What is the best design pattern for achieving the following (which doesn't work)?
var obj = (function() {
// code defining private variables and methods
var _obj = {
property: value,
method1: function() {
// do stuff
},
method2: function() {
// use property
var prop = _obj.property; // obviously doesn't work
// call method1
obj.method1(); // "obj" not finished being defined yet!
}
};
// obviously now I could do...
var prop = _obj.property;
return _obj;
})();
// and I could now do...
obj.method1();
A variation which I think should work is
var obj = (function() {
var property = value,
method1 = function() {
// do stuff
},
method2 = function() {
// use property
var prop = property;
// call method1
method1();
},
_obj = {
property: property,
method1: method1,
method2: method2
};
return _obj;
})();
Similarly, how does it work for objects meant to be created with the new
operator? Within the constructor function itself you can write this.method()
. But what if you want to keep the constructor small, only defining those things which will likely be customized upon creation, and then defining the rest in the prototype? (This seems to be the common pattern.) Can the properties / methods within the prototype interact in any way?
var MyObj = function(name) {
this.name = name;
};
var obj = new MyObj('Bob');
MyObj.prototype = {
called_often: function() {
// lots more code than just the following
return document.getElementById('someID').value;
},
global_default: 'value', // can be changed, so need to pull value when run
does_stuff: function(value) {
var str = global_default + value, // can't access global_default on its own
input = MyObj.called_often(), // doesn't work; MyObj.prototype.called_often() DOES
name = this.name; // 'this' used in the prototype doesn't work
// even within a created object
return name + input + str;
}
};
I'm sure there's better ways to achieve my result whenever I run into this problem. This code isn't situation specific and just illustrates the general problem. So you won't be able to give me an alternative for those specific situations I run into. But maybe you can help my overall thinking.
You can call a function inside an object by declaring the function as a property on the object and invoking it, e.g. obj. sum(2, 2) . An object's property can point to a function, just like it can point to a string, number or other values.
Objects in JavaScript are collections of key/value pairs. The values can consist of properties and methods, and may contain all other JavaScript data types, such as strings, numbers, and Booleans.
How can we make methods available on all objects? Explanation: It is possible to add methods to Object. prototype, making them available on all objects. This is not recommended, however, because prior to ECMAScript5, there is no way to make these add-on methods non enumerable, and if you add properties to Object.
A member function of a class is a function that has its definition or its prototype within the class definition like any other variable. It operates on any object of the class of which it is a member, and has access to all the members of a class for that object.
Well, from your first example:
var _obj = { property: value, method1: function() { // do stuff }, method2: function() { // use property var prop = this.property; // call method1 this.method1(); } };
That's what the this
value is for.
Now, what you cannot do is refer to a property of an "under construction" object from elsewhere in the object literal syntax. (It's hard to give an example because it's just not syntactically possible.) In cases where you want to do that, you do need one or more separate assignment statements.
Guess what? You are making simple stuff complex. Pointy's answer is good, but the prototype way is better for several reasons. That's why I am describing (rather, making corrections in) the last method. Check this fiddle.
var MyObj = function(name) {
this.name = name;
};
MyObj.prototype = {
called_often: function() {
// lots more code than just the following
return 'VALUE'; //document.getElementById('someID').value;
},
global_default: 'value', // can be changed, so need to pull value when run
does_stuff: function(value) {
var str = this.global_default + value, // can't access global_default on its own
input = this.called_often(), // doesn't work; MyObj.prototype.called_often() DOES
name = this.name; // 'this' used in the prototype doesn't work
// even within a created object
return name + input + str;
}
};
var obj = new MyObj('Bob');
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