Counting trial repetitions

[Euramsia]

Hi, I need a trial counter but my ideas keep not working,

I have a situation where two things need to change independent of each other.
1 different images where an object changes its positions
2 display duration of image

The images have the same difficulty, but should be displayed randomly
The display duration increases gradually and makes the experiment more difficult
One image however, should not be tied to a certain duration

Here an example with the two images that are displayed in one trial for two (out of dozens) of trials.
I thought I would create a counter variable z, a variable durations that contains the times, and then set the counter variable higher each time so that the trial_durations can increase linearly, while the images keep changing randomly.

Also, I would feed the duration to the data, by setting the parameter in the data to the duration that was used, but I'm not sure whether that can work. Otherwise I would need to calculate and round the duration from time_elapsed.

When I do it like below (code doesn't work), the variable z is updated exactly one time from 0 to 2 and then stays at 2. When I try do durations[z] for trial_duration I get an error. And I didn't even try setting the data parameter with durations[z] yet.
(the third segment where participants need to answer is not shown)

	var z = 0;
        var durations = [1000, 1500, 3000, 3500];
	var UFOV_lvl3 = {
        timeline: [
	{
            type: 'image-keyboard-response',
            stimulus: jsPsych.timelineVariable('blank4'),
            choices: jsPsych.NO_KEYS,
	    trial_duration: durations[z],
	    data: jsPsych.timelineVariable('data_blank4'),
	    stimulus_height: 360,
            stimulus_width: 640
         },
	{
            type: 'image-keyboard-response',
            stimulus: jsPsych.timelineVariable('stim4'),
            choices: jsPsych.NO_KEYS,
	    trial_duration: durations[z+1],
	    data: jsPsych.timelineVariable('data_stim4'),
	    stimulus_height: 360,
	    stimulus_width: 640
            on_finish: z += 2
        },
          timeline_variables: [
                 {blank4: 'img/UFOV_blank4.jpg', data_blank4: {test_part: 'blank4', duration: durations[z]}, stim4: 'img/UFOV_stim4.jpg', data_stim4: {test_part: 'stim4', duration: durations[z+1]}},
		 {blank4: 'img/UFOV_blank4.jpg', data_blank4: {test_part: 'blank4', duration: durations[z]}, stim4: 'img/UFOV_stim4.jpg', data_stim4: {test_part: 'stim4', duration: durations[z+1]}}

PS: very nice library and good documentation