inner object mthods return undefined?

const typeStore = {
  b2_uint16_1p: arr => {
    let dv = new DataView(new Uint8Array(arr).buffer);
    let decim = dv.getUint16(0);
    return (decim / 10).toFixed(1);
  },
  b4_float32_: arr2 => {
    let dv = new DataView(new Uint8Array(arr2).buffer);
    let decim = dv.getFloat32(0);
    return decim;
  },
}

I invoke method like this:

inter.prototype.interpret = function (dataTypeString, dataArr) {
  return typeStore[dataTypeString](dataArr);
};

let md=new Md;
md.interpret[b4_float32_](99)

{
  VERSION: "2v04.332",
  GIT_COMMIT: "57da2618",
  BOARD: "STM32F4DISCOVERY",
  FLASH: 1048576, RAM: 196608,
  SERIAL: "32005a00-0c513832-36393630",
  CONSOLE: "USB",
  MODULES: "Flash,Storage,hea" ... "S,WIZnet,neopixel",
  EXPTR: 536871244 }

I can't invoke the second method, in this position, I just can only use the first method, the others just return UNDEFINED。