Return empty arrays for performance.getEntries, other relevant methods

[flotwig]

Purpose (TL;DR)

Fix issues caused by applications which expect performance.getEntries, performance.getEntriesByName, and performance.getEntriesByType to return arrays (such as Facebook SDK - downstream issue: cypress-io/cypress#3625)

Right now, lolex clobbers these with () => undefined. Just using () => [] instead would fix this.

Background (Problem in detail)

MDN page for Performance functions: https://developer.mozilla.org/en-US/docs/Web/API/Performance

Only 5 have non-undefined return types. The other 2 (besides getEntries.*) are:

• now() - already done by lolex
• toJSON() - not handled, not sure if there's a good way to mock this value

Solution

Just have those 3 methods return [] when lolex.install() is called, instead of undefined

[fatso83]

I am just wondering about the cleanup bit. Will the original functions now be "restored"?

[flotwig]

Just added a test that the original functions are restored, it does appear to work.

I also have a test for Cypress that runs in actual Chrome that shows the restoration working IRL: https://github.com/cypress-io/cypress/pull/4108/files#diff-c0a131a0b58412db68e166e9d7a16383R75

[SimenB]

I think he asked about the restoration within tests since you override and delete from prototype

[flotwig]

Hmm, I don't see what you mean. Is there something else that needs to be tested?

[SimenB]

dunno, maybe something like

const origGetEntries = Performance.prototype.getEntries;
const origGetEntriesByName = Performance.prototype.getEntriesByName;
const origGetEntriesByType = Performance.prototype.getEntriesByType; =

then restore them instead of the delete? not sure. If the Performance object is already a fake, then it doesn't matter and I probably misunderstood 🙂

[flotwig]

I could be wrong, but it looks like lolex just uses the prototype to know what methods to override on the global performance, so I don't think anything additional needs to be tested here.

[SimenB]

@fatso83 is this ready?

[fatso83]

Sure. If you and Ben thinks this is fine, go ahead. I didn't mean to be a blocker (too busy these days with 👶)

[benjamingr]

I think this is fine :]

[benjamingr]

I can do a release next week when I'm back from DevDays

[flotwig]

@benjamingr Ping, would be awesome to see this released so we can incorporate this fix downstream :)

[fatso83]

doing it