I just had a really interesting meeting with some of the scripting community MVPs. I may write up some notes on that meeting here next week, so watch this space.
In other news, someone asked me last night (and I quote)
Oh ECMA guru, can you provide a succinct explanation of why the following code works, but if I replace x with this it fails?
Number.prototype.times = function(f) {
for(var i = 0; i < this; i++)
f();
}
Number.prototype.raiseTo = function(n) {
var answer = 1;
var x = this;
n.times(function(){ answer = x * answer;}); // succeeds
//n.times(function(){ answer = this * answer;}); // fails!
return answer;
}
(3).raiseTo(2).raiseTo(5);
Well I'd hardly describe myself as a guru, but I take the questioner's point -- surely if you set
x to this then x and this should be aliases for each other, right?
Often. But not in this case.
The inner function's execution context contains a scope chain that includes the variable object of the outer function, so the inner function can see
x by looking up the scope chain. But this is not a variable, so it is not member of the outer function's variable object. Rather, it is a member of the outer function's execution context.
Therefore the inner function's execution context gets the global
this, in accordance with the ECMAScript specification, revision 3, section 10.2.3:
The caller provides the
this value. If the this value provided by the caller is not an object (including the case where it is null), then the this value is the global object.
The global object is not something that can be multiplied, so the alternate version of the program fails.
The code above was of course not real production code, it was just code that came up while testing an engine implementation. Obviously JScript already has a built-in power function that works much better than this crazy thing. But I'm in an expansive mood, so next time I'll talk a bit about some of the shortcomings of this programming style, and some ideas for improving it.
Comment Notification
If you would like to receive an email when updates are made to this post, please register here
Subscribe to this post's comments using
About Eric Lippert
Eric Lippert is a senior developer on the Microsoft C# compiler team. Before that he worked on the framework of Visual Studio Tools For Office. Before that, he worked on the compilers, runtimes and tools for VBScript, JScript, Windows Script Host and other Microsoft Scripting technologies. He lives in Seattle and spends his free time editing books about programming languages, playing the piano, and trying to keep his tiny sailboat upright in Puget Sound.
