为什么我在Javascript中收到错误消息对象没有方法？

Question

I have the following code however am getting the error Uncaught TypeError: Object #<addThis> has no method 'returnValue' (anonymous function)

function addThis() {
    this.value1 = 1;
    this.value2 = 2;

    var returnValue = function () {
        return (this.value1 + this.value2);
    }
}

//Instantiate object and write response
var simpleObject = new addThis();
document.write(simpleObject.returnValue());

Answer 1

when you use this. it is public in scope. when you use var , it is private . since you used var returnValue , it is private, and thus not exposed for use.

In fact, i'm guessing you wanted to hide the values and expose the getter, so reverse what you did..

function addThis() {
    var value1 = 1;
    var value2 = 2;

    this.returnValue = function () {
        return (this.value1 + this.value2);
    }
}

Answer 2

var will declare a variable local to the function. I think you meant to assign it to this.returnValue :

function addThis() {
    this.value1 = 1;
    this.value2 = 2;

    returnValue = function () {
        return (this.value1 + this.value2);
    };
}

// Instantiate object and write response
var simpleObject = new addThis();
document.write(simpleObject.returnValue());

Answer 3

Because returnValue is just a local variable in the addThis function, it doesn't end up in the object that is created.

Assign the function to a property of the object:

function addThis() {
  this.value1 = 1;
  this.value2 = 2;

  this.returnValue = function() {
    return this.value1 + this.value2;
  };
}

Or use the prototype for the object:

function addThis() {
  this.value1 = 1;
  this.value2 = 2;
}

addThis.prototype.returnValue = function() {
  return this.value1 + this.value2;
};