JS how to gradually increase/decrease a value

Question

var x = 1000;

setInterval(function(){
x -= 50;
}, x)

In the above code, I want the interval to take the variable that’s inside the function, but it won’t work. If I defined the variable inside the function, then it will be undefined.

Here are a few real life examples of what I’m trying to do:

- Car is parked. Slowly but surely starts going faster and faster, and faster.

- soccer ball is kicked. goes really fast, rolls, slows down, slowly, finally stops.

https://code.sololearn.com/WTB2kqV57G1l/?ref=app

Answer 1

https://code.sololearn.com/WvCLvZ3HlY0e/?ref=app

Answer 2

var x = 1000;
setInterval(func(){
x -= 50;
}, x);

i know the code doesn’t work for what i’m trying to make it do, but here’s the expected output;

every x time, do something;
every 1950 sec do ...
every 1900 sec do ...
every 1850 sec do ...

so by the time it’s 1000 milliseconds, the variable x decreases by 50. So mow it becomes 1950.

then by the time it reaches 1950, decrease again by 50. it becomes 1900. etc