Typing indexOf redefinition with Closure compiler

Question

I am trying to redefine Array.prototype.indexOf for old IE versions. I am having trouble typing it correctly according to Google Closure Compiler.

It says that the type of @this is wrong.

if (!Array.prototype.indexOf) {
  /**                                                                                                                                                 
   * @this {Array}                                                                                                                                    
   * @param {*} item                                                                                                                                  
   * @param {number=} from: ignored 
   * @return {number}                                                                                                                                 
   */
  Array.prototype.indexOf = function(item, from) {
    // ...
  }
}

I get the following output

test.js:12: WARNING - variable Array.prototype.indexOf redefined with type \
function (this:Array, *, number=): number, original definition at \
externs.zip//es3.js:633 with type function (this:Object, *, number=): number
Array.prototype.indexOf = function(item, from) {
^

Surprisingly, changing @this {Array} by @this {Object} (though it does not make much sense) returns this even more obscure message:

test.js:12: WARNING - variable Array.prototype.indexOf redefined with type \
function (this:Object, *, number=): number, original definition at \
externs.zip//es3.js:633 with type function (this:Object, *, number=): number
Array.prototype.indexOf = function(item, from) {
^

Any hint on how to do it properly?

Answer 1

You can use @suppress {duplicate} to ignore this warning:

/**
 * @this {Array}
 * @param {*} item
 * @param {number=} from: ignored
 * @return {number}
 * @suppress {duplicate}
 */
Array.prototype.indexOf = function(item, from) {
    // ...
}

I am not sure about the implications redefining the method has on optimizations from the Closure Compiler in ADVANCED mode, though.

Answer 2

Array methods are generic, they should in fact take an Array-like value. The latest Closure Compiler defines it as:

/**
 * Available in ECMAScript 5, Mozilla 1.6+.
 * @param {T} obj
 * @param {number=} opt_fromIndex
 * @return {number}
 * @this {{length: number}|Array.<T>|string}
 * @nosideeffects
 * @template T
 * @see http://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Array/indexOf
 */
Array.prototype.indexOf = function(obj, opt_fromIndex) {};

Simply assigning the value works:

if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function(item, from) {
     // ...
  }
}

Consider upgrading to a recent release of the Compiler.