why this logs 1 and not 2 ? i am trying to pass this scope to initData function but no success. after running init i should trigger iniData but this should refer to the object itself not to a new scope
let startFunction = function (){
let initData = () => {
this.x = 2
}
return {
x:1,
init:initData.bind(this)
};
};
obj=startFunction()
obj.init();
console.log(obj.x); // should be 2
update:
let startFunction = function (){
let initData = function(){
this.x = 2
}
return {
x:1,
init:initData
};
};
obj=startFunction()
obj.init();
console.log(obj.x); // should be 2
and why this works ? shouldn't the function create a new scope ?
There are two issues here. The first issue is that arrow functions are immutably bound to their execution context. Function.prototype.bind has no effect on an arrow function. The second issue here is what this refers to when you call startFunction. When you invoke a function freely, without a binding to a particular object, this refers to the object in which the function is invoked. So this refers to the global object in initData permanently.
So let's think about the value of obj and the global object's x value after each line.
After obj=startFunction:
// global x is undefined
// obj
{
x: 1,
init: initData // with the same binding to the global object
}
After obj.init(), initData sets the global object's x value to 2:
// global x is 2
// obj
{
x: 1,
init: initData // with the same binding to the global object
}
Problem you are facing is the x in the object has nothing to do with the this.x that is being assigned in the init method. You need to set an object and in the init code adjust that object.
let startFunction = function() {
const initData = () => {
data.x = 2
};
var data = {
x: 1,
init: initData,
};
return data;
};
const obj = startFunction()
obj.init();
console.log(obj.x);
Better solution, use a proper class.
class MyClass {
x = 1;
init() {
this.x = 2;
}
}
const obj = new MyClass();
obj.init();
console.log(obj.x);
class MyClass2 {
x = 1;
constructor() {
this.x = 2;
}
}
const obj2 = new MyClass2();
console.log(obj2.x);
Related
I have a constructor and am trying to have an array of functions inside its prototype, but i need the functions to have the scope of the object created by the constructor, and not the array's scope.
I tried using .bind(this) or .bind(_p) but "this" is the node server's scope and _p is just the prototype without the variables.
function BoardModel() {
this.x = 3
this.y = 2
}
_p = BoardModel.prototype;
_p.skillFunctions = [
function(){
console.log(this.x); //undefined
},
function(){
console.log(this.y); //undefined
},
];
Why not use an own method for each property, instead of an array?
function BoardModel() {
this.x = 3
this.y = 2
}
_p = BoardModel.prototype;
_p.skillFunctionsX = function (){
console.log(this.x);
};
_p.skillFunctionsY = function (){
console.log(this.y);
};
var item = new BoardModel;
item.skillFunctionsX();
item.skillFunctionsY();
Arrow functions use this from the enclosing context so how about this (pun intended)?
function BoardModel() {
this.x = 3
this.y = 2
this.skillFunctions = [
() => { console.log(this.x) },
() => { console.log(this.y) },
];
}
let board = new BoardModel()
board.skillFunctions.forEach((skillFunction) => { skillFunction() })
I am trying to simulate a simple Holder "class" in JavaScript with a "private" property that holds something and "public" getter and setter "methods" to access the value.
The approach exhibited by HolderA below is mentioned e.g. here. The other approach I more or less arrived at by mutation but I guess it must be recognizable as an idiom as well. I like it because it contains no this or prototype stuff and seems very elementary and functional. Is there a difference between the two?
The test code (I run it under nodejs) seems to suggest that the two approaches are identical except that in the first case the objects I get have typeof object whereas in the second function.
var test = function(o) {
var initialValueCorrect = (!(typeof o.getX()==='undefined'))&&(o.getX()===0);
var VALUE_TO_SET = 10;
o.setX(VALUE_TO_SET);
var getSetWorks = o.getX()===VALUE_TO_SET;
var xIsPrivate = (typeof o.x === 'undefined');
var xHasCorrectValue;
if (!xIsPrivate)
xHasCorrectValue = o.x === VALUE_TO_SET;
return {initialValueCorrect: initialValueCorrect,
getSetWorks : getSetWorks,
xIsPrivate: xIsPrivate,
xHasCorrectValue: xHasCorrectValue};
};
var HolderA = (function() {
function foo(x) {
this.getX = function() {
return x;
};
this.setX = function(_x) {
x = _x;
};
};
return foo;
})();
var createHolderB = (function() {
var x;
function foo(_x) {
x = _x;
return foo;
}
foo.getX = function() {
return x;
};
foo.setX = function(_x) {
x = _x;
};
return foo;
})();
var objects = [{object: new HolderA(0), name: "approach with constructor-invocation and 'this'"},
{object: createHolderB(0), name: "approach with normal function invocation and closed variable"}];
for (var i = 0; i<objects.length ; i++) {
var testResult = test(objects[i].object);
console.log('['+objects[i].name+']: the object is a: '+(typeof objects[i].object)
+'\n\n\t\t\t'+JSON.stringify(testResult)+'\n\n\n\n\n');
}
update
As Bergi has pointed out function createHolderB in my code above is plain wrong and only creates a singleton object. So, is not really a "constructor" function. To that end I've now created createHolderC which can be used to really create multiple objects with a hidden private property like this:
var objectC1 = createHolderC()(0);
Now, is there any material difference between HolderA and the createHolderC function or is the difference purely stylistic?
