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2011-10-23

Printing objects on Node.js

Problem: If a data structure is too deeply nested, Node.js stops showing the details. This post presents two solutions.

Display a value as JSON data

Node.js stops displaying a data structure after two levels of nesting:
    > [[[ "test" ]]]
    [ [ [ 'test' ] ] ]

    > [[[[ "test" ]]]]
    [ [ [ [Object] ] ] ]
In this case [Object] stands for [ "test" ].

Option 1: a single line of JSON.

    > console.log("%j", [[[[ "test" ]]]]);
    [[[["test"]]]]
Option 2: Use JSON.stringify() [1] to print an indented tree.
    var tree = function(x) { console.log(JSON.stringify(x, null, 4)) };
    > tree([[[[ "test" ]]]]);
    [
        [
            [
                [
                    "test"
                ]
            ]
        ]
    ]
Warning: Anything non-JSONy will not be printed.
    > console.log("%j", { foo: function() {}, bar: "abc" })
    {"bar":"abc"}

Use util.inspect()

You can also use util.inspect(), but you need to specify that the nesting depth should be unlimited (the default is 2). It is also better not to show non-enumerable properties to avoid recursing into the properties of a function.

Option 1: Use util.inspect() directly.

    var util = require("util");
    > function ins(value) { console.log(util.inspect(value, false, null)); }

    > ins([[[[ "test" ]]]])
    [ [ [ [ 'test' ] ] ] ]

    > ins({ foo: function() {}, bar: "abc" });
    { foo: [Function], bar: 'abc' }

Option 2: Use console.dir(), which internally calls util.inspect(). Drawback: Stops after a nesting depth of 2.

    > console.dir([[[[ "test" ]]]])
    [ [ [ [Object] ] ] ]

    > console.dir({ foo: function() {}, bar: "abc" });
    { foo: [Function], bar: 'abc' }

Further reading

  1. JavaScript’s JSON API

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