Unexpected behavior of JSON.stringify()

[adius]

JSON.stringify({[new Date()]: 'test'})

yields

{
  "Sat Nov 26 2016 13:46:13 GMT+0000 (UTC)": "test"
}

but I would expect it to yield

{
  "2016-11-26T13:46:13.923Z": "test"
}

[ljharb]

new Date().toString() produces the former, the latter is new Date().toISOString(). The conversion happens when the Date object is coerced to a string to become the object property key. Object.keys({[new Date()]: 'test'})[0] will produce the same string value - JSON.stringify doesn't come into it.

[ljharb]

new Date().toString() produces the former, the latter is new Date().toISOString(). The conversion happens when the Date object is coerced to a string to become the object property key. Object.keys({[new Date()]: 'test'})[0] will produce the same string value - JSON.stringify doesn't come into it.

[adius]

I just realized that, too 🙈.

What I was thinking of in the first place was rather: new Map([[new Date(), 'test']])
…and actually the yield of JSON.stringify(new Map([[new Date(), 'test']])) is also not optimal.

It should rather be {"2016-11-26T14:15:55.380Z": "test"} instead of [["2016-11-26T14:15:55.380Z","test"]].

[adius]

I just realized that, too 🙈.

What I was thinking of in the first place was rather: new Map([[new Date(), 'test']])
…and actually the yield of JSON.stringify(new Map([[new Date(), 'test']])) is also not optimal.

It should rather be {"2016-11-26T14:15:55.380Z": "test"} instead of [["2016-11-26T14:15:55.380Z","test"]].

[ljharb]

In fact, it can't, because Maps can have objects as keys, and object keys themselves can only be strings or Symbols. An array of entries (where an "entry" is an array of key, value) is the only way a Map can be represented in a serializable fashion.

[ljharb]

In fact, it can't, because Maps can have objects as keys, and object keys themselves can only be strings or Symbols. An array of entries (where an "entry" is an array of key, value) is the only way a Map can be represented in a serializable fashion.

[adius]

is the only way a Map can be represented in a serializable fashion

Yeah, I guess you're right. Otherwise you would have to serialize simple maps as JSON Objects and complex maps as JSON Arrays, which would be very confusing.

Maybe there should be a JSON.stringifyMap() method which simply stringifies each key…

[adius]

is the only way a Map can be represented in a serializable fashion

Yeah, I guess you're right. Otherwise you would have to serialize simple maps as JSON Objects and complex maps as JSON Arrays, which would be very confusing.

Maybe there should be a JSON.stringifyMap() method which simply stringifies each key…

[littledan]

Keys of objects are already strings or symbols when constructed. So it would be a little late for that

[littledan]

Keys of objects are already strings or symbols when constructed. So it would be a little late for that

[benjamn]

There are a variety of libraries that attempt to improve on JSON by supporting stringification of objects with prototypes other than Object.prototype or Array.prototype, along with circular and repeated references, another common JSON.stringify gotcha.

Two that come to mind are lave and arson (full disclosure: I am the author of arson).

Here's how the example might work:

> const { encode, decode } = require("arson")
undefined
> map = new Map
Map {}
> map.set(new Date, "test")
Map { 2016-11-26T17:34:28.448Z => 'test' }
> encode(map)
'[["Map",1],[2,3],["Date",4],"test","2016-11-26T17:34:28.448Z"]'
> decode(_)
Map { 2016-11-26T17:34:28.448Z => 'test' }

More examples in the README.md.

[benjamn]

There are a variety of libraries that attempt to improve on JSON by supporting stringification of objects with prototypes other than Object.prototype or Array.prototype, along with circular and repeated references, another common JSON.stringify gotcha.

Two that come to mind are lave and arson (full disclosure: I am the author of arson).

Here's how the example might work:

> const { encode, decode } = require("arson")
undefined
> map = new Map
Map {}
> map.set(new Date, "test")
Map { 2016-11-26T17:34:28.448Z => 'test' }
> encode(map)
'[["Map",1],[2,3],["Date",4],"test","2016-11-26T17:34:28.448Z"]'
> decode(_)
Map { 2016-11-26T17:34:28.448Z => 'test' }

More examples in the README.md.

[adius]

Thanks for the tip!

[adius]

Thanks for the tip!