Convert string parameter to an array of one element

You can use Array.concat(), since concat accepts both arrays and non arrays:

const names = (v) => [].concat(v).map(name => name.toUpperCase())

console.log(names(['Bart', 'Lisa'])) // [ 'BART', 'LISA' ]
console.log(names('Homer')) // ['HOMER']

Short solution:

[names].flat()

If names is an array, it will be left as-is. Anything else will be converted to an array with one element.

This works because .flat() only flattens one level by default.

If names is not an array, [names] makes it an array with one element, and .flat() does nothing more because the array has no child arrays. If names is an array, [names] makes it an array with one child array, and .flat() brings the child array back up to be a parent array.

Alternative

This is more self-explanitory:

names instanceof Array ? names : [names]

This uses a simple ternary statement to do nothing to it if it is an array already or make it an array if it is not already.