In nodejs 'inspect' acts like a reserved word

Question

In nodejs 'inspect' acts like a reserved word

In node.js calling console.log on an object that has an element called inspect prints undefined even though it still works as an object. I assume this is because node uses inspect internally to print stuff out.

var thingTwo = {
  num: 1,
  func: function (x) { 2; },
  inspect: function (x) { "hi"; },
};

console.log(thingTwo);  // undefined

To avoid this trap in the future is there a list of other words that break standard functionality?

javascript
node.js

Answer 1

Cool, this piqued my curiosity and indeed, there is an undocumented feature where an object can provide it's own inspect method. The relevant code in util.js:

function formatValue(ctx, value, recurseTimes) {
  // Provide a hook for user-specified inspect functions.
  // Check that value is an object with an inspect function on it
  if (value && typeof value.inspect === 'function' &&
      // Filter out the util module, it's inspect function is special
      value.inspect !== exports.inspect &&
      // Also filter out any prototype objects using the circular check.
      !(value.constructor && value.constructor.prototype === value)) {
    return String(value.inspect(recurseTimes));
  }

I.e., it's only when inspect exists and is a function that triggers the behavior.

Answer 2

If an object has an inspect property and it is a function, console.log prints its return value.

$ node
> console.log({ inspect: function() { return 'hi'; } });
hi

If we look at the source, we'll see that there's no way to avoid this behavior. If an object's inspect is a function, console.log will print its return value.

Note that in Node 0.6.x and before, console.log doesn't actually print undefined, it just prints a blank line. You'd only see undefined if you're doing this in the REPL.