Pass Variable in Javascript CustomEvent

I recently stumbled across events in Javascript and thought it would be a pretty good method passing Data on certain situations. Now I got the problem, that I dont know how to pass Variables through CustomEvent details correctly. I'd really appreciate if you'd take a look and maybe help me :)

var testVar = 5;
const myEvent = new CustomEvent('testEvent', {
    detail: {
        myVar: testVar
    }
} )
addEventListener('testEvent', (e) =>{
    console.log("testVar:" + e.detail.myVar)
})
dispatchEvent(myEvent)
testVar = 2
dispatchEvent(myEvent)

When I use the function dispatchEvent(myEvent) i thought I'd always get the current Number stored in testVar.

Expected Output:

"testVar : 5"

"testVar : 2"

Actual Output:

"testVar : 5"

"testVar : 5"