# The jsPsych core library

---
## jsPsych.addNodeToEndOfTimeline
```
jsPsych.addNodeToEndOfTimeline(node_parameters, callback)
```

### Parameters

Parameter | Type | Description
--------- | ---- | -----------
node_parameters | object | An object defining a timeline. It must have, at a minimum, a `timeline` parameter with a valid timeline array as the value for that parameter.
callback | function | An optional callback function. If adding the node to the timeline requires any preloading of media assets, this callback will be triggered after preloading is compelte.

### Return value

None.

### Description

Adds the timeline to the end of the experiment.

### Examples

#### Without callback

```javascript
var trial = {
  type: 'html-keyboard-response',
  stimulus: 'This is a new trial.'
}

var new_timeline = {
  timeline: [trial]
}

jsPsych.addNodeToEndOfTimeline(new_timeline)
```

### With callback

```javascript
var first = {
  type: 'html-keyboard-response',
  stimulus: 'first trial; new trial added when on_finish is called',
  on_finish: function(){
    jsPsych.pauseExperiment();
    jsPsych.addNodeToEndOfTimeline({
      timeline: [{
        type: 'image-keyboard-response',
        stimulus: 'img/happy_face_4.jpg'
      }]
    }, jsPsych.resumeExperiment)
  }
}
```

---
## jsPsych.currentTimelineNodeID

```
jsPsych.currentTimelineNodeID()
```

### Parameters

None.

### Return value

Returns the ID of the TimelineNode that is currently active.

### Description

Gets the ID of the active TimelineNode. The ID is a string that follows a specific format:

* `"0.0"` is the ID of the first top-level TimelineNode
* `"1.0"` is the ID of the second top-level TimelineNode
* `"2.0"` is the ID of the third top-level TimelineNode, and so on...

If a TimelineNode iterates multiple times (using the loop function, for example), then the iterations are indicated in the second number:

* `"0.0"` is the ID of the first top-level TimelineNode during the first iteration
* `"0.1"` is the ID of the first top-level TimelineNode during the second iteration
* `"0.2"` is the ID of the first top-level TimelineNode during the third iteration, and so on...

If TimelineNodes are nested in other TimelineNodes, then the hierarchical structure is shown with `"."`:

* `"0.0-1.0"` is the ID of the second TimelineNode on the timeline of the first top-level TimelineNode.
* `"0.0-2.0"` is the ID of the third TimelineNode on the timeline of the first top-level TimelineNode, and so on...

The rules about iterations apply throughout the hierarchical ID:

* `"0.2-1.3"` is the ID of the second TimelineNode, executing for the fourth time, on the timeline of the first top-level TimelineNode, executing for the third time.


### Example

```javascript
var id = jsPsych.currentTimelineNodeID();

console.log('The current TimelineNode ID is '+id);
```

---
## jsPsych.currentTrial

```
jsPsych.currentTrial()
```

### Parameters

None.

### Return value

Returns the object describing the current trial. The object will contain all of the parameters associated with the current trial.

### Description

Get a description of the current trial

### Example

```javascript

var trial = jsPsych.currentTrial();

console.log('The current trial is using the '+trial.type+' plugin');
```
---
## jsPsych.endCurrentTimeline

```
jsPsych.endCurrentTimeline
```

### Parameters

None.

### Return value

None.

### Description

Ends the current timeline. If timelines are nested, then only the timeline that contains the current trial is ended.

