push() won't work as expected in reduce()

Question

Why doesn't a.push(b) work in my Array.reduce()? a=a.push(b) where b is a string, turns a to an integer.?!

 getHighestValuesInFrequency: function(frequency) {
 //Input:var frequency = {mats: 1,john: 3,johan: 2,jacob: 3};
 //Output should become ['John','jacob']

var objKeys = Object.keys(frequency);
var highestVal = objKeys.reduce((a,b)=>
            {highestVal = (frequency[b] > a)? frequency[b] : a;
             return highestVal;},0);

var winner = objKeys.reduce((a,b)=>
      { a = (frequency[b] === highestVal)? a.push(b) : a;
        return a},[]);

return winner;  
 }                                  

Answer 1

Since push() returns the new length of the array, you're assigning the length to a. Instead of a conditional operator, use an if statement.

var winner = objKeys.reduce((a, b) => {
    if (frequency[b] === highestVal) {
        a.push(b);
    }
    return a;
}, []);

Answer 2

The push() returns the new length. You can use ES2015 spread syntax:

var winner = objKeys.reduce((a, b)=> {
    a = (frequency[b] === highestVal)? [...a, b] : a;
    return a
}, []);