So I can define a property on a function from the outside such as: createNewPerson.hmm = 3; and I get the result 3 when I use the alert. However if I try to do the same but from within a function (as in define a property within a function) it doesn't work.. I tried all the commented lines below. Is there a different way to achieve this or is it simply not possible? If it's not possible, then how does javascript append the .hmm property to the function and later call it?
Full code:
function createNewPerson(name) {
//var hello = "hgfgh";
//hello: "hgfgh"
//hello = hgfgh;
//this.hello = hgfgh;
}
createNewPerson.hmm = 3;
alert(createNewPerson.hmm);
alert(createNewPerson.hello);
I think you are trying to create objects. In javascript, you do it this way:
function Test(){
this.foo = "bar";
this.func = function(){
console.log(this.foo);
}
}
const test = new Test();
test.foo = "baz";
test.func(); //Will print "baz".
note the use of "new". This is what enables the mechanism of allowing code to modify properties of object. The modified values of the properties is then accessible by the object itself.
Hi please try below code,
jsFiddle
function createNewPerson(name) {
//var hello = "hgfgh";
//hello: "hgfgh"
//hello = hgfgh;
this.hello = "hgfg";
this.name = name;
}
var cnp = new createNewPerson('de');
cnp.hmm = 3;
alert(cnp.hmm);
alert(cnp.hello);
alert(cnp.name);
Related
I am currently learning javascript and came across this example
var t = function()
{
this.name = "Jam";
no = "123";
}
console.log(t.no); //Undefined
var m = new t();
console.log(m.name);
Why is the first statement undefined ?
t is a function object. As any other object, the function may have properties assigned. So in order your code to work you shall assign "123" to no property of your function (line A):
var t = function()
{
this.name = "Jam";
}
t.no = "123"; // line A
console.log(t.no); // "123"
var m = new t();
console.log(m.name);
Why is the first statement undefined ?
Because t doesn't have a property no.
First of all, the code inside the function, namely
this.name = "Jam";
no = "123";
is only executed when the function is called. You are doing this with var m = new t();, which comes after console.log(t.no);.
Secondly, no = "123"; does not create a property on the function object. It will attempt to set the value of variable no. Since the variable doesn't exist in your example, that line will either create a global variable no, or throw in error if your code is in strict mode.
Consider the following example:
var no = 21;
function foo() {
no = 42;
}
console.log(no); // 21
foo();
console.log(no); // 42
Because t is a function, that would be executed by t();. no on the other hand is a global scooed variable that is reached without prefix from everywhere.
t is a function expression. You you can access a returned object of a function like t().no or you can create a new object by using the function as a constructor like this
myT = new t()
console.log(t.no);
But your no variable is just a global variable inside the function and it is not a part of what it returns nor it is not attached to the returning object of the constructor function.
Here is a very good tutorial which covers all these topics at depths.
I'm working on a debugger that injects code into already existing code. Which then use eval() to log and run it.
However eval() seems to work in only its existing scope. To overcome this I tried to add a class that i recreated in each local scope. But it doesnt work.
I've added code to ilustrate my problem. The first alert works as expected, I'm assuming that it is because MyClass is created in the same scope.
The second alert still display b as 10 even though I've set b in local scope to 20 inside the TestC function. And the last alert doesn't work at all eval() returns in console "Uncaught ReferenceError: c is not defined".
If I add the whole MyClass inside each function and the assign it, then it works, but that doesn't feel like an elegant solution. And can add 1000's of lines of code to a project.
http://jsfiddle.net/ekim/zryj3taq/2/
var MyClass = function()
{
this.MyAlert = function(codex)
{
eval(codex);
}
}
var b = 10;
var MyOne = new MyClass();
MyOne.MyAlert("alert(b);");
function TestC()
{
var b = 20;
var MyOne2 = new MyClass();
MyOne2.MyAlert("alert(b);");
var c = 20;
var MyOne2 = new MyClass();
MyOne2.MyAlert("alert(c);");
}
TestC();
You can use new Function(\[arg1\[, arg2\[, ...argN\]\],\] functionBody)
(function(){
var str = "return (A+B)*C";
var myFunc = new Function("A","B","C", str);
var result = myFunc(2,3,4);
console.log(result);
}());
/* Example with function */
(function(){
var fncDouble = function (X) {
return X*2;
}
var str = "return F(A)+B";
var myFunc = new Function("F","A","B", str);
var result = myFunc(fncDouble,3,1);
console.log(result);
}());
As far as I know, there is no easy solution to this, as the eval() function just creates a new, separate compiler.
Try this: MyOne2.MyAlert.call(this, "alert(b)");
I'm trying to learn some OOP, so bear with me. I need to use a variable I defined in one function, elsewhere. Here is my example code (I want INTERCEPT!! to be logged, but it returns undefined):
function Talk() {
var greeting;
var pleaseStop; // declare it
this.A = function () {
greeting = 'hello';
console.log(greeting);
var intercept = function () {
pleaseStop = 'INTERCEPT!';
}
}
this.B = function () {
greeting = 'goodbye';
console.log(pleaseStop); // this returns undefined!
console.log(greeting);
}
}
var activateTalk = new Talk();
activateTalk.A();
activateTalk.B();
This whole code logs the following:
hello
undefined
goodbye
I have also tried intercept.pleaseStop() but it still returns undefined. Would anyone know of a solution?
EDIT:
I've removed the var the second time, but it still returns undefined:
http://jsfiddle.net/d654H/2/
var pleaseStop = 'INTERCEPT!';
You're declaring a new, function-local variable here; drop the var to assign to the existing variable in scope.
