unusual behavior with object creation in Javascript

Question

I am not getting expected values through this code, please help me to identify the bug in this code. I am newbie to java script, I doubt the problem might be due to object creation or object scope. I debugged it in several ways but unable to identify the problem.

Also please tell me if the data I am creating is in JSON format or not. If not what is the equivalent JSON format ?

PS.id = -1;

PS.data = {
 formData:[]
};

PS.setDef = [];

PS.item = {};

if(cond){

PS.item.Name = "From";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[0].Name);

PS.item.Name = "To";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[1].Name);

}//here alerts are coming properly. getting 'from' and 'to' in alerts

PS.data.formData.push({"id":id++, "setDef":[PS.setDef[0]]});
PS.data.formData.push({"id":id++, "setDef":[PS.setDef[1]]});

//expected id 0 
alert(PS.data.formData[0].id);
//expected name 'from'
alert(PS.data.formData[1].setDef[0].Name);

//expected id 1
alert(PS.data.formData[1].id);
//expected name 'to'
alert(PS.data.formData[1].setDef[1].Name);

Instead of getting 0 and 1 as ID's and names as 'from' and 'to' I am getting following values

-1
0
to
to

Update 1 : here PS means page scope which is already declared, edited the alerts to print variables.

Where is your declaration of PS? And you are alerting strings and not the value of a variable. — Xotic750
– Xotic750, Commented Feb 24, 2014 at 6:37
Also when you execute PS.data.formData.push({"id":id++, "setDef":[PS.setDef[0]]}); the id variable hasn't been defined before. — kamilkp
– kamilkp, Commented Feb 24, 2014 at 6:43

6502 · Accepted Answer · 2014-02-24 07:05:21Z

1

Your code had a couple of logical errors and a typo. This is the corrected version (changes have been marked in comments with **HERE**)

PS.id = -1;

PS.data = {
    formData:[]
};

PS.setDef = [];

if(cond){
    // *** HERE ***
    // Your code was creating just one object instance in
    // PS.item, pushing it, mutating and pushing it again
    // The "alert" call was apparently correct just because
    // it was done before mutating the object. Just moving
    // both alerts at the end would have shown the problem.

    PS.setDef.push({Name: "From", Type: "STRING"});
    alert(PS.setDef[0].Name);

    PS.setDef.push({Name: "To", Type: "STRING"});
    alert(PS.setDef[1].Name);

}

// **HERE**
// id++ increments a value but returns the value BEFORE the increment.
// Thus formData[0] is expected to have .id == -1, not 0.
// If you want the value AFTER the increment then use ++id.
PS.data.formData.push({"id":++id, "setDef":[PS.setDef[0]]});
PS.data.formData.push({"id":++id, "setDef":[PS.setDef[1]]});

//expected id 0 
alert(PS.data.formData[0].id);
//expected name 'from'
// *** HERE ***
// In the original code there is a typo (formData[1] instead of formData[0])
alert(PS.data.formData[0].setDef[0].Name);

//expected id 1
alert(PS.data.formData[1].id);
//expected name 'to'
alert(PS.data.formData[1].setDef[1].Name);

answered Feb 24, 2014 at 7:05

6502

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3 Comments

user3131769 Over a year ago

Thanks for your time. Understood my mistakes, issue resolved. Can you please explain about second part of my question which is about JSON format ? Requirment is to convert this data to JSON format and to sent to backend using DWR and process this data backend using Java. I am confused about the syntax of the method arguments. Like for example like getJsonData(Map<String, Map<String,Map<String,String>>[]>)

user3131769 Over a year ago

is getJsonData(Map<String, Map<String,Map<String,String>>[]>[]) correct syntax in java to represent the above data format ?

6502 Over a year ago

@user3131769: Sorry but I don't know Java generics (I abandoned Java before they attempted introducing them). I'd say that no static type that isn't based on a generic "value" type (that can be null, a string, a number, an array or an object, where arrays and objects contains generic values) would however be able to represent the parsing of a generic JSON message.

SinaX · Accepted Answer · 2014-02-24 06:47:59Z

0

You just referenced PS.item twice in this part of your code:

PS.item.Name = "From";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[0].Name);

//changing the value of PS.item 
PS.item.Name = "To";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[1].Name);

You can chnage your code this way and go happy with the results:

//creating first instance
PS.item = {};

PS.item.Name = "From";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[0].Name);

//creating the second instance
PS.item = {};

PS.item.Name = "To";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[1].Name);

Got the idea?

answered Feb 24, 2014 at 6:47

SinaX

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2 Comments

user3131769 Over a year ago

Thanks @Sinax, issue resolved. can you please clarify the second part of my question about json format ?

SinaX Over a year ago

Sorry but I can't find the JSON part in your code snippet. You are creating and populating some objects and arrays and you later can convert them to JSON format using JSON.stringify method in one call, giving you don't have any cyclic-reference.

rakhi4110 · Accepted Answer · 2014-02-24 06:54:35Z

0

There are 2 things here. First for id, change the code to the following,

PS.data.formData.push({"id":++PS.id, "setDef":[PS.setDef[0]]});
PS.data.formData.push({"id":++PS.id, "setDef":[PS.setDef[1]]});

Since your id starts with -1 when u assign id++ to id, it firsts assigns the value currently in id and then increments the value of id, hence you get -1 and 0.

Second, when you push PS.item, you are actually passing the reference to the object and not the actual object. Hence when you are updating the value of PS.item, it gets reflected in both the places. Try to create a new object and push it, instead of overwriting the old one.

Use PS.item = {}; to create a new object, before assigning the values. This should work.

edited Feb 24, 2014 at 6:54

answered Feb 24, 2014 at 6:43

rakhi4110

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1 Comment

user3131769 Over a year ago

Thanks for your answer, can you please post the code to create a new object. I know the constructor way of object creation but here since I have to use PS scope I am struck. Thanks in advance.

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