jquery .val() and google closure compiler

Question

I want to select the first option in a control so I write:

$("#MySelect").val($("#MySelect option:first").val());

Now go ahead and copy-paste the following into the google closure compiler :

// ==ClosureCompiler==
// @output_file_name default.js
// @compilation_level ADVANCED_OPTIMIZATIONS
// @externs_url http://closure-compiler.googlecode.com/svn/trunk/contrib/externs/jquery-1.7.js
// ==/ClosureCompiler==

$("#MySelect").val($("#MySelect option:first").val());

You'll get this error:

I don't see why the compiler is complaining! What's the problem?

Thanks for your suggestions.

Answer 1

You could do this $("#MySelect").prop("selectedIndex", 0)​​​​ . It's simpler and passes closure compiler.

Answer 2

Many of the jQuery methods return different types based on the number and type of the input parameters. This behavior is akin to function overloading in a traditional language. However, JavaScript doesn't support traditional function overloading and jQuery mimics the behavior by inspecting the function arguments.

For .val , here's how the method would be annotated if function overloading was supported:

/** @return {number|string|Array.<string>} */
jQuery.prototype.val = function() {};

/**
 * @param {number|string|Array.<string>|function(number, *)} newVal
 * @return {!jQuery}
 */
jQuery.prototype.val = function(newVal) {};

Since there isn't function overloading, the actual signature for .val is a combination of both uses:

/**
 * @param {(number|string|Array.<string>|function(number, *))=} newVal
 * @return {!jQuery|number|string|Array.<string>|function(number, *)}
 */
jQuery.prototype.val = function(newVal) {};

Because of this, if you wish to use the return value of .val as the input for a separate call to .val , you must type cast the original return value to specify which usage you expect:

$("#MySelect").val(
    /** @type {number|string|Array.<string>} */
    ($("#MySelect option:first").val()) //note the extra parens
);

This behavior is described in a comment at the top of the jQuery externs file: http://code.google.com/p/closure-compiler/source/browse/trunk/contrib/externs/jquery-1.7.js#20