如何定义数组对象的可变类型的新对象？

Question

jQuery has made my life easier but I'm still very beginner to JavaScript. So, may be, I'm asking a very stupid question here:

var t = {
    rows: 3,
    columns: 5,
    getCellCount: function () {
        return this.rows * this.columns;
    }
};
var tn = t;
tn.rows = 6;
document.write(tn.rows + " , " + t.rows);   // returns 6 , 6

I also tried var tn = new t(); // but seems wrong var tn = new t(); // but seems wrong

So, How to retrieve old intrinsic value from object so that it results 6 , 3

Answer 1

tn and t are both pointing to the same object, that's why when you change tn.rows it also changes t.rows . There is no such thing as old intrinsic value .

You must copy the object in order to keep the old value. There are two: shallow copy and deep copy .

Copying the key-value pairs is pretty easy with Object.create .

var newObj = Object.create(oldObj);

Now if you change the values in newObj , it will not change the original one:

var a = {a:1}, b = Object.create(a);
b.a = 2;
console.log(a.a, b.a);  //1,2

However, to perform a complete copy is really complicated. See more: How do I correctly clone a JavaScript object?

PS: The new keyword you mentioned is for creating an object as in class es.

function Car(n, y){                              //This is called a "constructor"
    var name = n, year = y;                      //Private variables
    return {
        getName: function(){ return name; },     //Getters
        getYear: function(){ return year; }
    };
}
var myNewCar = new Car("Toyota", 2010);
myNewCar.getName();  //"Toyota"
myNewCar.getYear();  //2010

_{(This is also how you create objects in Java, if you have taken those CS courses you would recognize this pattern.)}

Answer 2

var tn = t;

simply makes both tn and t to point to the same object in memory. So, change in one object will reflect in other as well. You can clone the object, like this

function copyObject(sourceObject) {
    var result = {};
    for (var key in sourceObject) {
        if (sourceObject.hasOwnProperty(key)) {
            result[key] = sourceObject[key];
        }
    }
    return result;
}
var tn = copyObject(t);

Answer 3

You are asking to clone a JSON object.

var tn={}; 
for (key in t) tn[key]=t[key];

There may be other "prettier" ways, but this guarantees the clone.

Answer 4

The = operator in javascript just changes what the object points to, so it will not create a copy of the original object. You can take a look here to see possible ways to create a clone of the object.

Answer 5

If you want to create a Javascript object, the conventional way is to create a function:

// Use capitalized names for Object-creating functions.
// I guessed that you wanted a matrix of some sort.
function Matrix(rows, columns) {
    this.rows = rows;
    this.columns = columns;
    this.cellCount = function() {
        return this.rows * this.columns;
    }
    this.dimensions = function() {
        return this.rows, + ", " + this.columns;
    }
    this.copy = function() {
        return new Matrix(this.rows, this.columns);
    }
}

var t = new Matrix(6, 3);

There are more sophisticated ways to do this, using Object.create . Look at Javascript: The Good Parts .