Turning a column name into a number for indexing in the range [1, →)

columnindex.ts

function parseHeader(letters: string): number {
    function letterValue(letter: string): number {
        return parseInt(letter, 36) - 9
    }
    
    let result = 0
    let factor = 1
    
    if (letters.length === 1) {
        result = letterValue(letters[0])
    } else {
        for (let i = letters.length - 1; i >= 0; i -= 1) {
            result += factor * letterValue(letters[i])
            factor *= 26
        }
    }
    
    return result 
}