BaseToDecimal.js

const value = prompt("Input required")
const base = prompt("Base required")
const adapters = { 16: { "A": 10, "B": 11, "C": 12, "D": 13, "E": 14, "F": 15 } }

const formula = value
  .split("")
  .reverse()
  .map((item, index) => `${(adapters[base] || {})[item] || item} * ${base} ^ ${index}`)
  .reverse()
  .join(" + ");

alert(formula)

log.txt

Input required: FACA
Base required: 16

Output: 15 * 16 ^ 3 + 10 * 16 ^ 2 + 12 * 16 ^ 1 + 10 * 16 ^ 0