When you encounter the “TypeError: Cannot read properties of undefined (reading ‘filename’)” error message in JavaScript, it means that you are trying to access a property or method of an undefined value or variable.
To better understand this error, let’s consider a few examples.
Example 1:
const person = {};
console.log(person.address); // TypeError: Cannot read properties of undefined (reading 'address')
In this example, we have an empty object assigned to the variable person. When we try to access the address property of person, which does not exist, we get the mentioned error. This happens because the address property is undefined.
Example 2:
const numbers = [1, 2, 3];
console.log(numbers.length); // 3
console.log(numbers.filename); // TypeError: Cannot read properties of undefined (reading 'filename')
In this example, we have an array of numbers assigned to the variable numbers. We can access the length property of the array without any issues. However, when we try to access the non-existent filename property, we encounter the error. Again, this happens because the filename property is undefined.
To avoid this error, you should ensure that you are accessing properties or calling methods on defined values. You can use conditional statements, such as if statements, to check if a value is defined before accessing its properties.
if (person !== undefined) {
console.log(person.address); // may or may not log the address depending on its existence
}
In the above example, we check if the person object is not undefined before accessing the address property. This helps prevent the TypeError.
Overall, the “TypeError: Cannot read properties of undefined (reading ‘filename’)” error message indicates that you are trying to work with properties or methods on undefined values. By checking if a value is defined before using it, you can avoid this error.