问在方程中访问对象的值问题

EN

  function orbitalPeriod(arr) {
  const GM = 398600.4418;
  const earthRadius = 6367.4447;

//Step 1: get two times pi
let stepOne = 2 * Math.PI;

//Step 2: find a appears to be e radius + avg alt
/*the issue is in this line. I know I am calling the avgAlt wrong, but I'm not sure how it is wrong. 
The console log returns undefined, but it should be returning a number */ 
let stepTwo = earthRadius + arr.avgAlt;

console.log(arr.avgAlt);

//Step 3: get a^3
let stepThree= Math.pow(stepTwo, 3);

//Step 4: divide step 2 by GM defined above
let stepFour= stepTwo/GM;

//Step 5: Combo it all and round to find t
let t = Math.round(stepOne*stepFour);

//return the array w/ the new key/value pair & t is the value
 let newArr= arr.map(function(el){
    return {name:el.name,
            orbitalPeriod: t}          
  });

  return newArr;
};
orbitalPeriod([{name : "sputnik", avgAlt : 35873.5553}]);