The trick to making console.log play nice with complex objects

#javascript #typescript #node #webdev

console.log is useful in a lot of situations (although there are usually better ways to debug if that's what you're doing).

In the browser console.log works well with objects, you can drill down as much as you need. But in Node.js, when you look at the output of a nested object, you'll often see this:

$ console.log({a: {b: { c: {d: {}}}}})

{ a: { b: { c: [Object] } } }

d: {} was replaced with [Object]. But why?

It's because the command line / terminal doesn't have a nice UI for drilling down, so Node attempts to print up to three levels deep. Beyond three levels it just prints [Object].

This is controlled by a variable in the node 'util' module, called depth, which defaults to 2. You can set it yourself here:

require('util').inspect.defaultOptions.depth = 0; // top level only, e.g.:
// { a: [Object] }
require('util').inspect.defaultOptions.depth = null; // print everything, e.g.: 
// {
//  a: { b: { c: { d: {} } } }
// }

It's not a great idea to change an underlying variable, as it might come back to bite later. So a cleaner way is to convert the JSON object to a string and log that. We can use node's built in JSON class and the stringify method:

complexObject = {a: {b: { c: {d: {}}}}}
console.log(JSON.stringify(complexObject, null, 2))

// {
//   "a": {
//     "b": {
//       "c": {
//         "d": {}
//       }
//     }
//   }
// }

Note that the 3rd parameter of JSON.stringify, Number 2, controls the number of spaces of indentation, and the 2nd parameter can be used to filter or adjust the objects and properties shown.

Now you can really see what's in those deep Objects.

Top comments (8)

Peter Franken • Oct 21 '21 • Edited

Or better yet console.dir(obj, { depth: null }) :)

Tracy Gilmore • Oct 23 '21 • Edited

Using JSON.stringify is useful in most cases but has limitations. There are some data types that JSON does not support well such as Dates. In complicated scenarios there is also a risk the object could contain a circular reference. I.e. a reference to a child object within the parent.
Reference MDN for more details on the Console API. developer.mozilla.org/en-US/docs/W...

Jeremy Bradbury • Oct 23 '21

I'm with Peter on console.dir & TJ prettifying JSON can cause other problems.

The depth to null, is not great, but perhaps 5... You shouldn't rly allow infinite depth.

And ideally just log obj.a.b instead or use console log how it was intended: console.log('label for line', obj.a, obj.a.b, obj.a.b.c, obj.a.b.c.d)