console.log formatting for dates and objects

Question

How can we format a value into a string to be consistent with how console.log(value) does it?

I need to append extra details to each value I am printing, without breaking the default formatting provided by console.log. It is easy for simple values, but for such types as Date and Object it is more complicated.

Is there a way I can make it consistent with console.log? In other words, how to format a value to be the same string as we get from console.log(value)?

I'm trying to implement a simple library manakin. Here's its complete source code:

'use strict';

(function () {

    var error = console.error, warn = console.warn;

    console.error = function () {
        var a = arguments;
        error.apply(this, Object.keys(a).map(function (key) {
            return "\u001b[31m" + a[key] + "\u001b[0m";
        }));
    };

    console.warn = function () {
        var a = arguments;
        warn.apply(this, Object.keys(a).map(function (key) {
            return "\u001b[33m" + a[key] + "\u001b[0m";
        }));
    };

})();

The problem I am having is that for types Date and Object the output is different:

// without manakin:
console.warn({value: 1});
//=> { value: 1 }

// with manakin:
console.warn({value: 1});
//=> [object Object]

And I want to make it consistent.

Simply put; if console.log is like a printf, then OP wants a sprintf. — noisypixy
– noisypixy, Commented Jun 19, 2016 at 23:36

omarjmh · Accepted Answer · 2016-06-19 23:50:06Z

1

I believe if you modify the code in warn to be:

console.warn = function () {
    var a = arguments;
    warn.apply(this, Object.keys(a).map(function (key) {
      if (a[key] instanceof Date) {
        return "\u001b[33m" + a[key].toString() + "\u001b[0m";
      }
      return "\u001b[33m" + JSON.stringify(a[key]) + "\u001b[0m";
    }));
};

you have what you're looking for. I tested it out by just pasting the new code in the console and logging the examples you have, hope that works for you!

edited Jun 19, 2016 at 23:50

answered Jun 19, 2016 at 23:37

omarjmh

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4 Comments

noisypixy Over a year ago

I thought this approach too, but for dates, JSON.stringify returns them in RFC3339 format, while console.log prints them in what seems to be RFC1123. Maybe checking for a[key] instanceof Date and, if true, use a[key].toString() instead.

vitaly-t Over a year ago

But the output from JSON.stringify is still very different from console.log formatting.

vitaly-t Over a year ago

I think I will make it for Node.js only, and then use this: stackoverflow.com/questions/10729276/…. Because otherwise there seems to be no way to provide consistent formatting.

vitaly-t Over a year ago

In the end I got it all working everywhere - see the update ;)

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