React Native - Difference between onChange vs onChangeText of TextInput

UPDATE 26.08.2019

Since the initial version of the answer, TextInput's API has changed, and answer below is no longer valid. I haven't worked with react-native for more than 2 years now, so I can't really tell which version had these changes.

Regarding the answer, onChangeText is still a simple prop, that gives whatever is the value of the input field on every change.

onChange on the other hand, passes an event with { nativeEvent: { eventCount, target, text} } (as mentioned in the comment to this answer). Now, I cannot tell with confidence, why do you need eventCount and target. I can only state, that eventCount is increased every time you interact with TextInput component (character added, removed, all deleted, value pasted) and target is a unique integer for that TextInput field. And text is the same value as in onChangeText

So basically, I would suggest to use onChangeText, as a more straight forward prop.

If you want to accomplish functionality like in the old answer(below), you can create custom component, that wraps TextInput and receives custom properties and passes them to the onChange prop later. Example below:

const MyTextInput = ({ value, name, type, onChange }) => {
  return (
    <TextInput
      value={value}
      onChangeText={text => onChange({ name, type, text })}
    />
  );
};

And then use it whenever you need to use TextInput

handleChange(event) {
    const {name, type, text} = event;
    let processedData = text;
    if(type==='text') {
        processedData = value.toUpperCase();
    } else if (type==='number') {
        processedData = value * 2;
    }
    this.setState({[name]: processedData})
}

<MyTextInput name="username" type="text" value={this.state.username} onChange={this.handleChange}}>
<MyTextInput name="password" type="number" value={this.state.password} onChange={this.handleChange}}>

OLD ANSWER

onChangeText is basically a simplified version of onChange, so you can easily use it, without the hassle of going through event.target.value to get changed value.

So, when should you use onChange and when onChangeText?
If you have simple form with few textinputs, or simple logic, you can go straight away and use onChangeText

<TextInput value={this.state.name} onChangeText={(text) => this.setState({name: text})}>

If you have more complicated forms and/or you have more logic in handling data (like handling text differently from number) when user changes input, then you are better of with onChange, because it gives you more flexibility. For example:

handleChange(event) {
    const {name, type, value} = event.nativeEvent;
    let processedData = value;
    if(type==='text') {
        processedData = value.toUpperCase();
    } else if (type==='number') {
        processedData = value * 2;
    }
    this.setState({[name]: processedData})
}

<TextInput name="username" type="text" value={this.state.username} onChange={this.handleChange}}>
<TextInput name="password" type="number" value={this.state.password} onChange={this.handleChange}}>