JavaScript object set and function call

Question

I created the following javascript object.

var obj = function () {
    var a = 6;
    var b = 7;
    var sumFunction;

    function sum() {
        alert(a);
        alert(b);
        sumFunction();
    }

    return {
        a: a,
        b: b,
        sum: function (f) {
            sumFunction = f;
            sum();
        }
    }
}

var o = new obj;

function Calculate() {
    o.a = 3;
    o.b = 4;

    o.sum(function () {
        alert('finish');
    });
}

When the Calculate method is called, the two alert box show 6 and 7. It should be 3 and 4. I don't know what wrong my object. How can I correct this? Thanks.

the reason why a and b are calling 6 and 7 is because you're not specifying this. it should be this.a, and this.b — kennypu
– kennypu, Commented May 10, 2014 at 10:31

kennypu · Accepted Answer · 2014-05-10 10:36:11Z

1

First thing, you don't need to give a return value for functions used with new. Next, when you're refering to instance variables, you should use this.

var obj = function () {
    this.a = 6;
    this.b = 7;
    this.sumFunction;

    this.sum = function(f) {
        alert(this.a);
        alert(this.b);
        this.sumFunction = f;
        this.sumFunction();
    }


}



function Calculate() {
    var o = new obj;
    console.log(o);
    o.a = 3;
    o.b = 4;

    o.sum(function () {
        alert('finish');
    });
}
   Calculate();

example fiddle: http://jsfiddle.net/3WgKK/1/

answered May 10, 2014 at 10:36

kennypu

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2 Comments

a better oliver Over a year ago

you don't need to give a return value for functions used with new - While your solution may work for the OP I would refrain from making such a generic assumption. It's a common pattern after all and for good reasons.

Bergi Over a year ago

I don't think sumfunction should be a property of the object at all, it's just a callback.

a better oliver · Accepted Answer · 2014-05-10 10:35:46Z

1

function sum() {
  alert(a);

References the variable a, that still has the value 6. Your construct makes it somewhat difficult to reference the property a. The following approach does what you want:

function sum() {
    alert(this.a);
    alert(this.b);
    sumFunction();
}
    return {
    a: a,
    b: b,
    sum: function (f) {
        sumFunction = f;
        sum.call(this);
    }

answered May 10, 2014 at 10:35

a better oliver

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