Use a regex and String.replace:

const inp = "ab#c";
const match = /.#/;
const out = inp.replace(match, "");

console.log(out);

Note that this removes any character preceding a '#' in your original string, but only a single occurrence. Add the g flag to match and replace all occurrences, and use [a-zA-Z] to just match letters.

If you want to work the other way (character following, not preceding), and want to replace #c but not \#c, just add that as well:

const inp = "ab\\#c3#cc";
const match = /[^\\]{1}#./;
const out = inp.replace(match, "");

console.log(out);

You can use global regex replace

const inputStr = "ab#c de#f gh#i";

const regxStr = /.#/g;
const outputStr = inputStr.replace(regxStr, "");

console.log(outputStr);