understanding decision table in javascript

Question

I was just going through the code of Jbox and cam across the following snippet of code:

// Internal functions, used to easily get values
    this._getOpp = func         


        

Answer 1

It's a technique that I know as a decision table (a wrote a blog post about them a while ago), but other might call it by different names.

It uses the square bracket notation in the same way you'd reference the index of an array:

var arr = [0, 2, 3];
return arr[1]; // return the second value in the array

Because object properties are key/value pairs it works like this:

  return {
    left: 'right', 
    right: 'left', 
    top: 'bottom', 
    bottom: 'top',
    x: 'y', 
    y: 'x'
  }[opp]

It will actually return the string value, to the key opp.

i.e. if opp is 'right', it will return 'left'.

n.b. the wiki page on decision tables is also worth a read

Answer 2

It creates an object with values first, then return a specific attr in the object whose key is the given opp. And if no such key is founded, undefined is returned.

You can see them as :

var states= {
    left: 'right', 
    right: 'left', 
    top: 'bottom', 
    bottom: 'top',
    x: 'y', 
    y: 'x'
};
return settings[opp];

The snippet shows how it would act.

var getOpp = function(opp) { 
      return {left: 'right', 
      right: 'left', 
      top: 'bottom', 
      bottom: 'top',
      x: 'y', 
      y: 'x'}[opp]; 
};

console.log(getOpp('right'));
console.log(getOpp('oops'));