JavaScript algo: decompress a compressed string

I wanted to write a function that takes a compressed string and outs the decompressed string.

A compressed string like a2b2c3 and the decompress string is aabbccc More examples would be

`a` -> `a`
`ab12` -> `abbbbbbbbbbbb`
`a3b2a2` -> `aaabbaa

I tried to implement it but it is really messy and buggy for compressed strings like ab12

function isNumeric(num) {
    if (num === '') return false
    if (num === null) return false
    return !isNaN(num)
  }
  

function decompress(compressedStr) {
    const array = compressedStr.split('')
    let prevChar, str = ''
    for(let i = 0; i < array.length; i++) {
        if(i === 0) {prevChar = array[i]}
        if(isNumeric(array[i])) {
            str += prevChar.repeat(Number(array[i]))
            prevChar = null
        } else {
            if(!prevChar) prevChar = array[i] 
            else {
                str += prevChar
                prevChar = array[i] 
            }
        }
    }

    return str
}

Now it works for a3b2a2 but it is buggy for cases like ab12 . Need help to rewrite this function to make it work.

from JavaScript algo: decompress a compressed string