Fixing 'TypeError: InnerDecoder' In Polkadot-API

Encountering errors while working with blockchain technologies is a common challenge for developers. One such error that you might stumble upon while using Polkadot-API is the infamous TypeError: innerDecoder is not a function. This error can be cryptic and frustrating, but with a clear understanding of its causes and potential solutions, you can effectively resolve it.

What Does This Error Mean?

When you encounter the TypeError: innerDecoder is not a function error in the Polkadot-API context, it typically indicates an issue with the decoding process of data structures. Polkadot-API relies heavily on scale-ts, a library for encoding and decoding data in the SCALE (Substrate Codec Library) format, which is fundamental to Substrate-based blockchains like Polkadot. This error usually arises when the decoder for a specific data type, particularly within complex structures like enums or variants, is either missing or incorrectly configured.

In simpler terms, the API is trying to interpret a piece of data but can't find the right instructions (the decoder) to understand it. This often happens when there's a mismatch between the expected data structure and the actual data received, or when the necessary codec definitions are not correctly loaded or specified.

Diving Deeper into the Causes

Several factors can contribute to this error. Let's explore some of the most common scenarios:

1. Mismatched Types and Codecs: At the heart of this error is often a mismatch between the data type defined in your application and the codec (encoder/decoder) that's supposed to handle it. If you're working with custom types or complex structures, ensuring that the correct codecs are registered and used is crucial.

2. Incorrect Metadata: Polkadot-API uses metadata to understand the structure of the blockchain's state and transactions. If the metadata is outdated, corrupted, or doesn't accurately reflect the current state of the chain, it can lead to decoding errors. Metadata discrepancies can occur after a chain upgrade or if the API is not properly synchronized with the chain's latest state.

3. Enum and Variant Issues: Enums and variants, which are data structures that can hold one of several possible types, are common culprits. The error often points to issues within the scale-ts library's Enum.ts or Variant.ts files because these structures require precise decoding logic to handle their multiple potential forms.

4. Library Version Conflicts: Incompatibility between different versions of scale-ts, Polkadot-API, and other related libraries can also trigger this error. Ensuring that your project's dependencies are aligned and compatible is essential for smooth operation.

5. Custom Type Definitions: When you introduce custom types or chain extensions, you need to provide explicit type definitions and codecs. If these definitions are missing or incorrectly implemented, the decoder won't know how to handle the custom data, leading to the innerDecoder error.

Diagnosing the Error

Before diving into solutions, it's important to diagnose the specific cause of the error in your context. Here are some steps to help you pinpoint the problem:

1. Examine the Stack Trace: The stack trace provides valuable clues about where the error is occurring. Pay close attention to the file names and line numbers mentioned, as they can indicate the specific part of your code or the libraries where the issue lies. In the provided example, the stack trace points to scale-ts's Enum.ts and Variant.ts, suggesting a problem with enum or variant decoding.

2. Review Your Type Definitions: If you're using custom types, carefully review their definitions to ensure they match the chain's specifications. Check for typos, incorrect type mappings, and missing codecs.

3. Check Metadata Consistency: Ensure that the metadata you're using is up-to-date and consistent with the chain's current state. You can fetch the latest metadata from the chain using Polkadot-API and compare it with your local copy.

4. Inspect the Data: If possible, inspect the raw data that's being decoded. This can help you identify if the data is malformed or doesn't match the expected structure.

5. Isolate the Issue: Try to isolate the specific part of your code that's causing the error. Simplify your code by removing non-essential parts and see if the error persists. This can help you narrow down the source of the problem.

Solutions and Workarounds

Once you have a better understanding of the error's cause, you can start applying solutions. Here are some strategies to address the TypeError: innerDecoder is not a function error:

1. Update Dependencies: Ensure that you're using the latest versions of Polkadot-API, scale-ts, and other related libraries. Outdated dependencies can often lead to compatibility issues. Use your package manager (e.g., npm, yarn) to update your dependencies to the latest stable versions.

   npm update
   # or
   yarn upgrade
   

2. Verify Custom Type Definitions: If you're working with custom types or chain extensions, double-check your type definitions. Make sure they accurately reflect the chain's specifications and that you've provided the necessary codecs. You might need to register custom codecs with scale-ts if they're not automatically recognized.

