在 JavaScript 对象数组中查找第一个未使用的属性值

Question

In looking for a way to efficiently determine the lowest positive integer that is not used as a value of a specific property in any object within an array of objects.

In other words, I'm looking for a function/algorithm that for these arrays:

var example1 = [{ id: 1 }, { id: 2 }, { id: 3 }],
    example2 = [{ id: 6 }, { id: 4 }, { id: 2 }],
    example3 = [{ id: 2 }, { id: 1 }, { id: 4, otherProp: 3 }];

Would return 4, 1 and 3 respectively. (Obviously using the id-property, in this example.)

I considered using Underscore.js for this, but I couldn't find a way to do it without some ugly nested loops. Does anybody have a better idea?

Answer 1

function next(prop) {
    return function(arr) {
        var used = arr.reduce(function(o, v) {
            o[v[prop]] = true;
            return o;
        }, {});
        for (var i=1; used[i]; i++);
        return i;
    }
}
var nextId = next("id");

nextId([{ id: 1 }, { id: 2 }, { id: 3 }]) // 4
nextId([{ id: 6 }, { id: 4 }, { id: 2 }]) // 1
nextId([{ id: 2 }, { id: 1 }, { id: 4, otherProp: 3 }]) // 3

Answer 2

One possible approach:

function aiKey(arr, prop) {
  var indices = [];
  arr.forEach(function(el) {
    indices[ el[prop] ] = true;
  });
  for (var i = 1, l = indices.length; i < l; i++) {
    if (indices[i] === undefined) {
      break;
    }
  }
  return i; 
}

aiKey(example1, 'id'); // 4
aiKey(example2, 'id'); // 1
aiKey(example3, 'id'); // 3