Just add

.toLowerCase()

after

.replace(/(Value)$/, '')

Here:

// Input: { toastDurationValue: 3000, toastPositionValue: 'bottom' }
const options = Object.keys(element.dataset)
  .filter(k => k.startsWith(prefix))
  .reduce((prev, curr) => ({
    ...prev,
    [curr.slice(prefix.length).replace(/(Value)$/, '').toLowerCase()]: element.dataset[curr]
  }), {});
console.log(options);

Output:

{ duration: 3000, position: 'bottom' }

May below snippet can contribute to your solution.

const obj = { toastDurationValue: 3000, toastPositionValue: 'bottom' }
const prefix = 'toast';

Object.keys(obj).forEach(key => {
    obj[key] = obj[key.substring(0, key.lastIndexOf('Value')).replace(prefix, '').toLowerCase()] = obj[key];
    delete obj[key];
})

console.log(obj);

This can be done in one statement, in an elaborate (difficult to read) way using slice(), replace() and toUpperCase(). The original string stays as it is:

let str = 'hellorandomWORDValue';
const prefix = 'hello';

const str2 = str.replace(prefix, "").slice(0,-5)[0].toUpperCase() + str.replace(prefix, "").slice(0,-5).slice(1)

console.log(str);
console.log(str2);

slice(0,5) -> removes the last 5 letter, so basically removes Value suffix.

replace(prefix,"") -> replaces first instances of the variable string prefix. So basically removes the prefix.

[0].toUpperCase() -> takes out the first character and makes it uppercase.

.slice(1) -> takes out rest of the string (from 1st index). Basically, rest of the string after first character.

const options = Object.keys(element.dataset)
  .filter(k => k.startsWith(prefix))
  .reduce((prev, curr) => ({
    ...prev,
    [curr.replace(prefix, "").slice(0,-5)[0].toUpperCase() + curr.replace(prefix, "").slice(0,-5).slice(1)]: element.dataset[curr]
  }), {});