cannot read properties of undefined reading length ') jquery

Cannot read property length of undefined

stackoverflow.com › questions › 7158439 › cannot-read-property-length-of-undefined

The id of the input seems is not WallSearch. Maybe you're confusing that name and id. They are two different properties. name is used to define the name by which the value is posted, while id is the unique identification of the element inside the DOM.

Other possibility is that you have two elements with the same id. The browser will pick any of these (probably the last, maybe the first) and return an element that doesn't support the value property.

Answer from GolezTrol on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 7158439 › cannot-read-property-length-of-undefined

Cannot read property length of undefined - javascript

Top answer

1 of 3

2 of 3

perhaps, you can first determine if the DOM does really exists,

function walkmydog() {
    //when the user starts entering
    var dom = document.getElementById('WallSearch');
    if(dom == null){
        alert('sorry, WallSearch DOM cannot be found');
        return false;    
    }

    if(dom.value.length == 0){
        alert("nothing");
    }
}

if (document.addEventListener){
    document.addEventListener("DOMContentLoaded", walkmydog, false);
}

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Rollbar

rollbar.com › home › how to fix typeerror: cannot read property length of undefined in javascript

How to Fix TypeError: Cannot Read Property Length of Undefined in JavaScript

03:57

Verify that nested properties exist ... TypeError: Cannot read property 'length' of undefined happens when you try to access the length property on something that doesn't exist....

Published: July 13, 2025

Discussions

Uncaught TypeError: Cannot read properties of undefined (reading 'length') — DataTables forums

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typeerror - JavaScript - Cannot read properties of undefined (reading 'length') - (Code does work and provided) - Stack Overflow

I have looked at many different methods here on SO, and neither of them will resolve the issue I am facing. If anything, they all still show the same Error. The count starts when you type a number ... More on stackoverflow.com

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JS error Uncaught TypeError reading length - Support - Themeco Forum

Hello, on the portfolio page when using the category filter I get this error: jquery-migrate.min.js?ver=3.3.2:2 Uncaught TypeError: Cannot read properties of undefined (reading ‘length’) at init.s.fx.s.Tween.run (jquery-migrate.min.js?ver=3.3.2:2:6779) at u (jquery.min.js?ver=3.6.0:2:59565) ... More on theme.co

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March 26, 2022

TrackJS

trackjs.com › javascript answers › how to fix `cannot read properties of undefined (reading 'length')`

How to fix `Cannot read properties of undefined (reading 'length')` • TrackJS

April 8, 2026 - Common JavaScript error when accessing length property on undefined values. Usually occurs with arrays, strings, or API responses that haven't loaded yet. Quick fixes: add null checks, use optional chaining, handle loading states.

Bobby Hadz

bobbyhadz.com › blog › javascript-cannot-read-property-length-of-undefined

Cannot read properties of undefined (reading 'length') in JS | bobbyhadz

The "TypeError: Cannot read properties of undefined (reading 'length')" error occurs when accessing the length property on an undefined value.

Coding Beauty

codingbeautydev.com › home › posts › 4 quick fixes for the “cannot read property ‘length’ of undefined” error in javascript

[SOLVED] Cannot Read Property 'length' of Undefined in JS

August 21, 2022 - To fix the “cannot read property ‘length’ of undefined” error, perform an undefined check on the variable before accessing the length property from it. There are various ways to do this.

reddit.com › r/learnjavascript › [deleted by user]

[deleted by user] : r/learnjavascript

July 15, 2022 - You’re getting this error when calling the length property on something. That means that something you have use .length on is undefined at some point.

DataTables

datatables.net › forums › discussion › 75966 › uncaught-typeerror-cannot-read-properties-of-undefined-reading-length

Uncaught TypeError: Cannot read properties of undefined (reading 'length') — DataTables forums

April 3, 2023 - TypeError no capturado: no se pueden leer las propiedades de undefined (leyendo 'estilo') en ee (tablas de datos.min.js:16:40103) en Jt (tablas de datos.min.js:16:32717) en t (tablas de datos.min.js:16:8587) en HTMLTableElement.<anónimo> (datatables.min.js:16:8673) en Function.each (jquery-3.6.0.min.js:2:3003) en S.fn.init.each (jquery-3.6.0.min.js:2:1481) en S.fn.init.w [como tabla de datos] (datatables.min.js:16:3564) en P.fn.DataTable (datatables.min.js:16:86687) en HTMLDocument.<anónimo> (admin.js:3:31) kthorngren Posts: 22,495Questions: 26Answers: 5,169 · April 2023 edited April 2023 · After making the version changes it gives the following error, and still does not show the data. TypeError not caught: can't read properties of undefined (reading 'style') Please see my previous comments: Use the browser's network inspector to view the JSON response.

