What's faster in JS: Object with keys or array with an if statement?

Question

Which of these structures is generally more performant in JS with the v8 engine?

Example 1:

const obj = {
  a: 'hello',
  b: 'world',
  c: 'etc',
};

function getVal(str) {
  return obj[str];
}

getVal('b');

Example 2:


function getVal(str) {
  if(str=='a') return 'hello';
  if(str=='b') return 'world';
  return 'etc';
}

getVal('b');

I would imagine example 2 is faster, but 1 is better code. I ask because I'm looking at chess AI, and can structure the position weights as objects:

const positions_w = {
    'p':[
            [ 100, 100, 100, 100, 105, 100, 100,  100],
            [  78,  83,  86,  73, 102,  82,  85,  90],
            [   7,  29,  21,  44,  40,  31,  44,   7],
            [ -17,  16,  -2,  15,  14,   0,  15, -13],
            [ -26,   3,  10,   9,   6,   1,   0, -23],
            [ -22,   9,   5, -11, -10,  -2,   3, -19],
            [ -31,   8,  -7, -37, -36, -14,   3, -31],
            [   0,   0,   0,   0,   0,   0,   0,   0]
        ],
    'n': // ...

and then get them with positions_w[piece.type][y][x] or structure them in arrays:

const p = [
            [ 100, 100, 100, 100, 105, 100, 100,  100],
            [  78,  83,  86,  73, 102,  82,  85,  90],
            [   7,  29,  21,  44,  40,  31,  44,   7],
            [ -17,  16,  -2,  15,  14,   0,  15, -13],
            [ -26,   3,  10,   9,   6,   1,   0, -23],
            [ -22,   9,   5, -11, -10,  -2,   3, -19],
            [ -31,   8,  -7, -37, -36, -14,   3, -31],
            [   0,   0,   0,   0,   0,   0,   0,   0]
        ];

and then get them with if(piece.type=='p')return p[y][x]

Why don't you simply test this yourself? Half of the code is already there, you just need to take the time and create some benchmark loops. — Rob
– Rob, Commented Mar 3, 2021 at 8:27

geoffrey · Accepted Answer · 2021-03-03 14:48:10Z

1

If the object is small and was created with all of its keys, the speed of property access should be comparable to a switch statement.

One thing to consider however is that you will be updating your position weights pretty often. For this reason I think one big "3D" Int16Array would make a greater difference overall.

In any case, I would make sure no part of my app is dependant on the concrete data structure so that it is possible to delay this decision until later when you can actually profile your AI speed.

answered Mar 3, 2021 at 14:48

geoffrey

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2 Comments

Justin Over a year ago

Thank you for the insightful comment. I figured in JS objects are probably structured like hashtables and should be O(1) to search given the key, but if statements are pretty darn fast too. I'll try making a 3d array in a bit and see if that helps. Thank you.

geoffrey Over a year ago

Oh, V8 is smarter about it: it creates hidden classes in order to optimise property access. Hidden classes describing the same structure are shared. Properties added during the creation are stored in the most efficient way. Property offsets are cached after a couple of successful repetitive lookups (as long as the structure has not changed). However I do remember that rule of thumb: when you have less than about 10 cases, a switch statement is better than an object. With only 32 pieces in a chess game, I don't think average time complexity is the best tool to judge performance.

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