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TypeScript: can only be default-imported using the 'esModuleInterop' flag

TypeScript: can only be default-imported using the 'esModuleInterop' flag

Matthew C.

The Problem

When using the TypeScript compiler to create JavaScript files from TypeScript files, in a process known as transpiling, you may get the following error in your terminal:

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This module is declared with 'export =', and can only be used with a default import when using the 'esModuleInterop' flag.

This error occurs when the TypeScript config esModuleInterop flag either isn’t set or is set to false, and a non-ES module package is imported, by default, in an ES module file.

TypeScript treats CommonJS, AMD, and UMD modules similarly to how it treats ES modules. This can cause issues with importing modules from CommonJS modules into ES modules, as there may be inconsistencies with the default export type. The default export is the exports object from Node version 14.13.0. However, different JavaScript bundlers may use exports.default as the default export.

The Solution

Set the esModuleInterop flag to true in your tsconfig.json file. Its default value is true if the TypeScript config option module is node16 or nodenext.

Setting the esModuleInterop flag to true in your tsconfig.json file also adds extra interoperability code to the transpiled JavaScript to handle different types of default imports from CommonJS modules.

However, you may have esModuleInterop set to true but still find that you get the error when you run the tsc command and specify the input file as follows:

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tsc server.ts

This is because when you specify input files, tsconfig.json files are ignored.

Next, set the output folder for the transpiled JavaScript:

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"outDir": "./dist",

Transpile using the tsc command and run the transpiled JavaScript code:

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tsc && node ./dist/server.js

You can also use Nodemon to transpile and run your TypeScript code. It automatically restarts your Node.js application when file changes are detected:

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nodemon ./server.ts

Nodemon version 1.19 added inbuilt support for TypeScript files. It uses ts-node to run your Node.js application.

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