Moment.js Sandbox

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.2.1/moment.min.js"></script>

JavaScript

// With a moment instance, concatenation vs interpolation gives a different result:
const mmt = moment();
console.log('concatenated: ' + mmt); // "concatenated: 1651070909974"
console.log(`interpolated: ${mmt}`); // "interpolated: Wed Apr 27 2022 10:48:29 GMT-0400"
console.log('mmt.valueOf(): ' + mmt.valueOf()); // "mmt.valueOf(): 1651070909974"
console.log('mmt.toString(): ' + mmt.toString()); // "mmt.toString(): Wed Apr 27 2022 10:48:29 GMT-0400"


// I suspected this is because of .toString() vs .valueOf(), so I made
// an object to test that and it showed the same thing:
const obj = {
  toString: () => 'toString',
  valueOf: () => 'valueOf',
};

console.log('concatenated: ' + obj); // "concatenated: valueOf"
console.log(`interpolated: ${obj}`); // "interpolated: toString"
console.log('obj.valueOf(): ' + obj.valueOf()); // "obj.valueOf(): valueOf"
console.log('obj.toString(): ' + obj.toString()); // "obj.toString(): toString"


// However, a Date instance does *not* give a different result in concatenation
// vs interpolation, despite having .toString() vs .valueOf() giving different results:
const dte = new Date();
console.log('concatenated: ' + dte); // "concatenated: Wed Apr 27 2022 10:48:29 GMT-0400 (Eastern Daylight Time)"
console.log(`interpolated: ${dte}`); // "interpolated: Wed Apr 27 2022 10:48:29 GMT-0400 (Eastern Daylight Time)"
console.log('dte.valueOf(): ' + dte.valueOf()); // "dte.valueOf(): 1651070909974"
console.log('dte.toString(): ' + dte.toString()); // "dte.toString(): Wed Apr 27 2022 10:48:29 GMT-0400 (Eastern Daylight Time)"