Alpha numeric id generator

alphanumericgenerator.js

class AlphaNumericIdGenerator {
    constructor() {
        const alpha = Array.from({ length: 26 }, (_, i) => String.fromCharCode(i + 97))
        const numo = Array.from({ length: 10 }, (_, i) => i.toString())
        this.alphanumeric = numo.concat(alpha)
        this.alphaIndx = this.alphanumeric.reduce((obj, cur, i) => ({ ...obj, [cur]: i }), {});
    }

    between(after, before) {
        this._validate(after);
        this._validate(before);

        if(before <= after) {
            throw new Error('before has to be bigger then after')
        }

        const valuesBetween = [];
         while(after < before && valuesBetween.length < 100000){
            valuesBetween.push(before);
            before = this.increment(before,-1)
          }
         const mid = Math.floor(valuesBetween.length/2)
      
         if(valuesBetween.length <= 1) return false;

        return valuesBetween[mid];
      }
    _validate(val){
        if(typeof val !== 'string' || val<'0' || val>'zzzzzzzzz' || val.length>9) {
         throw new Error('Must be string between "a" and "zzzzzzzzz" ')
        }
    }
    after(after) {
        this._validate(after);
        return this.between(after,'zzzzzzzzz')
    }
    before(before) {
        this._validate(before);
        return this.between('0',before)
    }
    increment(val, incrementer) {
        let nextVal = val.split('');
        let i = val.length-1
        for(;i>=0 && incrementer !== 0; i--){
            const cur = nextVal[i];
            if(cur === 'z' && incrementer === 1){
                nextVal[i] = '0';
            } else if(cur === '0' && incrementer === -1){
                nextVal[i] = 'z';
            } else {
                const idx = this.alphaIndx[cur]
                nextVal[i] = this.alphanumeric[idx+incrementer];
                incrementer = 0
            }
      }
      nextVal = nextVal.join('')

      if(incrementer === 1){
        nextVal = val + '0';
      }
      if(incrementer === -1){
        nextVal = val.slice(1)
      }
      return nextVal;
    }
}





let stringGenerator = new AlphaNumericIdGenerator();

let a = ['0', '01']

let id = stringGenerator.between(...a);
let idAfter = stringGenerator.after(a[1]);
 let idBefore = stringGenerator.before(a[1]);

//  a.push(id);
a.push(idAfter);
a.push(idBefore);
a.sort();