Avoid using .onload, .onclick, etc. in a userscript. (This is considered bad practice even on a regular web page).
The reason for this recommendation is that userscripts are restricted within a sandbox ("isolated environment"), which limits the ability to set or access page-wide JavaScript objects in a Chrome userscript or content-script.
Instead, always opt for using addEventListener() (or similar functions from libraries like jQuery's .on()). Additionally, it is advisable to establish load listeners prior to injecting <script> elements into the DOM.
If you need to reach variables in the page scope (such as B here), you will need to insert code that facilitates this operation. (Alternatively, you can consider switching to Tampermonkey and utilizing unsafeWindow, though there have been reported issues with this approach due to Chrome 29 compatibility problems.)
Consider employing something along these lines:
addJS_Node (null, "http://localhost/test/js/load.js", null, executeAfterLoad);
function executeAfterLoad () {
addJS_Node ("console.log (B);");
}
//-- The addJS_Node function follows standard practices
function addJS_Node (text, s_URL, funcToRun, runOnLoad) {
var D = document;
var scriptNode = D.createElement ('script');
if (runOnLoad) {
scriptNode.addEventListener ("load", runOnLoad, false);
}
scriptNode.type = "text/javascript";
if (text) scriptNode.textContent = text;
if (s_URL) scriptNode.src = s_URL;
if (funcToRun) scriptNode.textContent = '(' + funcToRun.toString() + ')()';
var targetElement = D.getElementsByTagName ('head')[0] || D.body || D.documentElement;
targetElement.appendChild (scriptNode);
}
Alternatively:
addJS_Node (null, "http://localhost/test/js/load.js", null, executeAfterLoad);
function executeAfterLoad () {
addJS_Node (null, null, myScriptThatUsesPageJS);
}
function myScriptThatUsesPageJS () {
console.log (B);
//--- ADDITIONAL FUNCTIONALITY CAN BE INCLUDED HERE.
}
... ...