fixation cross in jsPsych

Question

I'm trying to add a fixation cross to an experiment with jsPsych. I'm pretty new to js.

Here is what I have:

var test_stimuli = [
      {
        image: "img/blau_links.png",
        data: { correct_response: 37 }
      },
      {
        image: "img/blau_rechts.png",
        data: { correct_response: 37 }
      },
      {
        image: "img/gruen_links.png",
        data: { correct_response: 39 }
      },
      {
        image: "img/gruen_rechts.png",
        data: { correct_response: 39 }
      }
];



var all_tests = jsPsych.randomization.repeat(test_stimuli, 50, true);

var post_test_gap = function() {
    return Math.floor( Math.random() * 1500 ) + 750;
}

var test_block = {
    type: "single-stim",
    stimuli: all_tests.image,
    choices: [37, 39],
    data: all_tests.data,
    timing_response: 1500,
    timing_post_test: post_test_gap
};

var fixation_trial = {
    type: 'single-stim',
    stimuli: ['<p style='text-align:center'>+</p>'],
    is_html: true,
    timing_response: 500,
    timing_post_trial: 0,
    choices: 'none'
}

var experiment = [];
    experiment.push(instructions_block);
    experiment.push(trial_block);
    experiment.push(posttrial_block);
    experiment.push(fixation_trial);
    experiment.push(test_block);
    experiment.push(debrief_block);

    /* start the experiment */
    jsPsych.init({experiment_structure: experiment,
      on_finish: function() {
        jsPsych.data.displayData();
      }

What I want now is to add a fixation cross before every trial of the testblock. In the current form it is only displayed once after all stimuli. Any help is appreciated.