Stack Overflow

stackoverflow.com › questions › 34287402 › datatables-cannot-read-property-length-of-undefined

jquery - DataTables: Cannot read property 'length' of undefined - Stack Overflow

Top answer

1 of 12

143

It's even simpler: just use dataSrc:'' option in the ajax defintion so dataTable knows to expect an array instead of an object:

    $('#pos-table2').DataTable({
                  processing: true,
                  serverSide: true,
                  ajax:{url:"pos.json",dataSrc:""}
            }
    );

See ajax options

2 of 12

CAUSE

This errors TypeError: Cannot read property 'length' of undefined usually means that jQuery DataTables cannot find the data in the response to the Ajax request.

By default jQuery DataTables expects the data to be in one of the formats shown below. Error occurs because data is returned in the format other than default.

Array of arrays

{ 
   "data": [
      [
         "Tiger Nixon",
         "System Architect",
         "$320,800",
         "2011/04/25",
         "Edinburgh",
         "5421"
      ]
   ]
}

Array of objects

{ 
   "data": [
      {
         "name": "Tiger Nixon",
         "position": "System Architect",
         "salary": "$320,800",
         "start_date": "2011/04/25",
         "office": "Edinburgh",
         "extn": "5421"
      }
   ]
}

SOLUTION

Use default format or use ajax.dataSrc option to define data property containing table data in Ajax response (data by default).

See Data array location for more information.

LINKS

See jQuery DataTables: Common JavaScript console errors for more details.

Find elsewhere

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Stack Overflow

stackoverflow.com › questions › 78264410 › javascript-cannot-read-properties-of-undefined-reading-length-code-does

typeerror - JavaScript - Cannot read properties of undefined (reading 'length') - (Code does work and provided) - Stack Overflow

Themeco

theme.co › support

JS error Uncaught TypeError reading length - Support - Themeco Forum

March 26, 2022 - Hello, on the portfolio page when using the category filter I get this error: jquery-migrate.min.js?ver=3.3.2:2 Uncaught TypeError: Cannot read properties of undefined (reading ‘length’) at init.s.fx.s.Tween.run (jquery-migrate.min.js?ver=3.3.2:2:6779) at u (jquery.min.js?ver=3.6.0:2:59565) at Function.S.fx.tick (jquery.min.js?ver=3.6.0:2:64608) at ot (jquery.min.js?ver=3.6.0:2:58967) And the filter is not working Disabled all other plugins, changed back from child theme, disabled cachin...

Stack Overflow

stackoverflow.com › questions › 14199949 › jquery-and-ajax-cannot-read-property-length-of-undefined

JQuery and Ajax: Cannot read property 'length' of undefined - Stack Overflow

Top answer

1 of 2

You should validate your data before performing operations on it:

UPDATED

 $.ajax({
            url: 'http://www.domain.pro/api/get_post/?post_id=' + postid + '&post_type=news',
            async: false,
            callback: 'callback',
            crossDomain: true,
            contentType: 'application/json; charset=utf-8',
            type: 'POST',
            dataType: 'jsonp',
            timeout: 5000,
            success: function (data, status) {
               if(data != undefined && data.post != undefined){
                 $('#news').append('<div>' + data.post.title + '</div><div>' + data.post.content + '</div><hr/>');
                }
            },
            error: function () {
              output.html('<h1 class="error">There was an error loading the data.</h2>');
            }
          });

2 of 2

you applying $.each on data.post which could be undefined if your data is null. $.each expects and array or collection that is why you getting an error for length. since your data is null hence data.post would be undefined. you could check before applying $.each like

if(typeof data == "object" && data.post)
    //Your code here.

this check should be applied whenever you expect some data from ajax.

WordPress Trac

core.trac.wordpress.org › ticket › 57412

#57412 (Uncaught TypeError: Cannot read properties of undefined (reading 'length'): jquery migrate) – WordPress Trac

I've been trying to fix this for ... jquery migration plugins, but none solve the error on my site. probably a plugin needed for the theme to work is in conflict. this plugin is helvig core. if I go to the web page the problem does not exist, but when I enter the menu and try to go to one of the menu pages it doesn't work and gives me the following error: Uncaught TypeError: Cannot read properties of undefined (reading 'length'...

Velog

velog.io › @easyliving › jquery-오류-Cannot-read-properties-of-undefined-reading-length

jquery - 오류: Cannot read properties of undefined (reading 'length')

Cannot read properties of undefined ... });//function · 위의 경우 $("#id").val() 값에 데이터가 없다면 undefined로 뜨고, 그럼 그 값의 length는 측정이 불가능해진다....

