Set undefined javascript property before read

Three solutions:

• Implement your object as a Proxy, which is designed to do exactly what you want. Yet, it is only a draft and currently only supported in Firefox’ Javascript 1.8.5 It was standardised with ES6, but might not yet be available in all environments.

• Always fill your translation object with a complete set of messages. When creating that “dictionary” (serverside or clientside), always include all needed keys. If no translation exists, you can use a fallback language, the message’s name or the string representation of undefined – your choice.

  But a non-existing property should always mean “there is no such message” instead of “no translation available”.

• Use a getter function with a string parameter instead of object properties. That function can look the messages up in an internal dictionary object, and handle misses programmatically.

  I would recommend a map object which is different from the dictionary, to allow “get” and co as message names:

var translate = (function(){
    var dict = {
        something: "somethingelse",
        ...
    };
    return {
        exists: function(name) { return name in dict; },
        get: function(name) { return this.exists(name) ? dict[name] : "undefined"; },
        set: function(name, msg) { dict[name] = msg; }
    };
})();