo = function(bar) { return true; }

var foo2 =
  /**
   * @param {string} bar
   * @return {boolean}
   */
  function(bar) { return true; }

Typedef

Complex types (including unions, and record types) can be aliased for convenience and maintainability using a typedef. These annotations can be long, but can be split over multiple lines for readability.

/** @typedef {{
 *             foo:string,
 *             bar:number,
 *             foobar:number|string
 *           }}
 */
var mytype;

For your original example, there are several possible ways to annotate such a function return value. One of the most specific and still convenient is the record type:

/** @return {{username:string, password:string, enabled:boolean}} */
function() {
  return {
    username: 'username',
    password: 'password',
    enabled:  true
  }
}

Note the extra {}. Also keep in mind that record types will not prevent property renaming.

This annotation tells the compiler that the function returns an anonymous type with username, password and enabled properties. Other valid options would be to define an interface elsewhere and typecast the return value to be that interface. The least specific annotation would be Object or *.

To see a wide range of possible annotations, take a look at the extern files in the Compiler project.