.map 的第一个参数是您要遍历的项目，而不是索引.

The first argument to .map is the item you're iterating over, not the index.

由于这里的每个项目都是一个数组，因此您可以 .pop 数组(将对现有数组进行 muting 更改)或 .slice 数组(不会改变现有数组).

Since each item here is an array, you can either .pop the array (which will mutate the existing array), or .slice the array (which will not mutate the existing array).

var matrixWithExtraInfo = [
    [1,2,3,4,"dog"],
    [5,6,7,8,"dog"],
    [9,10,11,12,"dog"],
    [13,14,15,16,"dog"],
    [17,18,19,20,"dog"]
];

var conciseMatrix = [
    [1,2,3,4],
    [5,6,7,8],
    [9,10,11,12],
    [13,14,15,16],
    [17,18,19,20]
]

var conciseMatrix = matrixWithExtraInfo.map((arr) => {
  arr.pop();
  return arr;
});
console.log(matrixWithExtraInfo);
console.log(conciseMatrix);

(上面是 weird -您需要的结构已经在 matrixWithExtraInfo 中，使保存它的另一个变量令人困惑，但这与您的原始代码最接近)

(the above is weird - the structure you need is already in matrixWithExtraInfo, making another variable to hold it is confusing, but this is the closest to your original code)

var matrixWithExtraInfo = [
    [1,2,3,4,"dog"],
    [5,6,7,8,"dog"],
    [9,10,11,12,"dog"],
    [13,14,15,16,"dog"],
    [17,18,19,20,"dog"]
];

var conciseMatrix = [
    [1,2,3,4],
    [5,6,7,8],
    [9,10,11,12],
    [13,14,15,16],
    [17,18,19,20]
]

var conciseMatrix = matrixWithExtraInfo.map(arr => arr.slice(0, -1));
console.log(matrixWithExtraInfo);
console.log(conciseMatrix);