Comments on: JavaScript: Variable declarations and scope

By: cin takvimi

cin takvimi — Tue, 05 Apr 2011 18:54:08 +0000

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By: Biuro Rachunkowe wrocław

Biuro Rachunkowe wrocław — Thu, 23 Dec 2010 04:37:47 +0000

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By: Heinz von Heiden

Heinz von Heiden — Tue, 14 Dec 2010 11:05:33 +0000

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By: Jack Tanner

Jack Tanner — Wed, 14 Jul 2010 20:44:56 +0000

This is called ‘hoisting’ the declaration.

For example, see http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting

By: Tjerk Wolterink

Tjerk Wolterink — Sat, 09 Jan 2010 14:37:34 +0000

Ah nice, i knew about function scope but i did not new about the javascript interpreter checking all declaration before execution. Again it’s all about semantics, that’s why a specification is so important. Too bad the ecmascript specifications are written in plain text. This increases the chances of different interpretations and ambiguity.

For example what do you think the value of x is after running this code?

var b=10;
var f=function() {
var b=5;
function g() {
var b=8;
return this.b;
}
return g();
};
var x = f();

Before running it try to run it in your head and write down the value of x. Then run it in a javscript engine ;-)

Semantics specifications *should* be written in a formal language like Structural Operational Semantics… but then only a few people can actually read them ;-)

Check SOS description of JS here! (the example was also from there)
http://www.stanford.edu/class/cs242/slides/OperSemanticsJS

But again thanks for this insight, i think it’s one of the bad things of javascript ;-)
Greetings to Nikolai!