您使用相同的 i 变量来控制循环，因此 i 仅在第一次迭代中超过了 sanitized_input。

// Below are the lists of the numbers (the original dataset)
var array_of_nums1 = [55.5, 51, 54.5, 53.5, 51, 59.5, 54, 42, 53.5, 40];
var array_of_nums2 = [60, 64.5, 66.5, 67, 62, 55, 62.5, 64, 65, 55];

// Here I collect how many times the user wants to loop over this mess.
var amount_of_loops = prompt("How many times would you like to loop over the list?", "Numbers only, please and thank you");
var sanitized_input = Number(amount_of_loops);

// This is SUPPOSED TO BE the array in which array that holds the final values are held in.
var list_of_outputted_numbers = [];

for (var i = 0; i < sanitized_input; i++) {

  // Below I am creating a new array that will contain the a randomized selection of 10 numbers from the original list.
  var random_array_of_nums1 = [];
  for (var j = 0; j < array_of_nums1.length; j++) {
    var rand_NUM_array1 = array_of_nums1[Math.floor(Math.random() * array_of_nums1.length)];
    random_array_of_nums1.push(rand_NUM_array1)
  }

  // Here I'm doing the same thing I did ealier, but with the second list
  var random_array_of_nums2 = [];
  for (var k = 0; k < array_of_nums2.length; k++) {
    var rand_NUM_array2 = array_of_nums2[Math.floor(Math.random() * array_of_nums2.length)];
    random_array_of_nums2.push(rand_NUM_array2)
  }

  // Here I'm finding the total of the randomized first list, than the total of the elements of the randomized second list.
  var rand_array_total1 = 0;
  for (var l = 0; l < random_array_of_nums1.length; l++) {
    rand_array_total1 += random_array_of_nums1[l];
  }
  var rand_array_total2 = 0;
  for (var m = 0; m < random_array_of_nums2.length; m++) {
    rand_array_total2 += random_array_of_nums2[m];
  }

  // Here I am finding the mean of all the numbers of the randomized elements list.
  var mean1 = rand_array_total1 / random_array_of_nums1.length;
  var mean2 = rand_array_total2 / random_array_of_nums2.length;

  // Difference between the means

  var mean_diff = mean2 - mean1;
  list_of_outputted_numbers.push(mean_diff);

}

console.log(list_of_outputted_numbers);

此代码应能按预期工作。

小心使用循环变量。您有以下问题：

1. 您的第一个 for 循环未声明变量 i ；当您的代码在非严格模式下运行时（确实如此），这意味着 i 将被定义为全局变量。
2. 您在内循环中使用相同的 i 变量；这意味着内循环和外循环将使用相同的 i 计数器变量，这不是您想要的。
3. 其他时候您使用 var i ；至少您在那里声明了变量，但 var 的行为违反直觉，因为变量是 函数作用域 ，而不是 块作用域 。通常您永远不应使用 var ；总是首选 let 或 const ，因为它们带来的意外更少。块范围变量的额外好处是，您可以重复使用相同的变量名称，而无需 覆盖 外部块变量的值（尽管这会让您的代码变得混乱，并且通常最好不要 遮蔽 变量）。

以下是您的循环应有的样子的简化版本：

for (let i = 0; i < sanitized_input; i++) {
  // ...
  for (let j = 0; j < array_of_nums1.length; j++) {
    // ...
  }
  // ...
  for (let j = 0; j < array_of_nums2.length; j++) {
    // ...
  }
  // etc.
}