javascript - 数组内边界的条件

Question

我有一个矩阵，我有一个函数可以使用以下代码随机选择数组的一个元素:

npcSelectShip() {
    let selectCol = () => {
        let colIndex = Math.floor(Math.random() * this.npcSelectionField.length);
        let selectCell = () => {
            let cellIndex = Math.floor(Math.random() * this.npcSelectionField[colIndex].length);
            if (this.npcSelectionField[colIndex][cellIndex].isEmpty === false) {
                selectCell();
            } else {
                this.npcSelectionField[colIndex][cellIndex].isEmpty = false;
                this.pickDirection(this.npcSelectionField, colIndex, cellIndex);
            }

        }

        selectCell();
    }

    selectCol();
}

在此之后，我有另一个函数来搜索随机选取元素的邻居(顶部、右侧、底部和左侧)，随机选取一个邻居并更改属性:

    pickDirection(field, col, cell) {
       let neighbors = [];
       neighbors.push(
           field[col - 1][cell],
           field[col + 1][cell],
           field[col][cell - 1],
           field[col][cell + 1]
       );
       let randDir = () => {
           let randIndex = neighbors[Math.floor(Math.random() * neighbors.length)];
           if (randIndex.isEmpty === false) {
               randDir();
           } else {
               randIndex.isEmpty = false;
           }
       }

       randDir();
}

我面临的问题是，当随机选取的元素的索引为 0 或等于数组长度时，因为如果它选取索引为 1 或索引+1 处的邻居，则它基本上是“越界”我收到这些错误:

TypeError: Cannot read property 'isEmpty' of undefined
TypeError: Cannot read property '9' of undefined

有没有一种方法可以解决这个问题，而不必编写大量的 ifs 和 elses ？

感谢您的帮助。

Answer 1

您可以使用Array.concat和一个默认模式，它返回一个空数组。

与concat结合使用，空数组充当中性值。

var neighbors = [].concat(
       (field[col - 1] || [])[cell] || [],
       (field[col + 1] || [])[cell] || [],
       (field[col] || [])[cell - 1] || [],
       (field[col] || [])[cell + 1] || []
   );

或者使用包装器来访问

function getCell(array, col, cell) {
    return (array[col] || [])[cell] || [];
}

使用

var neighbors = [].concat(
       getCell(field, col - 1, cell),
       getCell(field, col + 1, cell),
       getCell(field, col, cell - 1),
       getCell(field, col, cell + 1)
   );

How it works

(field[col - 1] || [])[cell] || []

It tries to get the value of

 field[col - 1]

and if the value is undefined,

 field[col - 1] || []

it returns an empty array, with the locical logical OR || operator. What we get is either the array of field[col - 1] or an empty array [].

For the next index, we use the same pattern and check if

(field[col - 1] || [])[cell]

exists and if not then we take another empty array as result

(field[col - 1] || [])[cell] || []

Now we have either a truthy value, like an object or an empty array.

This is necessary, because empty arrays are not added to an array with Array.concat.