You could split() the string on any ? or = and take the middle item ([1]) from the outcome array.

const data = "?retailerKey=A";
const result = data.split(/[\?=]/)[1];

console.log(result);

If you have multiple params, creating an object fromEntries() would be interesting.

const data = "?retailerKey=A?otherKey=B";

const keyVals = data.split(/[\?=]/).filter(x => x); // keys and values
const result = Object.fromEntries(keyVals.reduce((acc, val, i) => {
  // create entries
  const chunkI = Math.floor(i / 2);
  if (!acc[chunkI]) acc[chunkI] = [];
  acc[chunkI].push(val);
  return acc;
}, []));

console.log(result);

Using library could be a better choice but to do it from scratch : I suggest to use split with a regular expression.

// split for char equals to ? or & or =;
const url = '/toto?titi=1&tata=2';
const args = url.split(/[\?\&\=]/);
// shift the first element of the list since it the base url before "?"
args.shift();

// detect malformed url
if (args.length % 2) {
    console.error('malformed url', args);
}

const dictArgs = {};

for (let i = 0; i < args.length /2; i ++) {
     const key = args[2*i];
     const val = args[2*i+1];
     dictArgs[key] = val;
}

console.log(dictArgs);
    

use regex expression. Following will return the value between character ? and =

var result =  "?retailerKey=A".match(/\?(.*)\=/).pop();
console.log(result);