>>>>>>问题
"cannot read property of undefined" 是一个常见的 JavaScript 错误,包含我在内很多人都会遇到,表示你试图访问一个未定义(undefined)对象的属性。这通常是因为你在访问一个不存在的对象或者变量。为了解决这个问题,你需要检查你的代码,确保在访问对象属性之前,对象已经被正确定义。
How can I avoid 'cannot read property of undefined' errors?
如何避免“无法读取未定义的属性”错误?
Given that below object , not all object has same property , normally happens in JSON format , 如果阁下遇到以下问题,a中未必包含b,b中未必包含c,甚至a也不一定存在,应该如何优雅的判断呢。
// Where this array is hundreds of entries long, with a mix
// of the two examples given
var test = [{'a':{'b':{'c':"foo"}}}, {'a': "bar"}];
for (i=0; i<test.length; i++) {
// OK, on i==0, but 'cannot read property of undefined' on i==1
console.log(a.b.c);
}
>>>>>>解决方法
强力推荐!封装GetProperty方法,从对象中获取属性,属性不存在则返回默认值
This is a common issue when working with deep or complex JSON object, so I try to avoid try/catch or embedding multiple checks which would make the code unreadable. I usually use this little piece of code in all my projects to do the job.
/* Example: getProperty(myObj, 'aze.xyz', 0) // return myObj.aze.xyz safely
* accepts array for property names:
* getProperty(myObj, ['aze', 'xyz'], {value: null})
*/
function getProperty(obj, props, defaultValue) {
var res, isvoid = function(x) {return typeof x === "undefined" || x === null;}
if(!isvoid(obj)) {
if(isvoid(props))
props = [];
if(typeof props === "string")
props = props.trim().split(".");
if(props.constructor === Array) {
res = props.length>1 ? getProperty(obj[props.shift()], props, defaultValue) : obj[props[0]];
}
}
return typeof res === "undefined" ? defaultValue: res;
}
思路二,我的项目中用的就是这个方法 !!! 好用
//by zhengkai.blog.csdn.net
const temp = {};
console.log(getSafe(()=>a.b.c, '0'));
function getSafe(fn, defaultVal) {
try {
if (fn() === undefined || fn() === null) {
return defaultVal
} else {
return fn();
}
} catch (e) {
return defaultVal;
}
}
使用默认参数值
在函数定义时,为参数设置默认值,以确保即使没有传递参数,也不会出现未定义的属性。
function example(param = "default value") {
console.log(param);
}
example(); // 输出 "default value"
hasOwnProperty检查属性是否存在
const obj = {
key: "value"
};
if (obj.hasOwnProperty("key")) {
console.log(obj.key);
} else {
console.log("Key does not exist");
}
使用逻辑或操作符(||)
const obj = {
key: "value"
};
console.log(obj.key || "Default value"); // 输出 "value"
使用解构赋值
const obj = {
key: "value"
};
const { key = "Default value" } = obj;
console.log(key); // 输出 "value"
可选链操作符optional chaining语法(.?)
- If you use JavaScript according to ECMAScript 2020 or later, see optional chaining.
- TypeScript has added support for optional chaining in version 3.7.
// use it like this
obj?.a?.lot?.of?.properties
使用可选链操作符(?.):可选链操作符允许你在尝试访问对象的属性时提供一个后备值,以防属性不存在。
const obj = {
key: "value"
};
console.log(obj?.key ?? "Default value"); // 输出 "value"
不是很建议的try/catch
A quick workaround is using a try/catch helper function with ES6 arrow function:
function getSafe(fn, defaultVal) {
try {
return fn();
} catch (e) {
return defaultVal;
}
}
// use it like this
console.log(getSafe(() => obj.a.lot.of.properties));
// or add an optional default value
console.log(getSafe(() => obj.a.lot.of.properties, 'nothing'));
不够优雅的“多重判断”方法
ry this. If a.b
is undefined
, it will leave the if
statement without any exception.
if (a && a.b && a.b.c) {
console.log(a.b.c);
}