How does JSON.stringify automagically convert moment objects to iso strings?

Question

I'm curious how libs like moment, automagically convert from objects to strings when JSON.stringify is called on that object.

Example test in moment: https://github.com/moment/moment/blob/3147fbc486209f0b479dc0b29672d4c2ef39cf43/src/test/moment/format.js#L144-L146

Here's some example code that i'm curious how it works

const moment = require('moment');
const duration = moment.duration(1374);
console.log('duration = ', duration); // Prints a duration object
console.log('JSON.stringify(duration) = ', JSON.stringify(duration)); // Prints a string such as `P0D1T0H3` <-- Note: Not exact value, just similar format

Pretty sure JSON.stringify casts all non-primitives as strings, so custom toString method is the likely answer. — Jared Smith, Dec 13 '19 at 21:31

score 7 · Accepted Answer · answered Dec 13 '19 at 21:29

7

From MDN:

If the value has a toJSON() method, it's responsible to define what data will be serialized.

From moment:

proto.toJSON         = toISOString;

answered Dec 13 '19 at 21:29

Quentin

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score 3 · Answer 2 · answered Dec 13 '19 at 21:29

3

If an object has a .toJSON() method, JSON.stringify() will call that before attempting to do it's own thing with the object.

Date objects, and apparently Moment wrapper objects, have .toJSON() that calls .toISOString().

answered Dec 13 '19 at 21:29

Pointy

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How does JSON.stringify automagically convert moment objects to iso strings?

2 Answers2