Cypress, page content and variables - asynchronous

Question

Right or wrong: In Cypress, its impossible to read a value on page X, then keep this value and compare it to a value on page Y.
I can read a value from the page and log it:
cy.get('[data-e2e-selector=whatever]').then(elm => cy.log('Value from page X : ' + elm))
or, for instance, the number of elements with similar or partially matchin selectors:
cy.get('[data-e2e-selector=^whatever]').then(elm => cy.log('Number of elements like this on page X: ' + elm.length))
Hoever, I cannot create a variable of this, because of the asynchronous way Cypress runs. Right? Any value created to just be blank.
Nor can I pass the value read to a method, which in turn compares it to the value on the next page:
compareToValueOnNextPage(cy.get('[data-e2e-selector=^whatever]').then(elm => elm.length));
compareToValueOnNextPage(value: number) { // NB: Not sure if it's a number or string yet...
cy.get('[data-e2e-selector=^whateverNextPage]').then(elm => elm.length).should('have.length', 4)
}
Basically, if I want to compare values, the either have to be on the same page, or they need to be hard-coded. This is a huge limitation when actually end-to-end testing some applications. Very often, a value is created on page X, based on input which should in many cases ba random, thus creating a dynamic value in the test. Then, on page Y, that same value is fetch (from the backend) or shown in some other way, in a Summary etc. And, naturally, I want to compare the value shown on page X to the one shown on page Y/Summary. But this is not possible, due to the very unit-testing thinking that seems to be the foundation for Cypress.
Or am I missing something here? Are there ways around this that aren't ugly/smelly? I think it's possible to store the value on page X in a file, then read that file on page Y. However, Cypress seems to only have one option when reading the file, and that's reading the whole file and looking for a match. So that file would be a mess. Or I'd need several files.
I realize this is kind of trying to impose non-functional ways on a quite functional and asynchroeous technology. However, if it's not possible to "keep" a value and use it later, it's very limiting when it comes to end-to-end testing (even though it's not when the testing is unit-based on frontend components).
UPDATE:
As per Kerrry's suggestion in the answer below:
cy.get('[data-e2e-selector=dellan-accordion]')
.then(elm => cy.wrap(elm.length).as("myVariableName"));
-GO TO NEXT PAGE-
cy.get('[data-e2e-selector=betalingsplan-dellan]')
.then(elm => elm.length)
.should('have.length', myVariableName)
This yeilds "expected 4 to have property 'length'.
This means, obviously, that I cannot get the length of the length.
So I replace 'have.length' with 'eq':
cy.get('[data-e2e-selector=betalingsplan-dellan]')
.then(elm => elm.length)
.should('have.length', myVariableName)
And I get the following error:
expected 4 to equal 0
So, it seems that the first variable - myVariable - is gone after the first cy.get().
If I do everything inside one get (have another get inside that, where I go to the next page AND get the count of the elements), then it works. But the way Kerry shows it, it would be much more flexible. But, alas, the above error.

Answer 1

As jonrsharpe mentioned in the comments, please read the Cypress document on variables and aliases thoroughly. This is a core concept of Cypress, and it will give you a solid understanding of how to implement variables and carry the values between test steps.
The reader's digest example of what you how you can achieve is this:
cy.get('[data-e2e-selector=^whatever]')
.then(elm => cy.wrap(elm.length).as("myVariableName"));
What this is doing is cy.wrap will yield the value from elm.length in a Cypress command chain, which then allows you to assign it to an alias "myVariableName".
In your following test step where you want to compare the value on a separate page, you would then access the alias' value in one of two ways:
Using this.
cy.get('[data-e2e-selector=^whateverNextPage]')
.then(elm => elm.length)
.should('have.length', this.myVariableName)
OR
via cy.get()
cy.get("#myVariableName").then(function(variableValue){
cy.get('[data-e2e-selector=^whateverNextPage]')
.then(elm => elm.length)
.should('have.length', variableValue)
})