Use if else to declare a `let` or `const` to use after the if/else?

This is a good example of where a simple ternary assignment could suffice:

const classes = withBorder ?
 `${styles.circularBorder} ${styles.dimensions} ${styles.circularPadding} row flex-items-xs-middle flex-items-xs-center` : 
 `${styles.dimensions} ${styles.circularPadding} row flex-items-xs-middle flex-items-xs-center`

As specified in other comments/answers let and const are block scoped, so that’s why they don’t work in your example.

For DRYer code, you can use the ternary only for the part that depends on it:

 const classes = (withBorder ? `${styles.circularBorder} ` : "") +
 `${styles.dimensions} ${styles.circularPadding} row flex-items-xs-middle flex-items-xs-center`