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---
title: "on(event, callback)"
layout: default
section: api
---
__Description__ : Register a listener on an event.
__Arguments__
name | type | description
----------|----------|------------
event | string | the name of the event. Only 3 events are supported : `data`, `end` and `error`.
callback | function | the function called when the event occurs. See below for the arguments.
A `data` callback takes 2 parameters :
- the current chunk of data (in a format specified by the method which
generated this StreamHelper)
- the metadata (see each method to know what's inside)
A `end` callback does not take any parameter.
A `error` callback takes an `Error` as parameter.
The callbacks are executed in with the current `StreamHelper` as `this`.
__Returns__ : The current StreamHelper object, for chaining.
__Throws__ : An exception if the event is unkown.
__Example__
```js
zip
.generateInternalStream({type:"uint8array"})
.on('data', function (data, metadata) {
// data is a Uint8Array because that's the type asked in generateInternalStream
// metadata contains for example currentFile and percent, see the generateInternalStream doc.
})
.on('error', function (e) {
// e is the error
})
.on('end', function () {
// no parameter
})
.resume();
```
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