为什么我收到此错误：未捕获的TypeError：无法读取undefined的属性'john'

Question

I am new to JavaScript and may not be using correct terms so please bear with me.

http://jsbin.com/IPaDAJO/1/

I am expect the following be displayed on the page Hello my name is John Smith and I am 29 years old. Hello my name is Suzie Alport and I am 24 years old.

I want to understand why can't Suzie refer to John's greet method?

<script>
        var employees = {
            john: {
                name: "John Smith",
                age: 29,
                job: "web developer",
                greet: function(){
                    document.write("Hello my name is " + this.name + " and I am " + this.age + " years old. ");
                }
            },
            suzie: {
                name: "Suzie Alport",
                age: 24,
                job: "nursery assistant",
                greet: employees.john.greet
            }
        };
        employees.john.greet();
        employees.suzie.greet();
</script>

Thanks

Answer 1

As of the time that you're creating the object you're assigning to the suzie property, employees is undefined . The property initializers are evaluated before the overall expression is complete.

Here's the order in which things happen above:

1. The engine creates a variable called employees with the initial value undefined .
2. The engine creates a blank object (which will eventually be assigned to employees ).
3. The engine creates another blank object (which will eventually become the john property value).
4. It evaluates "John Smith" and creates a property on the blank object from #3 called name , giving it that value.
5. It repeats that process for age , job , and greet .
6. It assigns that object to the john property on the object from #2 above.
7. It creates another blank object (to become suzie ).
8. It evaluates "Suzie Alport" and creates a property on the object from #7 called name , giving it that value.
9. It repeats the process for age and job .
10. It tries to evaluate the expression employees.john.greet and fails because employees is undefined .

If you break it into two pieces, it works (until document.write blows things up):

var employees = {
    john: {
        name: "John Smith",
        age: 29,
        job: "web developer",
        greet: function(){
            document.write("Hello my name is " + this.name + " and I am " + this.age + " years old. ");
        }
    }
};
employees.suzie = {
    name: "Suzie Alport",
    age: 24,
    job: "nursery assistant",
    greet: employees.john.greet
};
employees.john.greet();
employees.suzie.greet();

That works because before we try to evaluate employees.john.greet , a reference to the object created in #2 above has been assigned to the employees variable.

---

Side note:

Avoid using document.write in modern web pages and applications. Instead, use DOM methods . document.write only works the way you likely want it to during the initial page load; after initial page load, it will wipe out the existing document entirely and start writing a new document in the same window.

For instance, you can append paragraphs to the document.body like this:

function display(msg) {
  var p = document.createElement('p');
  p.innerHTML = String(msg);
  document.body.appendChild(p);
}

Live Example with both of the above ( source )