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Test a Snap

You can test your Snap by hosting it locally using yarn start, installing it in Flask, and calling its API methods from a dapp.

For end-to-end Snap testing in a Jest environment, use the @metamask/snaps-jest package as follows.

Steps​

1. Install @metamask/snaps-jest​

Install the @metamask/snaps-jest package into your Snap project using Yarn or npm:

yarn add -D @metamask/snaps-jest

or

npm i @metamask/snaps-jest

2. Configure @metamask/snaps-jest​

The easiest way to configure this package is to add it to your Jest configuration as a preset. In the jest.config.js file, add the following:

jest.config.js
module.exports = {
  preset: "@metamask/snaps-jest",
}

This automatically configures Jest to use the @metamask/snaps-jest environment, and to use the @metamask/snaps-jest matchers. You can then run the jest command as usual.

note

@metamask/snaps-jest assumes the Snap is built in the directory you run Jest from. If you use a different directory, you can specify the path using the root option, or by running your own HTTP server. It's currently not possible to use @metamask/snaps-jest with a Snap that is not built.

If you don't use the package as a preset, you can alternatively add the @metamask/snaps-jest environment and matchers to your Jest configuration manually:

jest.config.js
module.exports = {
  testEnvironment: "@metamask/snaps-jest",
  setupFilesAfterEnv: ["@metamask/snaps-jest/dist/cjs/setup.js"],
}

You can pass any Jest options to the test environment by adding a testEnvironmentOptions property to your Jest configuration. For example:

jest.config.js
module.exports = {
  preset: "@metamask/snaps-jest",
  testEnvironmentOptions: {
    // Options go here.
  },
}

All options are optional.

3. Use @metamask/snaps-jest​

Use the package by calling any of the API methods. You can:

You can also use Jest matchers to assert that a response from a Snap matches an expected value.