### Example

#### Loop indefinitely until a particular key is pressed

```javascript

var images = [
  "img/1.gif", "img/2.gif", "img/3.gif", "img/4.gif",
  "img/5.gif", "img/6.gif", "img/7.gif", "img/8.gif",
  "img/9.gif", "img/10.gif"
];

var trials = [];
for (var i = 0; i < images.length; i++) {
  trials.push({
    stimulus: images[i]
  });
}

var block = {
  type: 'image-keyboard-response',
  choices: [89, 78], // Y or N
  prompt: '<p>Press Y to Continue. Press N to end this node of the experiment.</p>',
  on_finish: function(data) {
    if (data.key_press == 78) {
      jsPsych.endCurrentTimeline();
    }
  },
  timeline: trials
}

var after_block = {
  type: 'html-keyboard-response',
  stimulus: '<p>The next node</p>',
  is_html: true
}

jsPsych.init({
  timeline: [block, after_block],
  on_finish: function() {
    jsPsych.data.displayData();
  }
});

```

---
## jsPsych.endExperiment

```
jsPsych.endExperiment(end_message)
```

### Parameters

Parameter | Type | Description
--------- | ---- | -----------
end_message | string | A message to display on the screen after the experiment is over.

### Return value

None.

### Description

Ends the experiment, skipping all remaining trials.

### Example

#### End the experiment if a particular response is given

```javascript
var trial = {
  type: 'image-keyboard-response',
  stimulus: 'image1.jpg',
  choices: [89,78], // Y or N
  prompt: '<p>Press Y to Continue. Press N to end the experiment</p>',
  on_finish: function(data){
    if(data.key_press == 78){
      jsPsych.endExperiment('The experiment was ended by pressing N.');
    }
  }
}
```

---
## jsPsych.finishTrial

```
jsPsych.finishTrial(data)
```

### Parameters

Parameter | Type | Description
----------|------|------------
data | object | The data to store for the trial.


### Return value

Returns nothing.

### Description

This method tells jsPsych that the current trial is over. It is used in all of the plugins to end the current trial. When the trial ends a few things happen:

* The data is stored using `jsPsych.data.write()`
* The on_finish callback function is executed for the trial
* The on_trial_finish callback function is executed
* The progress bar is updated if it is being displayed
* The experiment ends if the trial is the last one (and the on_finish callback function is executed).
* The next trial, if one exists, is started.

### Example

```javascript

// this code would be in a plugin
jsPsych.finishTrial({correct_response: true});

```
---
## jsPsych.getDisplayElement

```
jsPsych.getDisplayElement()
```

### Parameters

None.

### Return value

Returns the HTML DOM element used for displaying the experiment.

### Description

Get the DOM element that displays the experiment.

### Example

```javascript
var el = jsPsych.getDisplayElement();

// hide the jsPsych display
el.style.visibility = 'hidden';
```

---
## jsPsych.getProgressBarCompleted

```
jsPsych.getProgressBarCompleted()
```

### Parameters

None.

### Return value

Returns a value between 0 and 1 representing how full the progress bar currently is.

### Description

Used to get the current value of the progress bar. Works for automated and manual control.

### Example

```javascript
var progress_bar_amount = jsPsych.getProgressBarCompleted();
```

---
## jsPsych.init

```
jsPsych.init(settings)
```

### Parameters

Parameter | Type | Description
----------|------|------------
settings | object | The settings object for initializing jsPsych. See table below.

The settings object can contain several parameters. The only *required* parameter is `timeline`.

