React native: How to combine external and inline styles?

This is the correct answer, tested in React Native 0.44.2:

<TouchableHighlight style={{...styles.button, ...{backgroundColor: '#f00'}}}>

Can use the spread syntax:

<TouchableHighlight style={{ ...styles.button, backgroundColor: '#f00'}}

Your brackets are wrong. If you intend to continue to use the array syntax, Use this instead:

<TouchableHighlight style={[styles.button,{ backgroundColor: '#f00'}]}