为什么我的代码没有产生预期的结果？ 它打印的比我想要的多

Question

Here is my code and I expect it prints out a number, instead, it prints out a number plus all my codes.

function Employee(salaryJan, salaryFeb, salaryMar){
    this.salaryJan = salaryJan;
    this.salaryFeb = salaryFeb;
    this.salaryMar = salaryMar;
}

var dennis = new Employee(6575, 7631, 8000);

Employee.prototype.sumAll = function(){
    var sum = 0;
    for (salary in this){
        sum += this[salary];
    }
    console.log(sum);
};

dennis.sumAll();

Currently my codes prints out:

22206function (){
    var sum = 0;
    for (salary in this){
        sum += this[salary];
    }
    console.log(sum);
}

I just want 22206 , and I have no idea why it also prints out some code.

I have a JSFiddle fiddle for this: http://jsfiddle.net/dennisboys/LZeQr/1/

Answer 1

Here's the problem:

for (salary in this)

This will loop through all properties of this . Let's see those properties:

this.salaryJan
this.salaryFeb
this.salaryMar
Employee.prototype.sumAll

You've got 4 properties which is what you see getting printed to the console.

You should use the hasOwnProperty method:

for (prop in this) {
    if (this.hasOwnProperty(prop)) 
        sum += this[prop];
    }
}

And here's a live demo .