How will we call a function written in a separate file from a Cypress test?

Cypress includes an external library file, located by default at (path to project)/support/index.js.

This file is included in all test files, so you can put shared code here.

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You can also include other files in the support folder with the following code in index.js:

import './MySupportFile.js'

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When including an external test file, simple function definitions do not seem to carry over. I'm not exactly sure why. Inserting functions into the cy object has worked for me:

cy.myFunction = greeting => {
    console.log(greeting);
};

cy.myNamespace = {};

cy.myNamespace.myNamespacedFunction = () => {
    console.log("hi");
};

These functions in the cy object will carry over into any /integration files you have.

Add a new file to cypress/support/, e.g. cypress/support/functions.js

cy.myproject = {
    makeUniqueUsername: () => {
        return 'cypress-test-' + Cypress.moment().format("YYMMDD-HHmmss");
    }
}

Include it by adding a reference to it in cypress/support/index.js

import './functions'

Then call your function from any test

describe('registration', function() {
    it('can register', function() {
        let username = cy.myproject.makeUniqueUsername();
        cy.visit('/register');
        cy.get('#username').type(username);
        // etc...
    })
});

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When considering plain functions versus cypress "custom commands" note that the cypress documentation encourage plain functions where custom commands aren't necessary in single spec files, though I don't agree with them that you shouldn't DRY test code.

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Thanks to the other answers for getting me to this. The main addition is making it step by step, and keeping custom functions out of the global cy.* namespace.

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Note: By doing it this way as opposed to registering a command via Cypress.Commands.add you lose logging and time-travel in the Cypress UI. (Thanks @conny). It is up to you and your team which you value more for each case, the explicit steps or the IDE/linter/etc support from using plain js. Registering a command makes it an explicit step in the test rather than just a supporting function.

From https://docs.cypress.io/api/cypress-api/custom-commands.html

Put this in your support/commands.js file:

Cypress.Commands.add('subValues', (a, b) => { return a - b });

Put this in your support/index.js file, if it isn't already there (it should be):

import "./commands";

Call it in your test like so:

describe ('Calling a function', function(){
  it('Call the Subtract function and asert the calculation', function(){
    cy
      .subValues(15, 8)
      .should('eq', 7) // true        
    });
});