Date assumes string conversion as a default

date-object-to-primitive-conversion.js

let d = new Date();

// get string representation
let str = d.toString();  // 'Wed Jan 17 2018 16:15:42'

// get numeric representation, num of milliseconds since Unix epoch
let num = d.valueOf();   // 1516198542525

// compare with a string representation
// true because d is converted to same string
console.log(d == str);   // true

// compare with numeric representation
// false, because d is not converted to a number via valueOf()
console.log(d == num);   // false

// Result is 'Wed Jan 17 2018 16:15:42Wed Jan 17 2018 16:15:42'
// '+' same to '==' triggers default conversion mode
console.log(d + d);

// Result is 0, since '-' operator explicitly triggers numeric conversion, not a default one
console.log(d - d);