html显示undefined,html - Javascript var returning "undefined" - Stack Overflow

The startApp function assigns to the constructor (the startApp function) - it's a property of the class, not a property of an instance. If you want to check the PACKAGE and PATH, check the properties of the constructor (either a.constructor or System.startApp):

var System = {

startApp:function(package) {

System.startApp.PACKAGE = package;

System.startApp.PATH = "@APP:/" + package;

},

};

var a = new System.startApp("Mr. T.");

console.log(a.constructor.PACKAGE + ", " + a.constructor.PATH);

Note that the var a will only be tied to anything useful if there are prototype functions on System.startApp, for example:

var System = {

startApp:function(package) {

System.startApp.PACKAGE = package;

System.startApp.PATH = "@APP:/" + package;

},

};

System.startApp.prototype.doSomething = () => {

console.log('doing something');

};

var a = new System.startApp("Mr. T.");

console.log(a.constructor.PACKAGE + ", " + a.constructor.PATH);

a.doSomething();

If you don't have that, it doesn't make sense for startApp to be called as a constructor, nor does it make much sense to assign properties to the function.

If you want every instance to have separate properties, then assign to properties of this:

var System = {

startApp:function(package) {

this.PACKAGE = package;

this.PATH = "@APP:/" + package;

},

};

var a = new System.startApp("Mr. T.");

console.log(a.PACKAGE + ", " + a.PATH);