Uncaught TypeError: Cannot read property 'length' of undefined

You are not passing the variable correctly. One fast solution is to make a global variable like this:

var global_json_data;$(document).ready(function() {    var json_source = "https://spreadsheets.google.com/feeds/list/0ApL1zT2P00q5dG1wOUMzSlNVV3VRV2pwQ2Fnbmt3M0E/od7/public/basic?alt=json";    var string_data ="";    var json_data = $.ajax({        dataType: 'json', // Return JSON        url: json_source,        success: function(data){            var data_obj = [];            for (i=0; i<data.feed.entry.length; i++){                var el = {'key': data.feed.entry[i].title['$t'], 'value': '<p><a href="'+data.feed.entry[i].content['$t']+'>'+data.feed.entry[i].title['$t']+'</a></p>'};                data_obj.push(el)};            console.log("data grabbed");              global_json_data =   data_obj;            return data_obj;        },              error: function(jqXHR, textStatus, errorThrown){                         $('#results_box').html('<h2>Something went wrong!</h2><p><b>' + textStatus  + '</b> ' + errorThrown  + '</p>');        }    });     $(':submit').click(function(event){        var json_data = global_json_data;        event.preventDefault();        console.log(json_data.length);        //function        if ($('#place').val() !=''){            var copy_string = $('#place').val();            var converted_string = copy_string;            for (i=0; i<json_data.length; i++){                //console_log(data.feed.entry[i].title['$t']);                converted_string = converted_string.replace(json_data.feed.entry[i].title['$t'],                     '<a href="'+json_data.feed.entry[i].content['$t']+'>'+json_data.feed.entry[i].title['$t']+'</a>');            }              $('#results_box').text(converted_string).html();        }    });});//document ready end 

console.log(typeof json_data !== 'undefined'    ? json_data.length : 'There is no spoon.');

...or more simply...

console.log(json_data ? json_data.length : 'json_data is null or undefined');

You are accessing an object that is not defined.

The solution is check for null or undefined (to see whether the object exists) and only then iterate.