How does console.log convert javascript date object into human readable format - Stack Overflow

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Output of console.log({a:new Date()}) is { a: 2019-05-19T11:30:57.514Z }

Value of JSON.stringify({a:new Date()}) is {"a":"2019-05-19T11:33:12.591Z"}

After overriding this: Date.prototype.toJSON = function(){ return this.toLocaleString(); }

Value of JSON.stringify({a:new Date()}) is {"a":"5/19/2019, 5:09:31 PM"}

But output of console.log({a:new Date()}) is still { a: 2019-05-19T11:41:31.256Z }

Tried overriding other Date.prototype methods like toISOString(), toSource, toString, toUTCString, valueOf and many others. But none helped.

Couldn't understand the native source code of v8 js engine.

Any way to override the behavior to get the desired result?

Output of console.log({a:new Date()}) is { a: 2019-05-19T11:30:57.514Z }

Value of JSON.stringify({a:new Date()}) is {"a":"2019-05-19T11:33:12.591Z"}

After overriding this: Date.prototype.toJSON = function(){ return this.toLocaleString(); }

Value of JSON.stringify({a:new Date()}) is {"a":"5/19/2019, 5:09:31 PM"}

But output of console.log({a:new Date()}) is still { a: 2019-05-19T11:41:31.256Z }

Tried overriding other Date.prototype methods like toISOString(), toSource, toString, toUTCString, valueOf and many others. But none helped.

Couldn't understand the native source code of v8 js engine.

Any way to override the behavior to get the desired result?

• javascript
• date
• overriding
• v8

Share Improve this question Follow asked May 19, 2019 at 11:49 Jayram KumarJayram Kumar 71244 gold badges1717 silver badges3131 bronze badges 3

• 1 json is not object, and imho diff them in console is a good thing. – apple apple Commented May 19, 2019 at 12:14
• if you want json representation, use console.log(JSON.stringify(...)) – apple apple Commented May 19, 2019 at 12:19
• You didn't actually specify what "the desired result" is. – jmrk Commented May 19, 2019 at 12:40

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3 Answers 3

Sorted by: Reset to default 4

It depends on the console implementation, which isn't in V8 but rather in the host (Chrome, Chromium, Node.js, ...).

Node.js used to look for a member called inspect (back in v4 at least) and use that if it was present, but that turned out to be a patibility problem and it doesn't do that anymore.

I don't think there's any way to override the console's rendering of built-in objects anymore, particularly not across host environments.

The console uses toString to convert dates (and other objects) to string for printing. So you can override Date.prototype.toString to change what gets printed. Demo (copy-pasted from a Chrome console session):

> var d = new Date()
> console.log(d)
< Sun May 19 2019 14:29:02 GMT+0200 (Central European Summer Time)
> Date.prototype.toString = Date.prototype.toLocaleString
< ƒ toLocaleString() { [native code] }
> console.log(d)
< 5/19/2019, 2:29:02 PM
> console.log({a:new Date()})
< {a: 5/19/2019, 2:31:21 PM}

A separate issue is that JSON.stringify({a: 1}) gives {"a":1}, whereas console.log({a: 1}) prints {a: 1} (without quotes around a). That's unrelated to dates, and is due to the fact that JSON is a very limited subset of JavaScript ({a: 1} is valid JavaScript but not valid JSON). Also, console.log is intended for human consumption, there is no requirement that the output can be parsed back to the original object(s) -- just consider what console.log(new Array(100)) does :-)

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Edit: The above is true for Chrome. Since the console object is not covered by the ECMAScript spec, every embedder can provide whatever they think is useful there. Node.js uses util.format, which in turn uses util.inspect, which in turn gets the toISOString function from the primordial Date.prototype, which means you can't override it (which is a feature because that means you can't break it, or a limitation because it means you can't customize it, depending on your point of view).

You are not converting the new date() into json when you are doing the console.log(). You can write it this way.

Date.prototype.toJSON = function(){ return this.toLocaleString(); }
console.log({a:JSON.stringify(new Date())})

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