Anyone who does advanced JavaScript work is familiar with the concept of scope. It causes interesting problems, particularly if you want your script to work with someone else’s. (Function names may be same, generating a strict warning… or variable names may be the same, resulting in complete silence…) This becomes especially annoying when you design your application to accept overlays from others…
Enter a little-known component called the JavaScript subscript loader. (I don’t know the official name for it.) Many thanks to timeless for suggesting this component.
The beauty of this component isn’t that it can load scripts for you… it’s that you can give it a “scope” to play with. That scope can be a simple JavaScript object
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. (Omitting the second argument means it will use whatever scope it’s being loaded from.)
Even better, these scope objects inherit functions & variables from the script that spawned them, but not from other scope objects.
Check this out:
var b = 4;
var url1 = "var a = 3; dump('url1:\\na: ' + typeof a + '\\nb: ' + typeof b + '\\n')"
url1 = "data:application/x-javascript," + encodeURIComponent(url1);
const jsLoader = Components.classes["@mozilla.org/moz/jssubscript-loader;1"]
.getService(Components.interfaces.mozIJSSubScriptLoader);
var scopeObj1 = {};
jsLoader.loadSubScript(url1, scopeObj1);
var url2 = "dump('url2:\\na: ' + typeof a + '\\nb: ' + typeof b + '\\n')"
url2 = "data:application/x-javascript," + encodeURIComponent(url2);
var scopeObj2 = {};
jsLoader.loadSubScript(url2, scopeObj2);
function foo(evt) {
dump("main scope:\n");
dump("a: " + typeof a + "\n");
dump("scopeObj1.a: " + typeof scopeObj1.a + "\n");
}
window.addEventListener("load", foo, true);
This generates, in my console:
url1: a: number b: number url2: a: undefined b: number main scope: a: undefined scopeObj1.a: number
So, if you want your application to accept overlays, you just need a scope registry to prevent conflicts (mostly). Lo and behold:
const scopeRegistry = {
subscriptLoader: Components.classes["@mozilla.org/moz/jssubscript-loader;1"]
.getService(Components.interfaces.mozIJSSubScriptLoader),
registeredScopes: {},
getScope: function getScope(aScopeId) {
if (typeof this.registeredScopes[aScopeId] == "undefined")
this.registeredScopes[aScopeId] = {};
return this.registeredScopes[aScopeId];
},
loadScriptByScope: function loadScriptByScope(aURL, aScopeId) {
if (aScopeId) {
var scopeObj = this.getScope(aScopeId);
this.subscriptLoader.loadSubScript(aURL, scopeObj);
} else {
this.subscriptLoader.loadSubScript(aURL);
}
}
}
I suppose I should have some try…catch blocks around the loadSubScript call. But if there’s an error loading a script through this, we want to know about it!