无法访问javascript变量中的对象属性

Question

I've struggled for a while seek for help. This is really strange. I want to access an object property, but this always throws an error:

TypeError : cannot read property template of undefined

But my application works properly. Just there is notification output if can't access template of undefined

 //this is my object variabel var login = {}; login.data = { checkInput : formValidation, userSchema : User, template : 'pages/users/login', } // so I add new method which I call in different files login.header = async(req, res, next) => { /// in this section function I want to read property of template but it always return undefined try { // I have some code with read databases here //some data i want to render var data = {}; res.render(this.data.template,data); // I've been also trying another way. var template = login.data.template !== undefined ? 'page/users/login' : login.data.template; res.render(login.data.template, data); // both of above always return output, but can't read template of undefined } catch(e) { throw new Error(e); } } 

Answer 1

It's because you're using an arrow function which loses the binding to login (the this you're accessing is an attempt to access the login object). Use a regular ES5 function.

login.header = async function(req, res, next) {...};

As found in the docs :

An arrow function expression is a syntactically compact alternative to a regular function expression , although without its own bindings to the this , arguments , super , or new.target keywords. Arrow function expressions are ill suited as methods, and they cannot be used as constructors.

Answer 2

i've been trying to access with property with no array function

 var login = {} login.data = { template : 'admin/pages/' body : {} }; login.getViews = function(req, res, next) { // it'l throw an error res.render(this.data.template, this.data.body); // than i try with another way, it works res.render(login.data.template, login.data.body); } 

so why i could't access with property this even i use no rows function??