Dayjs does not preserve timezone information when serializing to JSON string

When I do this

const timestamp = dayjs("2019-11-26T17:28:35.518+03:00")
const actualJson = JSON.stringify(timestamp)
const expectedJson = "\"2019-11-26T17:28:35.518+03:00\""

I want actualJson to be equal to expectedJson. However, actualJson is "\"2019-11-26T14:28:35.518Z\"". With Moment.js I can do this:

moment.fn.toJSON = function() { return this.format(); }

in order to get my expected JSON string with the preserved timezone information, see https://momentjs.com/docs/#/displaying/as-json/

How can I achieve this in dayjs? I am new to Javascript/Typescript and my compiler tells me that dayjs.fn does not exist. What would be the right workaround?

Information

• Day.js Version 1.8.28
• OS: Windows 8.1
• Time zone: CEST (Berlin)

[svdHero]

Hm, there seems to be a more fundamental problem. Apparently, already the parsing does not preserve the timezone information. I have the following Jest test code:

describe("dajys", () => {
    it("preserves timezone information on roundtrip", () =>{
        const originalISOString = "2019-11-26T18:28:35.518+04:00"
        const ISO_FORMAT = "YYYY-MM-DDTHH:mm:ss.SSSZ"
        // ###############
        const actualString = dayjs(originalISOString).format(ISO_FORMAT)
        // ###############
        expect(actualString).toEqual(originalISOString)
    })
})

When I run this, I get a failed test with output:

    Expected: "2019-11-26T18:28:35.518+04:00"
    Received: "2019-11-26T15:28:35.518+01:00"

The date strings are equivalent, but the parsing obviously returned a moment in my local timezone (GMT+01, Berlin). This is exactly how moment.js behaves. However, moment.js has moment.parseZone(...), see https://momentjs.com/docs/#/parsing/, which does exactly what I want and makes the above test code succeed.

Does day.js have anything similar?

[iamkun]

As JSON will serialize Dayjs object to an ISO 8601 string. https://day.js.org/docs/en/display/as-json

However, if you want to customize this function, you can write your own plugin to modify it. https://day.js.org/docs/en/plugin/plugin

const JsonPlugin = (option, dayjsClass, dayjsFactory) => {
  // overriding existing API
  dayjsClass.prototype.toJSON = function() {
      return this.format()
   }
}
dayjs.extend(JsonPlugin)