Parameter | Type | Description
--------- | ---- | -----------
timeline | array | An array containing the objects that describe the experiment timeline. See [Creating an Experiment: The Timeline](../overview/timeline.md).
display_element | string | The ID of an HTML element to display the experiment in. If left blank, jsPsych will use the `<body>` element to display content. All keyboard event listeners are bound to this element. In order for a keyboard event to be detected, this element must have focus (be the last thing that the subject clicked on).
on_finish | function | Function to execute when the experiment ends.
on_trial_start | function | Function to execute when a new trial begins.
on_trial_finish | function | Function to execute when a trial ends.
on_data_update | function | Function to execute every time data is stored using the `jsPsych.data.write` method. All plugins use this method to save data (via a call to `jsPsych.finishTrial`, so this function runs every time a plugin stores new data.
on_interaction_data_update | function | Function to execute every time a new interaction event occurs. Interaction events include clicking on a different window (blur), returning to the experiment window (focus), entering full screen mode (fullscreenenter), and exiting full screen mode (fullscreenexit).
on_close | function | Function to execute when the user leaves the page. Can be used, for example, to save data before the page is closed.
exclusions | object | Specifies restrictions on the browser the subject can use to complete the experiment. See list of options below.
show_progress_bar | boolean | If true, then [a progress bar](../overview/progress-bar.md) is shown at the top of the page.
message_progress_bar | string | Message to display next to the progress bar. The default is 'Completion Progress'. 
auto_update_progress_bar | boolean | If true, then the progress bar at the top of the page will automatically update as every top-level timeline or trial is completed.
show_preload_progress_bar | boolean | If true, then a progress bar is displayed while media files are automatically preloaded.
preload_audio | array | An array of audio files to preload before starting the experiment.
preload_images | array | An array of image files to preload before starting the experiment.
preload_video | array | An array of video files to preload before starting the experiment.
max_load_time | numeric | The maximum number of milliseconds to wait for content to preload. If the wait time is exceeded an error message is displayed and the experiment stops. The default value is 60 seconds.
max_preload_attempts | numeric | The maximum number of attempts to preload each file in case of an error. The default value is 10. There is a small delay of 200ms between each attempt.
use_webaudio | boolean | If false, then jsPsych will not attempt to use the WebAudio API for audio playback. Instead, HTML5 Audio objects will be used. The WebAudio API offers more precise control over the timing of audio events, and should be used when possible. The default value is true.
default_iti | numeric | The default inter-trial interval in ms. The default value if none is specified is 0ms.
experiment_width | numeric | The desired width of the jsPsych container in pixels. If left undefined, the width will be 100% of the display element. Usually this is the `<body>` element, and the width will be 100% of the screen size.
minimum_valid_rt | numeric | The minimum valid response time for key presses during the experiment. Any key press response time that is less than this value will be treated as invalid and ignored. Note that this parameter only applies to _keyboard responses_, and not to other response types such as buttons and sliders. The default value is 0.

Possible values for the exclusions parameter above.

Parameter | Type | Description
--------- | ---- | -----------
min_width | numeric | The minimum width of the browser window. If the width is below this value, a message will be displayed to the subject asking them to maximize their browser window. The experiment will sit on this page until the browser window is large enough.
min_height | numeric | Same as above, but with height.
audio | boolean | Set to true to require support for the WebAudio API (used by plugins that play audio files).

### Return value

Returns nothing.

### Description

This method configures and starts the experiment.

### Example

See any of the plugin examples in the [examples folder](https://github.com/jodeleeuw/jsPsych/tree/master/examples) in the GitHub repository.

---
## jsPsych.initSettings

```
jsPsych.initSettings()
```

### Parameters

None

### Return value

Returns the settings object used to initialize the experiment.

### Description

Gets the object containing the settings for the current experiment.

### Example

```javascript
var settings = jsPsych.initSettings();

// check the experiment structure
console.log(JSON.stringify(settings.timeline));
```

---
## jsPsych.pauseExperiment
```
jsPsych.pauseExperiment()
```

### Parameters

None.

### Return value

None.

### Description

Pauses the experiment. The experiment will finish the current trial, but will not execute any additional trials until `jsPsych.resumeExperiment()` is called.

### Example

```javascript
var trial = {
  type: 'html-keyboard-response',
  stimulus: 'Press p to take a 30 second break. Otherwise, press c to continue immediately.',
  choices: ['p','c'],
  on_finish: function(data){
    if(data.key_press == 80) { // 80 = p
      jsPsych.pauseExperiment();
      setTimeout(jsPsych.resumeExperiment, 30000);
    }
  }
}
```

---
## jsPsych.progress

```
jsPsych.progress()
```

### Parameters

None.

### Return value

Returns an object with the following properties:

Property  | Type | Description
----------|------|------------
total_trials | numeric | Indicates the number of trials in the experiment. Note that this does not count possible loops or skipped trials due to conditional statements.
current_trial_global | numeric | Returns the trial index of the current trial in a global scope. Every trial will increase this count by 1.
percent_complete | numeric | Estimates the percent of the experiment that is complete. Works as expected for experiments without conditional or looping timelines. For complex timelines, the percent is an approximation.


