Question

如何在使用序数访问不存在的键时避免错误[重复]

2021-10-01

366

javascript arrays json undefined

以下代码的结果是 Cat 和 undefined。 *如果键不存在，结果将为“undefined”。

var animals = {"mammals":["Cat","Dog","Cow"]};
   var groupA  = animals.mammals[0];
   var groupB  = animals.birds;
   console.log(groupA );
   console.log(groupB);

但是，以下代码的结果是“error”而不是“undefined”。

*Uncaught TypeError: Cannot read properties of undefined (reading '0')

var animals = {
  "mammals": ["Cat", "Dog", "Cow"]
};
var groupA = animals.mammals[0];
var groupB = animals.birds[0];

console.log(groupA);
console.log(groupB);

如果键具有不存在的序数，如何获取“undefined”？

Answer 1

您可以使用可选链接

The optional chaining operator (?.) enables you to read the value of a property located deep within a chain of connected objects without having to check that each reference in the chain is valid.

var animals = {
  "mammals": ["Cat", "Dog", "Cow"]
};
var groupA = animals.mammals?.[0];
var groupB = animals.birds?.[0];

console.log(groupA);
console.log(groupB);

Answer 2

您需要先评估属性，见下文：

var animals = {"mammals":["Cat","Dog","Cow"]};
var groupA  = animals.mammals[0];
var groupB  = (animals.birds || [])[0];

Answer 3

您可以检查属性并使用三元组明确设置该值：

var animals = {
  "mammals": ["Cat", "Dog", "Cow"]
};
var groupA = animals.mammals ? animals.mammals[0] : undefined;
var groupB = animals.birds ? animals.birds[0] : undefined;

console.log(groupA);
console.log(groupB);