Jquery

forum.jquery.com › portal › en › community › topic › cannot-read-property-length-of-undefined

Cannot read property 'length' of undefined

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Career Karma

careerkarma.com › blog › javascript › typeerror: cannot read property ‘length’ of undefined

Typeerror: Cannot Read Property ‘length’ of Undefined | Career Karma

January 4, 2021 - function readingErrorsAreImportant(a){ if(a.length === 5){ return "Equals five" } else if(a.length > 5) { return "Greater than five" } else { return "Less than five" } } console.log(readingErrorsAreImportant([1, 2, 3, 4, 5])) In a more advanced case where we are accessing an external API, we may want to indicate in our condition that if we get a length of undefined, then we can return something else to display to the user.

jQWidgets

jqwidgets.com › community › topic › cannot-read-property-length-of-undefined

Cannot read property 'length' of undefined - Angular, Vue, React, Web Components, Javascript, HTML5 Widgets

Uncaught TypeError: Cannot read property 'length' of undefined at i.jqx.dataAdapter.dataBind (jqx-all-3.8.0.js:7) at eval (eval at success (contents_1_1.js?dummy=20171017:133), <anonymous>:1:30) at Object.success (contents_1_1.js?dummy=20171017:133) at j (jquery-2.1.1.min.js:2) at Object.fireWith [as resolveWith] (jquery-2.1.1.min.js:2) at x (jquery-2.1.1.min.js:4) at XMLHttpRequest.<anonymous> (jquery-2.1.1.min.js:4)

DataTables

datatables.net › forums › discussion › 56615 › uncaught-typeerror-cannot-read-property-length-of-undefined

Uncaught TypeError: Cannot read property 'length' of undefined — DataTables forums

(anonymous) @ dashboard.js:6 (anonymous) @ dashboard.js:53 jquery.dataTables.min.js:49 Uncaught TypeError: Cannot read property 'length' of undefined at jquery.dataTables.min.js:49 at i (jquery.dataTables.min.js:35) at Object.success (jquery.dataTables.min.js:36) at c (jquery.min.js:2) at Object.fireWith [as resolveWith] (jquery.min.js:2) at l (jquery.min.js:2) at XMLHttpRequest.<anonymous> (jquery.min.js:2)

Stack Overflow

stackoverflow.com › questions › 76735014 › cannot-read-properties-of-undefined-reading-length-show-from-json

Cannot read properties of undefined (reading 'length') show from JSON

$.ajax({ method: 'GET', url: 'https://jsonplaceholder.typicode.com/users', success: function(response) { buildTable(response) } }) function buildTable(data) { var table = document.getElementById('myTable') for (var i = 0; i < data.length; i++) { var row = `<tr> <td>${data[i].name}</td> <td>${data[i].username}</td> <td>${data[i].address.city}</td> </tr>` table.innerHTML += row } } <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <table id="myTable"> </table>

Stack Overflow

stackoverflow.com › questions › 70733011 › can-not-read-properties-of-undefined-reading-length

Can not read properties of undefined (reading 'length')

// KÖLTSÉGVETÉS VEZÉRLŐ var koltsegvetesVezerlo = (function(){ var Kiadas = function (id, leiras, ertek) { this.id = id; this.leiras = leiras; this.ertek = ertek; } var Bevetel = function (id, leiras, ertek) { this.id = id; this.leiras = leiras; this.ertek = ertek; } var adat = { tetelek: { bev: [], kia: [] }, osszegek: { bev: 0, kia: 0 } } return { tetelHozzaad: function(tip, lei, ert) { var ujTetel, ID; ID = 0; // ID létrehozása if (adat.tetelek[tip].length > 0) { ID = adat.tetelek[tip][adat.tetelek[tip].length - 1].id + 1; }else { ID = 0; } // Új kiadás vagy bevétel létrehozás i

Stack Overflow

stackoverflow.com › questions › 72797130 › uncaught-typeerror-cannot-read-properties-of-undefined-reading-length-aut

Uncaught TypeError: Cannot read properties of undefined (reading 'length') . auto-suggestion in Jquery

Top answer

1 of 1

From https://jqueryui.com/autocomplete/ make sure you imported these two CDN

<script src="https://code.jquery.com/jquery-3.6.0.js"></script>
<script src="https://code.jquery.com/ui/1.13.1/jquery-ui.js"></script>

final code would like this

<!doctype html>
<html>

<head>
    <title>Online jQuery Editor</title>
    <script src="https://code.jquery.com/jquery-3.6.0.js"></script>
    <script src="https://code.jquery.com/ui/1.13.1/jquery-ui.js"></script>
</head>

<body>
    <input type="text" id="text" value="text" />
</body>
<script>
    $(document).ready(function () {
        $(function () {
            var list = [
                "one",
                "two",
                "three",
                "four",
            ];
            debugger;
            $("#text").autocomplete({
                minLength: 0,
                delay: 0,
                placeholder: "Select Transporter...",
                source: list
            });
        })
    });
</script>

</html>