Question
Answer and Explanation
The error "TypeError: cannot read property 'endsWith' of undefined in vendor.js at line 3159" typically arises in JavaScript code, especially when dealing with third-party libraries or frameworks, like those often bundled into a vendor.js file. Here's a breakdown of why this occurs and how to approach debugging it:
Understanding the Error
The error message itself is quite explicit: it's trying to call the endsWith() method on something that is undefined. This means that, at line 3159 of your vendor.js file, a variable expected to be a string is actually not defined or has a value of undefined.
Common Causes
1. Missing Data: The most common reason is that the code is expecting to receive some data (often from an API, a configuration file, or another part of your application), but that data is not available, or a property you're trying to access in an object is not present.
2. Asynchronous Issues: In many front-end applications, data is fetched asynchronously. If the code that attempts to call endsWith() runs before the data is fetched, you'll encounter this issue. Promises or async/await should be carefully managed.
3. Incorrect Data Structure: Sometimes, the data received from an API or data source might not be in the expected format. For instance, a property that should have been a string is missing or is an object or array instead.
4. Bug in a Third-Party Library: Rarely, a bug in a third-party library can cause this. The vendor.js is usually a compiled version of such a library.
Debugging Steps
1. Inspect the `vendor.js` Code: Open the vendor.js file (or the source map if available) in your browser's developer tools. Navigate to line 3159. Try to understand the context. What data is the endsWith method being called on? This is usually the variable immediately preceding .endsWith().
2. Use Breakpoints: Set a breakpoint at line 3159 (or near it) in the developer tools and reload the page. This will pause execution, allowing you to inspect the value of the variable that's throwing the error. Is it undefined? If so, you've found your culprit.
3. Trace the Source: Once you've found the undefined value, try to trace where it comes from. What part of your application code or the library is supposed to provide this data? There could be a configuration error or an issue with data loading.
4. Error Handling: Add error handling checks to your code, such as using conditional statements (if) to make sure your variable is defined before calling methods such as endsWith, or if it's an API call, using try/catch blocks, so the code handles undefined properties gracefully. For example:
if (myVariable && typeof myVariable.endsWith === 'function') {
myVariable.endsWith('.txt');
}
5. Check API Responses: If the data comes from an API call, check if it's returning the correct data structure. Use the browser's network tab to see the responses and use a json parser if necessary.
Example Scenario
Imagine the vendor.js contains something like:
var filePath = config.file.path;
if(filePath.endsWith('.txt')){//error here if filePath is undefined}
//other code
If config.file or config.file.path is missing, filePath will be undefined, causing the error. Debugging would involve ensuring config.file and config.file.path exist and are properly loaded.
By following these steps, you can effectively identify and resolve the "TypeError: cannot read property 'endsWith' of undefined" error.