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105 lines
3.8 KiB
Markdown
105 lines
3.8 KiB
Markdown
# Event-related callback functions
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jsPsych offers the ability to call arbitrary functions in response to certain events occurring, like the end of a trial or when new data is saved. This page summarizes the different opportunities for callback functions and how to specify them.
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---
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## on_data_update
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The `on_data_update` callback can be declared in the `jsPsych.init` method. The callback triggers at the end of a data update cycle. This happens after every trial, after the on_finish (trial) and on_trial_finish events execute, allowing you to modify the data in those callbacks, and then use this callback to store the data. The function will be passed a single argument, which contains the data that was written.
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#### Sample use
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```javascript
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jsPsych.init({
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timeline: exp,
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on_data_update: function(data) {
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console.log('Just added new data. The contents of the data are: '+JSON.stringify(data));
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}
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});
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```
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---
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## on_finish (trial)
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The `on_finish` callback can be added to any trial. The callback will trigger whenever the trial ends. The callback function will be passed a single argument, containing the data object from the trial.
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#### Sample use
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```javascript
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var trial = {
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type: 'image-keyboard-response',
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stimulus: 'imgA.png',
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on_finish: function(data) {
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console.log('The trial just ended.');
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console.log(JSON.stringify(data));
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}
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};
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```
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---
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## on_finish (experiment)
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The `on_finish` callback can be declared in the `jsPsych.init` method. The callback will trigger once all trials in the experiment have been run. The method will be passed a single argument, containing all of the data generated in the experiment.
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#### Sample use
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```javascript
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jsPsych.init({
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timeline: exp,
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on_finish: function(data) {
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console.log('The experiment is over! Here is all the data: '+JSON.stringify(data));
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}
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});
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```
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---
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## on_start (trial)
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The `on_start` callback can be added to any trial. The callback will trigger right before the trial begins. The callback function will be passed a single argument, containing the trial object. If any of the parameters of the trial are functions or timeline variables, these will be evaluated before `on_start` is called, and the trial object will contain the evaluated value. The trial object is modifiable, and any changes made will affect the trial.
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#### Sample use
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```javascript
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var trial = {
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type: 'image-keyboard-response',
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stimulus: 'imgA.png',
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on_start: function(trial) {
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console.log('The trial is about to start.');
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trial.stimulus = 'imgB.png'; // this will change what stimulus is displayed in the trial
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}
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};
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```
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---
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## on_trial_finish
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The `on_trial_finish` callback can be declared in the `jsPsych.init` method. The callback will trigger at the end of every trial in the experiment. If you want a callback to trigger only for the end of certain trials, use the [`on_finish`](#onfinishtrial) callback on the trial object. The callback function will be passed a single argument, containing the data object from the trial.
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#### Sample use
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```javascript
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jsPsych.init({
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timeline: exp,
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on_trial_finish: function(data) {
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console.log('A trial just ended.');
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console.log(JSON.stringify(data));
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}
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});
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```
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---
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## on_trial_start
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The `on_trial_start` callback can be declared in the `jsPsych.init` method. The callback will trigger at the start of every trial in the experiment. The function receives a single argument: a modifiable copy of the trial object that will be used to run the next trial. Changes can be made to this object to alter the parameters of the upcoming trial.
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#### Sample use
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```javascript
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var current_score = 0; // a variable that is updated throughout the experiment to keep track of the current score.
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jsPsych.init({
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timeline: exp,
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on_trial_start: function(trial) {
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trial.data.score_at_start_of_trial = current_score;
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console.log('A trial just started.');
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}
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});
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```
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