Typescript base64 to ascii String

base64decode.ts

export class base64decode{
    private PADCHAR: string = '=';
  private ALPHA: string = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

  private getByte(s: string, i: number): number {
    const x = s.charCodeAt(i);
    return x;
  }

  private getByte64(s: string, i: number): number {
    const idx = this.ALPHA.indexOf(s.charAt(i));
    return idx;
  }
  public decode (s: string): string {
    let pads = 0,
      i, b10, imax = s.length,
      x = [];

    s = String(s);

    if (imax === 0) {
      return s;
    }

    if (s.charAt(imax - 1) === this.PADCHAR) {
      pads = 1;
      if (s.charAt(imax - 2) === this.PADCHAR) {
        pads = 2;
      }
      imax -= 4;
    }

    for (i = 0; i < imax; i += 4) {
      b10 = (this.getByte64(s, i) << 18) | (this.getByte64(s, i + 1) << 12) | (this.getByte64(s, i + 2) << 6) | this.getByte64(s, i + 3);
      x.push(String.fromCharCode(b10 >> 16, (b10 >> 8) & 255, b10 & 255));
    }

    switch (pads) {
      case 1:
        b10 = (this.getByte64(s, i) << 18) | (this.getByte64(s, i + 1) << 12) | (this.getByte64(s, i + 2) << 6);
        x.push(String.fromCharCode(b10 >> 16, (b10 >> 8) & 255));
        break;
      case 2:
        b10 = (this.getByte64(s, i) << 18) | (this.getByte64(s, i + 1) << 12);
        x.push(String.fromCharCode(b10 >> 16));
        break;
    }

    return x.join('');
  }
}