why does jquery proxy have this?

Question

in jquery proxy fn

proxy: function( fn, context ) {
 var tmp, args, proxy;
 if ( typeof context === "string" ) {
 tmp = fn[ context ];
 context = fn;
 fn = tmp;
 }
 // Quick check to determine if target is callable, in the spec
 // this throws a TypeError, but we will just return undefined.
 if ( !jQuery.isFunction( fn ) ) {
 return undefined;
 }
 // Simulated bind
 args = slice.call( arguments, 2 );
 proxy = function() {
 return fn.apply( context || this, args.concat( slice.call( arguments ) ) );
 };
 // Set the guid of unique handler to the same of original handler, so it can be removed
 proxy.guid = fn.guid = fn.guid || jQuery.guid++;
 return proxy;
},

what is the main usage for the syntax 2, (context, string)?

• Arun P Johny: see jsfiddle.net/arunpjohny/Yv8hY/1 - api.jquery.com/jQuery.proxy/#example-1
• user400654: See the second example on the documentation for jQuery.proxy.

Answer 1

It's ensuring the passed in context variable is present as an argument (and is also a string) before assigning the data within the fn object with a key of context to the tmp variable.

Without doing this, if we just called $.proxy(fn) with no context argument, the fn variable would end up as undefined and the proxy function itself would return undefined instead of simply ignoring the context variable altogether and processing as if it weren't passed in in the first place.