How does the node.js REPL stringify output?

Question

How does the node.js REPL stringify the objects that it prints? I became curious after running into this oddity with an array containing a key-value pair (?!):

> var arr = [1]
undefined
> arr
[ 1 ]
> arr.key = "value"
'value'
> arr
[ 1, key: 'value' ]

The typical functions don't appear to be generating exactly [ 1, key: 'value' ].

> arr.toString()
'1'
> JSON.stringify(arr)
'[1]'
> require("util").inspect(arr, {showHidden: true})
'[ 1, [length]: 1, key: \'value\' ]'

The last one's almost it, but has an additional [length] (to clarify, the quotes obviously don't matter). I'm running node v0.10.33.

Answer 1

You're almost there; util.inspect returns a string and is called without showHidden:

> require("util").inspect(arr)
'[ 1, key: \'value\' ]'
> console.log( require("util").inspect(arr) )
[ 1, key: 'value' ]

If you log a string to the console, it'll be wrapped in quotes, but console.log does not do this. Write inspect's output to a file if you want to verify that it's exactly the same as console.log's output:

> require("fs").writeFileSync("dump.txt", require("util").inspect(arr));

In your console:

$ cat dump.txt
[ 1, key: 'value' ]

---

Regarding the first paragraph:

array containing a key-value pair (?!):

It's not that weird - arrays are objects, so it's perfectly valid. But it might be confusing to other people looking at your code. :-)

Doing that is not much different from this:

var fn = function() {}
fn.key = "value";

It'll look just like your array example if you were to log this.