Use length property with the isArray() method to check an array is undefined in JavaScript.
if (!Array.isArray(array) || !array.length) {
// array does not exist, is not an array, or is empty
// ⇒ do not attempt to process array
}Or do the check for undefined first, array empty or does not exist
if (array === undefined || array.length == 0) {
// array empty or does not exist
}Use Array some method to check array has an undefined value, it returns true if any item in the array is undefined.
JavaScript array is undefined
Simple example code.
<!DOCTYPE html>
<html>
<body>
<script>
const arr = [1, 2, 3, 4, 5, 6, 7, 8, undefined, null, 0, "", -0];
var res = arr.some(item => item === undefined);
console.log("Array has undefined values",res)
</script>
</body>
</html>
Output:
Filter undefined values from an array in Javascript
The filter() method creates a new array with all elements that pass the test implemented by the provided function. So, if x !== undefined, the object becomes part of the new array. If x === undefined, it is left out of the new array.
<script>
const arr = [1, 2, 3, 4, 5, 6, 7, 8, undefined, null, 0, "", -0];
const filter = arr.filter((x) => Boolean(x));
console.log(filter)
</script>Output: [ 1, 2, 3, 4, 5, 6, 7, 8 ]
If you’re encountering an error where a JavaScript array is undefined, it means that the variable you’re trying to access or manipulate as an array doesn’t have a defined value.
Uninitialized array variable: Ensure that you have properly declared and initialized the array variable before using it.
var myArray = []; // Initialize an empty array
console.log(myArray[0]); // Output: undefined
Incorrect array index: Double-check that you’re using a valid index to access elements within the array. Remember that array indices start from 0, so trying to access an element at an index beyond the array’s length will result in undefined.
var myArray = [1, 2, 3];
console.log(myArray[3]); // Output: undefined (array has length 3, so index 3 is out of range)
Array element assignment: If you’re trying to assign a value to an element within an array but encounter undefined, verify that the array exists and has sufficient length to accommodate the assigned index.
var myArray = [];
myArray[0] = 42; // Assign a value to the first element
console.log(myArray); // Output: [42]
Comment if you have any doubts or suggestions on this JS Array topic.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version