In JavaScript, you can define a callback handler in regex string replace operations:

str.replace(/str[123]|etc/, replaceCallback); 

Imagine you have a lookup object of strings and replacements.

var lookup = {"str1": "repl1", "str2": "repl2", "str3": "repl3", "etc": "etc" };

and this callback function:

var replaceCallback = function(match) { 
  if (lookup[match])
    return lookup[match]; 
  else
    return match;
}

How would you assess the performance of the above callback? Are there solid ways to improve it? Would

if (match in lookup) //....

or even

return lookup[match] | match;

lead to opportunities for the JS compiler to optimize, or is it all the same thing?