var createHolderC = function () {
return (function() {
var x;
function foo(_x) {
x = _x;
return foo;
};
foo.getX = function() {
return x;
};
foo.setX = function(_x) {
x = _x;
};
return foo;
})();
};
createHolderB does not create new holders like HolderA does. It's essentially a singleton pattern. You might also want to call it a module. Notice that createHolderB() === createHolderB.
createHolderC is still different from HolderA in that it returns function objects, not instances. You may see the differences better when you strip out the unnecessary IEFEs:
function HolderA(x) {
this.getX = function() {
return x;
};
this.setX = function(_x) {
x = _x;
};
// implicit `return this;` when called via `new`
}
function createHolderC() {
var x;
function foo(_x) {
x = _x;
return foo;
};
foo.getX = function() {
return x;
};
foo.setX = function(_x) {
x = _x;
};
return foo;
}
A typical factory would rather look like this:
function createHolderD(x) {
var foo = {};
foo.getX = function() {
return x;
};
foo.setX = function(_x) {
x = _x;
};
return foo;
}
(or even with return {getX: …, setX: …};), the only difference to HolderA is the prototypical inheritance then.
Basically both exhibits the private access behavior to the x variable. But the difference between those two are
Constructor function
var HolderA = (function() {
function foo(x) {
this.getX = function() {
return x;
};
this.setX = function(_x) {
x = _x;
};
};
return foo;
})();
This is a self executing function, where it returns Constructor foo.
That is the reason you are using new while creating this type of holder {object: new HolderA(0)
A function
var createHolderB = (function() {
var x;
function foo(_x) {
x = _x;
return foo;
}
foo.getX = function() {
return x;
};
foo.setX = function(_x) {
x = _x;
};
return foo;
})();
Even this is also a self executing function, but while execution it creates x variable and returns function foo, and foo access x by closure.
You are creating this type of holder just by normal function call object: createHolderB(0).
Both cases are really bad in practice.
If you're making closures (anonymous functions) that hold your "private variable" scope, it means you're creating those functions for every instance of the class. That costs memory and performance. I used to create classes like this, but then once I was making large number of instances and found out that it hits performance hard.
Just as in Python, make private variables private by some convention, like underscore in _name. Then you can break it for debug reasons (because variables in closure scope are not accessible).
This is how I would do it:
function MyClass(arg) {
this._privateVar = arg;
}
MyClass.prototype.getPrivateVar = function() {
return this._privateVar;
}
You can also make getters and setters:
Object.defineProperty(MyClass.prototype, "privateVar", {
get: function() {
return this._privateVar;
}
}
But don't try to push javascript into things it's not designed to, or you'll pay with performance, code readability and debugging complexity. Applies to other languages too.
I'm writing up a small game using CraftyJS. Here's what I want to write:
Crafty.scene('MainMap', function() {
this.player = Crafty.e('Player');
this.player.move(5, 5);
this.game_objects = [this.player];
isOccupied: function(x, y) {
for (var i = 0; i < this.game_objects.length; i++) {
// ...
}
}
if (!this.isOccupied(5, 5)) { ... }
// ...
}
Unfortunately, this doesn't work as expected; it's not an anonymous object, but it's a function. I have to use a different syntax, and pass in my objects, like so:
function isOccupied(x, y, game_objects) { ... }
// Same place as previous call to isOccupied
if (!isOccupied(x, y, this.gameObjects) { ... }
It's clear to me why I have to declare it as function isOccupied and not isOccupied: function (because it's inside a function, not an object) but it's not clear to me what the scope of this is. It doesn't get passed into the function.
Is it possible to somehow keep the objects within some non-global scope, and not need to pass them into isOccupied?
You could assign the parent scope to another variable, so it will be available in your closure. Like so...
Crafty.scene('MainMap', function() {
var self = this;
this.player = Crafty.e('Player');
this.player.move(5, 5);
this.game_objects = [this.player];
function isOccupied (x, y) {
for (var i = 0; i < self.game_objects.length; i++) {
// ...
