base26ToBase10

const input = 'AAA';

const mapping = {
  A:1,B:2,C:3,D:4,E:5,F:6,G:7,H:8,I:9,J:10,K:11,L:12,M:13,N:14,O:15,P:16,Q:17,R:18,S:19,T:20,U:21,V:22,W:23,X:24,Y:25,Z:26
} as any

function base26ToBase10(input: string): number {
  const split = input.split('').reverse();

  let output = 0
  for(let i = 0; i < split.length; i++) {
    const multiplier = 26 ** i;
    output += multiplier * mapping[split[i]]
  }
  return output;
}


console.log(base26ToBase10(input));