Understanding the 'import.meta' Meta-Property and its '--module' Requirement in TypeScript

What is import.meta?

import.meta is a special object in JavaScript that provides metadata about the current module. It was introduced with the ECMAScript Modules (ESM) and has become a common tool in modern JavaScript and TypeScript environments. Some typical use cases for import.meta include accessing information like the URL of the module or platform-specific metadata.

However, because import.meta is relatively new, it's only supported in specific module systems, and this is where the TypeScript --module option comes into play.

The TypeScript --module Option

The TypeScript compiler (tsc) translates your TypeScript code into JavaScript. During this process, it needs to know which module system to use, and this is determined by the --module option in your tsconfig.json or command-line configuration.

There are several module systems available, such as:

• CommonJS (default in Node.js prior to ES modules)
• ESM (ES2015/ES6 modules)
• AMD
• System
• Node16 and NodeNext (for Node.js module resolution)

The import.meta meta-property is only available in module systems that support ECMAScript Modules (ESM). These include:

• es2020
• es2022
• esnext
• system
• node16
• nodenext

Why the Error Happens

If you're using import.meta in your code but the --module option in your tsconfig.json is set to a module system that doesn't support it (like commonjs), you'll see this error:

"The 'import.meta' meta-property is only allowed when the '--module' option is 'es2020', 'es2022', 'esnext', 'system', 'node16', or 'nodenext'."

This error indicates that TypeScript cannot understand the import.meta syntax unless the module system supports it.

How to Fix the Error

Here are some common solutions:

1. Update Your tsconfig.json to Use a Supported Module System

If you're seeing this error, the simplest fix is to change your tsconfig.json file to use one of the module systems that support import.meta.

Open your tsconfig.json and look for the module option. Update it to one of the supported values, such as es2020 or esnext.

{
    "compilerOptions": {
      "module": "esnext"
    }
  }

This change tells TypeScript to compile your code using an ECMAScript module system that supports import.meta.

2. Use ts-jest for Jest with TypeScript

If you're running tests with Jest and TypeScript, you might run into this issue, especially if you're using ts-jest. To fix it, you can configure ts-jest properly in your Jest configuration to support the correct module system. Make sure your tsconfig.json and Jest configurations are aligned.

The import.meta meta-property is a useful feature in modern JavaScript and TypeScript projects, but it requires the right module system to work. By adjusting your tsconfig.json to use a supported module system like es2020 or esnext, you can avoid the error and take full advantage of import.meta.

References

• https://stackoverflow.com/questions/69716191/the-import-meta-meta-property-is-only-allowed-when-the-module-option-is-ehttps://github.com/kulshekhar/ts-jest/issues/3888
• https://github.com/vuejs/vetur/issues/3708