if (typeof(km_scripts) == 'undefined') var km_scripts = new Object();
km_myclass_import('includes/mortgage_calculator.js');

function km_myclass_import(jsFile) {
if (km_scripts[jsFile] != null) return;
var scriptElt = document.createElement('script');
scriptElt.type = 'text/javascript';
scriptElt.src = jsFile;
document.getElementsByTagName('head')[0].appendChild(scriptElt);
km_scripts[jsFile] = jsFile; // or whatever value your prefer
}

function km_myclass_alert() {
alert(importedValue);
}