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JavaScript array length undefined | Code

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Objects don’t have a length property, that way most time developers get JavaScript array length undefined error.

A simple solution is, if you know you don’t have to worry about hasOwnProperty checks, would be to do this:



JavaScript array length undefined

A simple example code tries to get the length of an object and use it the loop through the object and get values.

<!DOCTYPE html>


    var coordinates = {
     "a": [
     [1, 2],
     [8, 9]

     "b": [
     [5, 8],
     [2, 4]


   for (var i in coordinates) {



JavaScript array length undefined

or Object.keys

var keys = Object.keys(coordinates);

for (var i = 0, len = keys.length; i < len; i++) {

If you’re encountering a situation where the length of a JavaScript array is returning undefined, it could be due to one of the following reasons:

1. Incorrect array initialization: Ensure that you have properly initialized the array using square brackets [] or the Array constructor.

// Correct initialization
const myArray = [];

2. Incorrect assignment of values: Make sure that you have assigned values to the array elements correctly. If you’re using index-based assignment, double-check that you’re not inadvertently assigning values to non-existent indices. Remember that array indices start from 0.

const myArray = [];
myArray[0] = "First element";
myArray[1] = "Second element";
// ...

3. Overwriting the length property: JavaScript arrays have a built-in length property that represents the number of elements in the array. If you accidentally assign a different value to the length property, it can result in unexpected behavior. Ensure that you’re not modifying the length property directly.

const myArray = [1, 2, 3];
myArray.length = 10; // Incorrect assignment

4. Shadowing or overwriting the length identifier: If you have declared a variable or function with the name “length” in the same scope as your array, it can cause confusion and lead to unexpected results. Avoid naming variables or functions the same as existing JavaScript properties or identifiers.

Do comment if you have any doubts or suggestions on this JS array length topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.

OS: Windows 10

Code: HTML 5 Version

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