# Event-related callback functions

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.

---

## on_data_update

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.

#### Sample use
```javascript
jsPsych.init({
  timeline: exp,
  on_data_update: function(data) {
    console.log('Just added new data. The contents of the data are: '+JSON.stringify(data));
  }
});
```
---

## on_finish (trial)

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.

#### Sample use
```javascript
var trial = {
  type: 'single-stim',
  stimulus: 'imgA.png',
  on_finish: function(data) {
    console.log('The trial just ended.');
    console.log(JSON.stringify(data));
  }
};
```
---

## on_finish (experiment)

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.

#### Sample use
```javascript
jsPsych.init({
  timeline: exp,
  on_finish: function(data) {
    console.log('The experiment is over! Here is all the data: '+JSON.stringify(data));
  }
});
```

---

## on_trial_finish

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.

#### Sample use
```javascript
jsPsych.init({
  timeline: exp,
  on_trial_finish: function(data) {
    console.log('A trial just ended.');
    console.log(JSON.stringify(data));
  }
});
```
---

## on_trial_start

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. There are no parameters passed to the callback function.

#### Sample use
```javascript
jsPsych.init({
  timeline: exp,
  on_trial_start: function() {
    console.log('A trial just started.');
  }
});
```