Slight quibble: his JS example is a bit contrived. He gives the following:

    js> a = 5;
    5
    js> f = function(x) { return x+a }; 
    function (x){return x+a}
    js> f(10)
    15
    js> a = 100
    100
    js> f(10)
    110

Well, yes, this is true, but it's not a problem with JS' closures, it's a problem with its variable declaration. a is declared as a global variable by default. Yes, this is bad, and everyone knows it's bad, but it's a bit orthogonal to closures. It's very simple to solve this in JS:

    js> a = 5;
    5
    js> f = function (x) { var a = 5; return x + a };
    function(x){var a;return x+a}
    js> f(10)
    15
    js> a = 100
    100
    js> f(10)
    15