Two movers and a counter

Challenge 15 of 18

Everything so far in one picture: a shape going right, a shape coming back, and a number counting that is not a position at all. All three use the same pattern of a start and a step.

Set the two speeds, one of them negative, and the counter's step.

Your code JavaScript

Set the two speeds, one of them negative, and the counter's step.

Console print() output

0 / 40 frames · 400×400

Target

Yours

Results

Press Run to check your code

Penalties: none this run

Report this challenge

Lesson Moving Right

A number that remembers.

var x = 0; above everyFrame makes the number once, before the first frame. x = x + 6; inside everyFrame changes it on every frame after that. That second line looks wrong the first time you read it, because it looks like a claim about what x equals. It is not — it is an instruction, and the right-hand side happens first: take what x is, add 6, and make that the new x. Read it right to left and it makes sense.

Above, not inside.

If the var line goes inside everyFrame, the number is made again from scratch before every frame, so it is set back to its starting value every time and the shape stands still. Challenge 4 shows both at once, side by side. There is another block that runs once, start(), which you met when you set the canvas size — that one is for setting the sketch up, not for making numbers. A number made in there is invisible to everyFrame.

Two things to look at.

Every challenge in this topic prints its number along the bottom, on the target as well as on yours, so you can compare numbers instead of squinting at pixels. Drag the bar under the target to any frame and read what the number had reached — challenge 13 cannot be done any other way. And Try runs your sketch on its own, with no marking and no time limit, if you just want to watch it go.