can only be default-imported using the 'esmoduleinterop' flag

The error message "can only be default-imported using the 'esmoduleinterop' flag" typically arises in TypeScript when you're trying to import a module that has a default export, but the import statement is not using the default import syntax. This can happen if the module's type definition file (.d.ts) specifies that it should only be imported using the default keyword.

Basic Concept

In JavaScript and TypeScript, modules can be imported in two main ways:

1. Default Imports: When a module exports something as its default export, you import it using the import keyword without curly braces.
2. Named Imports: When a module exports multiple items, you import them using curly braces and the specific name of the export.

Why This Error Occurs

The error occurs because the module's type definition might be written in a way that enforces the use of default imports. This is often done to ensure compatibility with how the module is actually structured or to align with certain coding standards.

How to Resolve This Issue

To resolve this issue, you need to adjust your import statement to use the default import syntax. Here’s how you can do it:

Example Scenario

Suppose you have a module myModule.js that looks like this:

// myModule.js
export default function() {
    console.log("This is the default export");
}

And you are trying to import it like this:

import { default as myFunction } from './myModule';

This will trigger the error. Instead, you should use:

import myFunction from './myModule';

Using esModuleInterop

If you prefer to stick with named imports or if the module's type definition insists on using esModuleInterop, you can enable this flag in your tsconfig.json:

{
  "compilerOptions": {
    "esModuleInterop": true
  }
}

With esModuleInterop enabled, TypeScript will automatically handle the interoperability between CommonJS and ES modules, allowing you to use named imports even for default exports:

import { default as myFunction } from './myModule'; // This will work with esModuleInterop enabled

Types of Modules and Their Imports

• ES Modules: Use import and export keywords.
• CommonJS Modules: Use require and module.exports.

Application Scenarios

• Libraries: Many modern JavaScript libraries are written using ES module syntax and thus benefit from clear import statements.
• Large Projects: Using consistent import styles can improve readability and maintainability.

Conclusion

Understanding how to correctly import modules, especially when dealing with default exports, is crucial for writing clean and maintainable TypeScript code. By adjusting your import statements or enabling the esModuleInterop flag, you can avoid common pitfalls related to module imports.

This approach ensures compatibility across different environments and adheres to best practices in modern JavaScript development.