javascript - I don’t understand a function, please give me some advice?

Question

function choose(arr, size) {
  var allResult = [];

  (function (arr, size, result) {
    var arrLen = arr.length;
    if (size > arrLen) {
      return;
    }
    if (size == arrLen) {
      allResult.push([].concat(result, arr))
    } else {
      for (var i = 0; i < arrLen; i++) {
        var newResult = [].concat(result);
        newResult.push(arr[i]);

        if (size == 1) {
          allResult.push(newResult);
        } else {
          var newArr = [].concat(arr);
          newArr.splice(0, i + 　1);
          arguments.callee(newArr, size - 1, newResult);
        }
      }
    }
  })(arr, size, []);

  return allResult;
}

Answer 1

This is a permutation and combination implementation using black magic. The functions achieved are roughly:

• choose([1, 2, 3], 1) gets [ [ 1 ], [ 2 ], [ 3 ] ]

• choose([1, 2, 3], 2) gets [ [ 1, 2 ], [ 1, 3 ], [ 2, 3 ] ]

• choose([1, 2, 3, 4], 3) gets [ [ 1, 2, 3 ], [ 1, 2, 4 ], [ 1, 3, 4 ], [ 2, 3, 4 ] ]

The inner anonymous function calls itself recursively through arguments.callee. Each time it recursively calls itself, the size parameter is reduced by one. Each newResult is a [length is the total number of combinations, and each element is the current combination. ] is a two-dimensional array. When the length of each item in newResult reaches size, the recursion ends and the final result is returned.

Answer 2

Closure writing method prevents the internal parameters of the function from being affected by external factors.