Question

I stumbled upon the function .globalEval() from browsing the jQuery source. There is very brief documentation which I don’t understand. Apparently, it is “important for loading external scripts dynamically”. Why? The source is also somewhat obscure:

globalEval: function( data ) {
    if ( data && rnotwhite.test( data ) ) {
        // We use execScript on Internet Explorer
        // We use an anonymous function so that context is window
        // rather than jQuery in Firefox
        ( window.execScript || function( data ) {
            window[ "eval" ].call( window, data );
        } )( data );
    }
},

Do people actually use this in real life? If so, for what?

Answer 1

It is used, as the name suggests, to execute the eval code in the global context. For example, consider the following (jsFiddle):

function example(){
  $.globalEval("var example1 = 'first';");
  eval("var example2 = 'second';");
    console.log("In function: " + example1); //Prints 'first'
    console.log("In function: " + example2); //Prints 'second'
}
example();
console.log("Global: " + example1); //Prints 'first'
console.log("Global: " + example2); //ReferenceError

Because example1 was defined using globalEval, it’s in the global scope. Using plain old normal eval, the variable is only availble in the scope in which eval is called.

It can be useful if you want to load another JS script, and you want to execute that script in the global context (for example, above, we might need example1 to be available outside of the example function, so we have to use globalEval.

I’m not sure why the jQuery source uses window[ "eval" ].call instead of just eval.call, but I’m sure someone could explain 🙂