如何返回JS文字对象的默认属性？

Question

One JS object is defined like this:

var obj = {
      key1 : {on:'value1', off:'value2'},
      key2 : {on:'value3', off:'value4'}
}

Is there a tricky way, to pick a default key1 on property, when the obj.key1 is passed without property?

var key1State = obj.key1; // I want to receive 'value1' here, not obj.key1{...}

Is it somehow generally possible to recognize in the object definition body which property (if at all) is passed/asked during the object call?

Answer 1

I'm not sure if this is what you need, but messing with valueOf or toString may be what you're looking for.

For example:

var obj = {
      key1 : {on:'value1', off:'value2', toString : function(){ return this.on; }},
      key2 : {on:'value3', off:'value4', toString : function(){ return this.on; }}
}

var key1State = obj.key1; // the object
var key1StateStr = '' + obj.key1; // string "value1"
obj.key1 == "value1"  // true
obj.key1.toString() === "value1"  // true

So if you were going to use the default value as a string somewhere this may be useful.

Answer 2

This does not fix your case, but you may find it useful.

Use getters.

var obj = {

  key1 : {on:'value1', off:'value2'},
  key2 : {on:'value3', off:'value4'}, 
  get fooBar () { return this.key1.on }

}

now obj.fooBar returns keyq.on

Answer 3

I am understand your idea, but think the better way here is to use function like getState for getting actual state of object.

var obj = {
  key1: {on:'value1', off:'value2'},
  key2: {on:'value3', off:'value4'}
};

var key1State = getState(obj.key1);
var key2State = getState(obj.key2.off);

function getState(obj) {
  return obj.on || obj;
}

console.log(key1State, key2State);

See codepen.