Port of C# code published by ESRI

decompress-geometry.ts

/*************************************************
 * see http://resources.esri.com/help/9.3/arcgisengine/ArcObjects/esrinetworkanalyst/INACompactStreetDirection_CompressedGeometry.htm
 *************************************************/
class DecompressGeometry {
    
    public ExtractPointsFromCompressedGeometry(compresedGeometry: string)
    {
      // initialize result storage
      let result = <Array<{x: number; y: number;}>>[],
      nIndex = {ref: 0},
      dMultBy = this.ExtractInt(compresedGeometry, nIndex),
      nLastDiffX = 0,
      nLastDiffY = 0,
      nLength = compresedGeometry.length;
      
      while (nIndex.ref != nLength)
      {
        // extract number
        let nDiffX = this.ExtractInt(compresedGeometry, nIndex);
        let nDiffY = this.ExtractInt(compresedGeometry, nIndex);
        
        // decompress
        let nX = nDiffX + nLastDiffX;
        let nY = nDiffY + nLastDiffY;
        let dX = nX / dMultBy;
        let dY = nY / dMultBy;
        
        // add result item
        let point = {x : dX, y: dY};
        result.push(point);
        
        // prepare for next calculation
        nLastDiffX = nX;
        nLastDiffY = nY;
      }
      // return result
      return result;
    }
    
    // Read one integer from compressed geometry string by using passed position
    // Returns extracted integer, and re-writes nStartPos for the next integer
    private ExtractInt(src: string, nStartPos: {ref: number})
    {
      let bStop = false,
      result = "",
      nCurrentPos = nStartPos.ref;
      
      while (!bStop)
      {
        let cCurrent = src[nCurrentPos];
        if (cCurrent == '+' || cCurrent == '-')
        {
          if (nCurrentPos != nStartPos.ref)
          {
            bStop = true;
            continue;
          }
        }
        result += cCurrent;
        nCurrentPos++;
        if (nCurrentPos == src.length) // check overflow
          bStop = true;
      }
      
      let nResult = Number.MIN_VALUE;
      if (result.length != 0)
      {
        nResult = parseInt(result, 32);
        nStartPos.ref = nCurrentPos;
      }
      return nResult;
    }
    
}

Usage Example:

let decomp = new DecompressGeometry();
console.log(decomp.ExtractPointsFromCompressedGeometry("+1m91-66mmp+1nfmv+0+0"));

A much better version can be found here