}
}
}
You have a syntax error in your Crafty scene.
The colon in this part should not be there. In JavaScript, colons are only used in objects.
// Wrong
isOccupied: function(x, y) {
for (var i = 0; i < this.game_objects.length; i++) {
// ...
}
}
// Right
function isOccupied(x, y) {
// ...
}
In your function, this refers to the global object (window).
EDIT:
To fix this, use Function.prototype.bind, like so:
function isOccupied(x, y) {
// ...
}.bind(this);
After doing more research, it appears that this is a well-known issue with the this keyword. It will be fixed in ECMAscript 5.
To summarize: the this keyword gets confused when you have more than one level of nesting:
obj = {
speak: function() {
alert(this); // obj
inner = function() {
alert("inner: " + this); // window
};
}
};
The work-around is to use a scope chain, by assigning a variable to this:
obj = {
speak: function() {
alert(this); // obj
var that = this;
inner = function() {
alert("inner: " + that); // obj instead of window
};
}
};
Unfortunately for me, CraftJS has me inside a function instead of an object. To declare a sub-function, I still must specify it as function isOccupied:
Crafty.scene('MainMap', function() {
self = this; // new
this.player = Crafty.e('Player');
this.player.move(5, 5);
this.game_objects = [this.player];
function isOccupied(x, y) { // game_objects no longer passed in
for (var i = 0; i < self.game_objects.length; i++) { // uses self
// ...
}
}
if (!this.isOccupied(5, 5)) { ... }
// ...
}
I'm new to the prototype structure, and I'm having trouble figuring this one out. Here's my JavaScript code.
var Game = function ()
{
//some variables
};
Game.prototype.block =
{
spawn: function () {
var t1 = new this.inst;
},
inst : {
x: 5,
y: 0,
type: ''
}
};
When I try to create a new object "inst" I get the following error:
TypeError: object is not a function. What am I doing incorrectly?
If you want to create objects that inherit from the inst object, you can do that using Object.create, with var t1 = Object.create(this.inst);.
var Game = function () {
//some variables
};
Game.prototype.block = {
spawn: function () {
var t1 = Object.create(this.inst);
},
inst : {
x: 5,
y: 0,
type: ''
}
};
So then your code would look something like this;
var game = new Game();
game.block.spawn();
And the .spawn() method would have a variable that references an object that inherits from the Game.prototype.block.inst object.
First of all, inst is not defined within the scope of Game. So, this which refers to Game doesn't have any properties called inst. Secondly, inst must be followed by () to indicate a call to the constructor, which you are missing here.
I guest you need a static factory method to create new "inst". is the below code what you need? you call the Game.spawn method to generate a new inst, and you can put this method in setInterval.
function Game() {
//some variables
}
Game.spawn = function() {
function Inst() {
this.x = 5;
this.y = 0;
this.type = '';
}
return new Inst;
}
var inst1 = Game.spawn();
inst1.x = 1; //test inst1
console.log(inst1.x);
var inst2 = Game.spawn();
inst2.x = 2; //test inst2
console.log(inst2.x);
var inst3 = Game.spawn();
inst3.x = 3; //test inst 3
console.log(inst3.x);
var foo = {
x: 1,
y: (function () {
return ++this.x;
})()
};
console.log(foo.y); // undefined rather than 2
Let's say that I want to be able to reference foo.y without using foo.y(). Is that possible?
The above obviously doesn't work, and I'm assuming it is because closure references a different this.
If you want to access y as a property and not a function, and have it return the current value of foo.x, the only way I can think of is using getters/setters, which is part of the ES5 spec, and adopted in most of the modern browsers (but not really useable if you're supporting older browsers).
var foo = {
x: 1,
get y() {
return this.x;
}
};
You don't declare y as function, but as a result of a function call. It should be:
var foo = {
x: 1,
y: function () {
return this.x;
}
};
Or, if you want just to assign to y value of x:
var foo = {
x: 1,
y: this.x
};
UPDATE: It is NOT possible to make y synonym of x. When you declare y - it can be a value or a function. It cannot be a reference to another value.
Why not just return x directly if you don't want to use y() as a function? The only reason why you would return a function is you actually need to modify x's result.
var foo = {
x: 1,
y: (function () {
return this.x;
})()};
console.log(foo.x); // 1
var foo = new Object();
foo.x = 1;
foo.y = function() { return this.x; };
console.log(foo.y());
or if you're trying to invoke y while you define foo:
var foo = new Object();
foo.x = 1;
foo.y = x;
console.log(foo.y);
foo.y isn't a function. foo.y is defined as foo.x.