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