how do you format a number to currency when using React native EXPO?

You can use toFixed method for showing 2 decimal point.

let num = 1000; 
console.log(num.toFixed(2)); // 1000.00

And you can use Regex like this

function currencyFormat(num) {
   return '$' + num.toFixed(2).replace(/(\d)(?=(\d{3})+(?!\d))/g, '$1,')
}
console.log(currencyFormat(2665)); // $2,665.00

You can use this library react-number-format. It has these features

1. Prefix, suffix and thousand separator.
2. Custom format pattern.
3. Masking.
4. Custom formatting handler.
5. Format number in an input or format as a simple text

Sample usage

<NumberFormat value={2456981} displayType={'text'} thousandSeparator={true} prefix={'$'} />

Output : $2,456,981