Then, you need to actually call intercept; at the moment you only define it.
It's your choice as to when you call that function; in this live example I simply do so immediately after the definition, for the purposes of exposition.
Remove var in front of the assignment to pleaseStop.
This assigns a new value to the pleaseStop declared inside the constructor, which is visible also from inside B:
var intercept = function () {
pleaseStop = 'INTERCEPT!';
}
This declares a new local variable pleaseStop, completely unrelated to the other pleaseStop, that is not visible outside intercept:
var intercept = function () {
var pleaseStop = 'INTERCEPT!';
}
If you do the latter instead of the former, you end up changing the value of another variable than the one you intended.
Your problem is you never set pleaseStop. You have declared intercept as a function, but you never called it. Therefore, pleaseStop is undefined.
Firstly you have't called intercept() anywhere and also u did something
var pleaseStop = 'INTERCEPT!';
which will create new variable instead of initializing global variable
You can do something like this
function Talk() {
var greeting;
var pleaseStop; // declare it
this.A = function () {
greeting = 'hello';
console.log(greeting);
var intercept = function () {
pleaseStop = 'INTERCEPT!';//changed
}
intercept(); //..Added
}
this.B = function () {
greeting = 'goodbye';
console.log(pleaseStop); // this returns undefined!
console.log(greeting);
}
}
var activateTalk = new Talk();
activateTalk.A();
activateTalk.B();
Without var keyword.
var pleaseStop = "A";
function foo(){
pleaseStop = "B"; // overwriting to "B"
}
foo();
alert(pleaseStop); // shows "B"
With var keyword.
var pleaseStop = "A";
function foo(){
var pleaseStop = "B"
// This defines a new variable 'pleaseStop'
// in the scope of function foo(){}.
}
foo();
alert(pleaseStop); // shows "A"
Variable Scope
JavaScript has function-level scope. In most languages which have block-level variable scope, variable are accessible whithin their block surrounded by curly brackets ({and}). But JavaSciprt doesn't terminate scopes at the end of blocks, but terminate them at the end of functions.
I'm sure there are many articles and documents about it. I googled it and found an intresting introductory article.
http://javascriptissexy.com/javascript-variable-scope-and-hoisting-explained/
Hope this helps.
I can't think of a way to explain what I'm after more than I've done in the title, so I'll repeat it. Is it possible for an anonymous function called from within an object to have access to that object's scope? The following code block should explain what I'm trying to do better than I can:
function myObj(testFunc) {
this.testFunc = testFunc;
this.Foo = function Foo(test) {
this.test = test;
this.saySomething = function(text) {
alert(text);
};
};
var Foo = this.Foo;
this.testFunc.apply(this);
}
var test = new myObj(function() {
var test = new Foo();
test.saySomething("Hello world");
});
When I run this, I get an error: "Foo is not defined." How do I ensure that Foo will be defined when I call the anonymous function? Here's a jsFiddle for further experimentation.
Edit: I am aware of the fact that adding the line var Foo = this.Foo; to the anonymous function I pass in to my instance of myObj will make this work. However, I'd like to avoid having to expose the variable inside the anonymous function--do I have any other options?.
Should be this.Foo:
var test = new myObj(function() {
var test = new this.Foo();
test.saySomething("Hello world");
});
http://jsfiddle.net/grzUd/5/
Or alternatively using with:
var test = new myObj(function() {
with (this) {
var test = new Foo();
test.saySomething("Hello world");
}
});
http://jsfiddle.net/grzUd/6/
Change var test = new Foo(); to var test = new this.Foo();.
Edit: Or you could pass it as a parameter.
function myObj(testFunc) {
this.testFunc = testFunc;
var Foo = function (test) {
this.test = test;
this.saySomething = function(text) {
alert(text);
};
};
this.testFunc(Foo);
}
var test = new myObj(function(Foo) {
var test = new Foo();
test.saySomething("Hello world");
});
You seem to be confused about the difference between identifier resolution on the scope chain and property resolution.
Foo is a property of an instance of myObj (i.e. it's an object property). Calling new Foo will resolve Foo as a variable on the scope chain, which isn't the right place to look for it. That's why Petah's answer tries to use with, to put the object properties of the this object on the scope chain.
I want to create a single object. Does the below code make sense?
singleObj = new function () {
// act as a constructor.
};
Am I hurting any good practice?
I need a constructor. A simple object literal would not be useful here.
If you want just a single object, in that you are never going to make one again, an object literal works perfectly here.
var x = { };
Will give you an object.
In order for
var F = function() {
};
to give you an object you will need to invoke F
var x = new F();
you could try someting like:
var objCreate = function() {
var obj = {};
// do constructor logic
return obj;
};
Just create a new object and then populate it. You don't need a contrustor to do this.
var singleObject = {};
singleObject.value1 = "whatever";
If you really want to use a function, then you need to actually call it.
var singleObj = new (function () {
// act as a constructor.
})();
We can use a self executing function by creating a anonymous function function(){}, and immediately calling it with an empty argument set.
http://www.w3schools.com/js/js_objects.asp
//Used to create an object
personObj=new Object();
personObj.firstname="John";
personObj.lastname="Doe";
personObj.age=50;
personObj.eyecolor="blue";
//Used as a constructor for the object
function person(firstname,lastname,age,eyecolor)
{
this.firstname=firstname;
this.lastname=lastname;
this.age=age;
this.eyecolor=eyecolor;
}
//how to declare objects via constructor template
var myFather=new person("John","Doe",50,"blue");
var myMother=new person("Sally","Rally",48,"green");