How do I get the JSDoc text (docstring) from a function inside the code in JavaScript?

Considering I have a simple function, I want to extract the JSDoc string inside the code and display it in the page, like so:

/**
 * Represents a book.
 * @constructor
 */
function Book(title, author) {
  // contents
}

<span>
Represents a book.
@constructor
</span>

Can this be done?

In JavaScript, comments are not a part of the function's AST (Abstract Syntax Tree), they're purely metadata and most JavaScript engines don't preserve them when the function is converted to a string.

If that was not the case, the following could would work:

function extractJSDoc(func) {
    var match = func.toString().match(/\/\*\*([\s\S]*?)\*\//);
    return (match && match.length > 1) ? match[1].trim() : '';
}

/**
 * Represents a book.
 * @constructor
 */
function Book(title, author) {
  // contents
}

document.getElementById("displayJSDoc").innerText = extractJSDoc(Book);

<span id="displayJSDoc"></span>

As an alternative, you could consider storing your JSDoc comments in a separate data structure or file which you can access when needed. Another possible approach is to make use of a build tool or transpiler (like Babel) to extract the comments during a build step and generate a separate documentation file or inject them into your code where needed.