What is the test command while creating package.json?

The test command is the command that is run whenever you call npm test.

This is important when integrating with continuous integration/continuous deployment tools (such as jenkins, codeship, teamcity).

Example:
- say you deploy a project to AWS or some other cloud hosting provider,
- you can set up your infrastructure to automatically run npm test.
- If there are problems within those tests, your ci/cd will automatically rollback before deploying.

To execute tests
You can use karma, jest, or selenium/nightmare/phantomjs or about any other test scripting library/framework that allows you to write and execute tests and then set the required command in scripts.test and finally run it from npm test.

Assuming you mean scripts.test:

"scripts" : {
  "test" : "echo \"Error: no test specified\" && exit 1"
}

This field contains the program(/command line) that should run when you call npm test. Typically, that program is a test-runner like mocha, ava, jest, ...

The default value is a placeholder that prints an error message (try running npm test in the same directory as your package.json).