How to Fix "Cannot use namespace 'jest' as a value" TypeScript Error?

Why Does This Happen?

When you use jest from the global namespace, you might see the "Cannot use namespace 'jest' as a value" TypeScript error:

// Error: Cannot use namespace 'jest' as a value
const foo = jest.fn();

This error happens because TypeScript is unable to find the appropriate type information for Jest. TypeScript needs the type information to understand the shape of the jest global. Without the appropriate type definitions, TypeScript doesn't know what properties and methods the jest global has.

How to Fix the Issue?

To fix this issue, you must install the Jest type definitions using the @types/jest package. You can use the following command to install these type definitions:

npm install --save-dev @types/jest

Or, with yarn:

yarn add --dev @types/jest

By adding @types/jest as a development dependency, you are including the necessary TypeScript type definitions for Jest in your project, allowing TypeScript to understand the structure of global variables and functions related to Jest. This, in turn, resolves the "Cannot use namespace 'jest' as a value" TypeScript error.