Should I use an empty property key?

Question

I\'ve tested this only in Firefox, but apparently you can use an empty string as a key to a property in an object. For example, see the first property here:

Answer 1

An object's key must be a string, and the empty string ('') is a string. There is no cross browser issue that I've ever come across with empty strings, although there have been very few occasions where I thought it was acceptable to use an empty string as a key name.

I would discourage the general usage of '' as a key, but for a simple lookup, it'll work just fine, and sounds reasonable. It's a good place to add a comment noting the exceptional circumstance.

Additionally, during lookup you may have issues with values that are cast to a string:

o = {...} //some object
foo = 'bar';

//some examples
o[foo] //will return o['bar']
o[null] //will return o['null']
o[undefined] //will return o['undefined']

If you'd like to have null and undefined use the '' key, you may need to use a fallback:

key = key || '';

If you might have non-string values passed in, it's important to cast too:

key = key || '';
key = '' + key;

note that a value of 0 will turn into '', whereas a value of '0' will stay '0'.

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In most cases, I find I'm picking a pre-defined value out of a hashtable object. To check that the value exists on the object there are a number of options:

//will be falsey if the value is falsey
if (o[key]) {...}

//will return true for properties on the object as well as in the prototype hierarchy
if (key in o) {...}

//returns true only for properties on the object instance
if (o.hasOwnProperty(key)) {...}