Why does the second assignment need return and the first returns variable total automatically?

const total = inventors.reduce(((total, inventor) => total += inventor.passed - inventor.born), 0);

const transportation = data.reduce(((prevVal, currentVal) => {
      if (!prevVal[currentVal]) {
        prevVal[currentVal] = 0;
      }
      prevVal[currentVal]++;
      return prevVal;
    }), {})

If return prevVal is omitted I get Uncaught TypeError: Cannot read properties of undefined

What is the difference in how reduce() acts with values. Is it effected by ternary operation? If so then why this returns 0?

const transportation = data.reduce(((prevVal, currentVal) => 
(!prevVal[currentVal]) ? prevVal[currentVal] = 0 : prevVal[currentVal]++), {})