Issues Stringifying a multidimensional array with json.js

Question

I have problems with .stringify(), but I think my JavaScript array must be wrong, here's my code:

var questions = new Array();

$('#Valid').hover(function(){
    for (i=0;i < $('.Questions').length;i++){
        questions[i]=new Array();
        questions[i]['numero']=$('.Numero:eq('+i+')').html();
        questions[i]['question']=$('.ItemInput:eq('+i+')').val();
        questions[i]['variable']=$('.VarName:eq('+i+')').val();
    }

    var stringJSON=JSON.stringify(questions)
    alert (stringJSON)
})

The stringJSON var returns :

[[]]

What am I doing wrong?

You should use [] instead of new Array(). Also you should always make your loop variable local using the var keyword! — ThiefMaster
– ThiefMaster, Commented Mar 2, 2011 at 13:56
i < $('.Questions').length is bad. You create a new jQuery object every loop. If your elements are descendants of .Questions, use $('.questions').each(callback) — ThiefMaster
– ThiefMaster, Commented Mar 2, 2011 at 14:00

Lightness Races in Orbit · Accepted Answer · 2011-03-02 13:57:51Z

22

Arrays have integer keys, not strings.

Use an object instead; objects in JS sort of look like associative arrays:

var questions = new Array();
$('#Valid').hover(function(){
    for (var i=0;i < $('.Questions').length;i++){
        questions[i]={};
        questions[i]['numero']=$('.Numero:eq('+i+')').html();
        questions[i]['question']=$('.ItemInput:eq('+i+')').val();
        questions[i]['variable']=$('.VarName:eq('+i+')').val();
    }

    var stringJSON=JSON.stringify(questions);
    alert(stringJSON);
});

Setting questions[i] to {} is the key.

You can shorten this syntax:

var questions = new Array();
$('#Valid').hover(function(){
    for (var i=0;i < $('.Questions').length;i++){
        questions[i] = {
            numero:   $('.Numero:eq('+i+')').html(),
            question: $('.ItemInput:eq('+i+')').val(),
            variable: $('.VarName:eq('+i+')').val()
        };
    }

    var stringJSON=JSON.stringify(questions);
    alert(stringJSON);
});

answered Mar 2, 2011 at 13:57

Lightness Races in Orbit

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3 Comments

Coronier Over a year ago

Thanks, it seems so obvious now !

Lightness Races in Orbit Over a year ago

@Coronier: No problem. Arrays vs objects in JS are often confused.

Slavisa Perisic Over a year ago

this saved my life, literally. THANKS

ThiefMaster · Accepted Answer · 2011-03-02 13:57:02Z

7

Fix:

Replace questions[i]=new Array(); with questions[i] = {}; (or = new Object(); if you really want this "ugly" syntax).

Explanation:

Arrays in JavaScript are like arrays in languages like C: They only support integer indexes in the interval [0, array.length).

For associative arrays, you use objects which are created using the object literal {}.

answered Mar 2, 2011 at 13:57

ThiefMaster

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Comments

Pointy · Accepted Answer · 2011-03-02 14:05:46Z

First, your "questions" variable appears to be intended as a real array. What you're putting inside the array, however, are not real arrays — they're just objects with properties.

var questions = [];

$('#Valid').hover(function(){
    for (var i=0;i < $('.Questions').length;i++){
        questions[i] = {
          'numero': $('.Numero:eq('+i+')').html(),
          'question': $('.ItemInput:eq('+i+')').val(),
          'variable': $('.VarName:eq('+i+')').val()
        };
    }

    var stringJSON=JSON.stringify(questions)
    alert (stringJSON)
});

(Note that I also added a var for the "i" loop variable - important!)

Now, as to why the thing is coming up empty, well, I suspect it's because $('.Questions') is coming up empty.

Oh and also, this is something you could use the jQuery ".map()" API for:

 $('#Valid').hover(function() {
   questions = $('.Questions').map(function(i, q) {
     return {
       'numero': $('.Numero:eq('+i+')').html(),
       'question': $('.ItemInput:eq('+i+')').val(),
       'variable': $('.VarName:eq('+i+')').val()
     };
   }).get();

   var stringJSON = JSON.stringify(questions);
   alert(stringJSON);
});

That's a little nicer because it gets around the ugly problem of re-evaluating $('.Questions') on every iteration of the loop.

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