>>>>>>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==1console.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);
}