How to Solve TypeError: Undefined is not a function in Hermes JS Engine

If you are a JavaScript developer, you may have encountered the “TypeError: Undefined is not a function” error at some point in your coding journey. This error typically occurs when you are trying to call a function or method that does not exist or is not defined.

While this error can be frustrating, it is important to understand that it is not insurmountable. With a bit of troubleshooting and debugging, you can identify and resolve the root cause of the error. In this article, we will specifically address how to solve this error in the Hermes JS engine.

Understanding the Error

When you encounter the “TypeError: Undefined is not a function” error in Hermes, it means that you are attempting to call a function that has not been defined. This could be due to a typo in the function name, a missing import statement, or a problem with the scope in which the function is being called.

Troubleshooting Steps

Here are some steps to follow in order to troubleshoot and resolve the “TypeError: Undefined is not a function” error in Hermes:

1. Check for typos: Double-check the spelling and capitalization of the function name to ensure that it matches the actual function definition.
2. Verify function availability: If the function is part of a library or module, make sure that it is properly imported or included in your code.
3. Examine the function scope: Ensure that the function is being called within the correct scope and that it is accessible from the calling code.
4. Use console.log: Insert console.log statements in your code to log the values of variables and objects leading up to the point of the error. This can help you pinpoint the source of the problem.
5. Debug with breakpoints: If you are using a development tool with a debugger, set breakpoints in your code and step through it to identify where the error occurs.

Sample Code

Here is a simple example of how the “TypeError: Undefined is not a function” error might occur in Hermes:

    
      const myObject = {
        name: 'John',
        sayHello: function() {
          console.log('Hello, ' + this.name);
        }
      };

      // Attempting to call the sayHello function
      myObject.sayHello(); // Throws TypeError: Undefined is not a function
    
  

In this example, the myObject object does not actually have a method named sayHello defined, which causes the error when it is called.

Final Thoughts

By following the troubleshooting steps outlined in this article, you should be able to identify and resolve the “TypeError: Undefined is not a function” error in Hermes. Remember to carefully review your code, use debugging tools, and consult the Hermes documentation and community resources for further assistance.