3. Refresh Metadata: Ensure that you're using the most recent metadata from the chain. You can fetch the latest metadata using Polkadot-API and update your local copy. This is particularly important after a chain upgrade.

   const metadata = await api.rpc.state.getMetadata();
   // Use the metadata in your API instance
   

4. Correct Enum and Variant Handling: If the error points to enum or variant decoding, carefully review how you're handling these data structures. Ensure that you're correctly specifying the variants and that the decoder knows how to handle each case. This might involve providing custom decoding logic for specific variants.

5. Address Version Mismatches: If you suspect version conflicts between libraries, try to align the versions. Check the compatibility matrix of Polkadot-API and scale-ts to ensure that you're using compatible versions. You might need to downgrade or upgrade certain libraries to resolve the conflict.

6. Provide Explicit Codecs: In some cases, you might need to provide explicit codecs for certain types, especially if they're complex or custom. This involves creating custom encoder and decoder functions that tell scale-ts how to handle the data. Refer to the scale-ts documentation for guidance on creating custom codecs.

7. Examine Chain Extensions: If you're using custom chain extensions, verify that they are correctly implemented and that the API is properly configured to handle them. This often involves providing specific logic for encoding and decoding the extension's data.

Applying the Solutions to the Provided Example

Let's revisit the code example provided and see how we can apply these solutions.

The code snippet demonstrates an attempt to submit a transaction with custom signed extensions on a People chain. The error occurs during the signing process, specifically when decoding the transaction data.

Based on the stack trace and the code, the issue likely stems from one of the following:

• Incorrect PeopleLiteAuth Value: The code explicitly sets PeopleLiteAuth's value to new Uint8Array([1]), which is flagged as "THE WRONG VALUE". This suggests a potential mismatch between the expected value and the actual requirement of the chain extension.

• Codec Mismatch for Custom Extensions: The custom signed extensions (VerifyMultiSignature, AsPerson, PeopleLiteAuth) might not have the correct codecs registered or might be using outdated definitions.

To resolve this, we can take the following steps:

1. Correct PeopleLiteAuth Value: Determine the correct value for the PeopleLiteAuth extension. This might involve consulting the chain's documentation or the developers of the People chain.

2. Verify Custom Extension Codecs: Ensure that the codecs for the custom extensions are correctly defined and registered with the API. This might involve creating custom encoder and decoder functions for these extensions.

3. Update Dependencies: As a general practice, update Polkadot-API, scale-ts, and other related libraries to their latest stable versions.

4. Refresh Metadata: Fetch the latest metadata from the People chain to ensure that the API is using the most up-to-date information about the chain's state and extensions.

Best Practices to Avoid This Error

Preventing the TypeError: innerDecoder is not a function error is often easier than fixing it. Here are some best practices to help you avoid this issue:

• Stay Updated: Regularly update your dependencies to the latest stable versions. This ensures that you're using the most recent bug fixes and improvements.

• Use Consistent Type Definitions: Maintain consistent type definitions throughout your project. If you're using custom types, define them clearly and use them consistently across your codebase.

• Validate Metadata: Periodically validate your metadata against the chain's current state. This helps you catch discrepancies early on.

• Test Custom Codecs: If you're creating custom codecs, thoroughly test them to ensure they correctly encode and decode data.

• Consult Documentation: Refer to the documentation of Polkadot-API, scale-ts, and other related libraries for guidance on type definitions, codecs, and metadata handling.

• Engage with the Community: If you're stuck, don't hesitate to reach out to the Polkadot community for help. There are many experienced developers who can offer guidance and support.

Conclusion

The TypeError: innerDecoder is not a function error in Polkadot-API can be a challenging issue to tackle, but with a systematic approach and a clear understanding of its causes, you can effectively resolve it. By carefully examining your type definitions, metadata, and dependencies, and by following the best practices outlined in this article, you can minimize the chances of encountering this error and ensure the smooth operation of your Polkadot-based applications.

For further information on Polkadot API and related topics, you can visit the official Polkadot Wiki.