How can I create a two-way mapping in JavaScript, or some other way to swap out values?

With an extra internal object for reverse mapping. Best if we add a utility class 😉 like the following:

ES6 syntax (scroll down for ES5 syntax)

class TwoWayMap {
    constructor(map) {
       this.map = map;
       this.reverseMap = {};
       for(const key in map) {
          const value = map[key];
          this.reverseMap[value] = key;   
       }
    }
    get(key) { return this.map[key]; }
    revGet(key) { return this.reverseMap[key]; }
}

Then you instantiate like this:

const twoWayMap = new TwoWayMap({
   '*' : '__asterisk__', 
    '%' : '__percent__',
   ....
});

Finally, to use it:

twoWayMap.get('*')   //Returns '__asterisk__'
twoWayMap.revGet('__asterisk__')  //Returns '*'

Bonus: If you also need set/unset methods, you can do it (inside the class) easily like:

set(key, value) { this.map[key] = value; }
unset(key) { delete this.map[key] }
// same for revSet and revUnset, just use this.reverseMap instead

Equivalent with ES5 (old js) syntax:

function TwoWayMap(map) {
   this.map = map;
   this.reverseMap = {};
   for(var key in map) {
      var value = map[key];
      this.reverseMap[value] = key;   
   }
}
TwoWayMap.prototype.get = function(key){ return this.map[key]; };
TwoWayMap.prototype.revGet = function(key){ return this.reverseMap[key]; };

Usage is the same:

var twoWayMap = new TwoWayMap({
   '*' : '__asterisk__', 
    '%' : '__percent__',
   ....
});
twoWayMap.get('*')   //Returns '__asterisk__'
twoWayMap.revGet('__asterisk__')  //Returns '*'

Hope this helps. Cheers