You should use the string replace function, with a single regex. Assuming by special characters, you mean anything that's not letter, here is a solution:

const str = "abc's test#s";
console.log(str.replace(/[^a-zA-Z ]/g, ""));

Simply replace it with nothing:

var string = 'F0123456'; // just an example
string.replace(/^F0+/i, ''); '123456'

Try with this function:

function removeSpecialChars(str) {
  return str.replace(/(?!\w|\s)./g, '')
    .replace(/\s+/g, ' ')
    .replace(/^(\s*)([\W\w]*)(\b\s*$)/g, '$2');
}

• 1st regex /(?!\w|\s)./g remove any character that is not a word or whitespace. \w is equivalent to [A-Za-z0-9_]
• 2nd regex /\s+/g find any appearance of 1 or more whitespaces and replace it with one single white space
• 3rd regex /^(\s*)([\W\w]*)(\b\s*$)/g trim the string to remove any whitespace at the beginning or the end.

var desired = stringToReplace.replace(/[^\w\s]/gi, '')

As was mentioned in the comments it's easier to do this as a whitelist - replace the characters which aren't in your safelist.

The caret (^) character is the negation of the set [...], gi say global and case-insensitive (the latter is a bit redundant but I wanted to mention it) and the safelist in this example is digits, word characters, underscores (\w) and whitespace (\s).