### Description

This method returns information about the length of the experiment and the subject's current location in the experiment timeline.

### Example

```javascript

var progress = jsPsych.progress();

alert('You have completed approximately '+progress.percent_complete+'% of the experiment');

```
---
## jsPsych.resumeExperiment
```
jsPsych.resumeExperiment()
```

### Parameters

None.

### Return value

None.

### Description

Resumes the experiment after a call to `jsPsych.pauseExperiment()`. If the post trial delay (`post_trial_gap`) has not yet been reached, then the experiment will not continue until the delay is finished. For example, if `post_trial_gap` was 10,000ms and `jsPsych.resumeExperiment()` was called 6,000ms after the previous trial finished, then the experiment would not continue for another 4,000ms.

### Example

```javascript
var trial = {
  type: 'html-keyboard-response',
  stimulus: 'Press p to take a 30 second break. Otherwise, press c to continue immediately.',
  choices: ['p','c'],
  on_finish: function(data){
    if(data.key_press == 80) { // 80 = p
      jsPsych.pauseExperiment();
      setTimeout(jsPsych.resumeExperiment, 30000);
    }
  }
}
```

---
## jsPsych.setProgressBar

```
jsPsych.setProgressBar(value)
```

### Parameters

Parameter | Type | Description
----------|------|------------
value | numeric | Proprotion (between 0 and 1) to fill the progress bar.


### Return value

None.

### Description

Set the progress bar to a custom amount. Proportion must be between 0 and 1. Values larger than 1 are treated as 1.

### Example

```javascript
jsPsych.setProgressBar(0.85);
```

---
## jsPsych.startTime

```
jsPsych.startTime()
```

### Parameters

None.

### Return value

Returns a `Date` object indicating when the experiment began.

### Description

Get the time that the experiment began.

### Example

```javascript
var start_time = jsPsych.startTime();
```

---
## jsPsych.timelineVariable

```
jsPsych.timelineVariable(variable, call_immediate)
```

### Parameters

Parameter | Type | Description
----------|------|------------
variable | string | Name of the timeline variable
call_immediate | bool | Typically this parameter is `false`, or simply ommitted. When `false`, the return value is a function that returns the timeline variable. This makes `jsPsych.timelineVariable` suitable for dynamic parameters by default. If `true` the function returns the value of the timeline variable immediately.

### Return value

Depends on the value of `call_immediate` parameter. See description above.

### Description

[Timeline variables](/overview/timeline/#timeline-variables) are a powerful technique for generating experiments with repetitive procedures but different parameter values. This function fetches the current value of a particular timeline variable. It must be used in conjunction with a timeline that has timeline variables. See the [timeline variable section](/overview/timeline/#timeline-variables) for details.

### Examples

#### Standard use as a parameter for a trial
```javascript
var trial = {
  type: 'image-keyboard-response',
  stimulus: jsPsych.timelineVariable('image')
}

var procedure = {
  timeline: [trial],
  timeline_variables: [
    {image: 'face1.png'},
    {image: 'face2.png'},
    {image: 'face3.png'},
    {image: 'face4.png'}
  ]
}
```

#### Invoking immediately in a function
```javascript
var trial = {
  type: 'html-keyboard-response',
  stimulus: function(){
    return "<img style='width:100px; height:100px;' src='"+jsPsych.timelineVariable('image', true)+"'></img>";
  }
}

var procedure = {
  timeline: [trial],
  timeline_variables: [
    {image: 'face1.png'},
    {image: 'face2.png'},
    {image: 'face3.png'},
    {image: 'face4.png'}
  ]
}
```


---
## jsPsych.totalTime

```
jsPsych.totalTime()
```

### Parameters

None.

### Return value

Returns a numeric value indicating the number of milliseconds since `jsPsych.init` was called.

### Description

Gets the total time the subject has been in the experiment.

### Example

```javascript

var time = jsPsych.totalTime();
console.log(time);

```