Consider this piece of code
var crazy = function() {
console.log(this);
console.log(this.isCrazy); // wrong.
}
crazy.isCrazy = 'totally';
crazy();
// ouput =>
// DOMWindow
// undefined
From inside crazy() 'this' refers to the window, which I guess makes sense because normally you'd want this to refer to the object the function is attached to, but how can I get the function to refer to itself, and access a property set on itself?
Answer:
Don't use arguments.callee, just use a named function.
"Note: You should avoid using arguments.callee() and just give every function (expression) a name." via MDN article on arguments.callee
I think you are asking for arguments.callee, but it's deprecated now.
https://developer.mozilla.org/en/JavaScript/Reference/Functions_and_function_scope/arguments/callee
var crazy = function() {
console.log(this);
console.log(arguments.callee.isCrazy); // right.
}
crazy.isCrazy = 'totally';
crazy();
// ouput =>
// DOMWindow
// totally
As rfw said, this is the most straight forward way to go if the function has one single name:
var crazy = function() {
console.log(crazy);
console.log(crazy.isCrazy);
};
crazy.isCrazy = 'totally';
crazy();
In case it may have different names, or you wanted to pass it around, it must be wrapped in a closure:
var crazy = (function(){
var that = function() {
console.log(that);
console.log(that.isCrazy);
};
return that;
})();
crazy.isCrazy = 'totally';
crazy();
Bind the function to itself (taking a hint from answers by #ArunPJohny and #BudgieInWA):
crazy = crazy.bind(crazy);
This will give you access from the function to its properties via this.
> crazy()
function () {
console.log(this);
console.log(this.isCrazy); // works now
}
This seems like a better solution than the accepted answer, which uses the callee feature which is deprecated and doesn't work in strict mode.
You could also now have the function call itself recursively with this() were you so inclined.
We will call this self-thisifying. Write a little utility function:
function selfthisify(fn) { return fn.bind(fn); }
crazy = selfthisify(crazy);
crazy();
Or, if you prefer more "semantic" names, you could call it accessOwnProps.
If you're a syntactic sugar type of person, you could add a selfthisify property to the Function prototype:
Object.defineProperty(Function.prototype, 'selfthisify', {
get: function() { return this.bind(this); }
});
Now you can say
crazy.selfthisify();
You have to give it its own name, so:
var crazy = function() {
console.log(crazy);
console.log(crazy.isCrazy);
}
crazy.isCrazy = 'totally';
crazy();
The variable this is only applicable in the scope of an object, for instance, if you invoked your version of the crazy function with crazy.call(crazy), it will call the function in the context of the function crazy and all would be well.
This has to deal with the scope of the function crazy. If can pass any scope to a function using the function call().
Instead of
crazy();
Use
crazy.call(crazy);
For details refer
http://odetocode.com/blogs/scott/archive/2007/07/05/function-apply-and-function-call-in-javascript.aspxhttps://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/Callhttp://devlicio.us/blogs/sergio_pereira/archive/2009/02/09/javascript-5-ways-to-call-a-function.aspx
You can use the call method
var crazy = function() {
console.log(this);
console.log(this.isCrazy);
}
crazy.isCrazy = 'totally';
crazy.call(crazy);
// calls crazy using crazy as the target, instead of window:
// functionToCall.call(objectToUseForThis);
Though if your function only ever has one name, you can do this:
var crazy = function() {
console.log(crazy);
console.log(crazy.isCrazy);
}
crazy.isCrazy = 'totally';
crazy();
Easiest way to make the function itself available in its body is to do
var crazy = function crazy2() { crazy2(); }, it's okay for crazy and crazy2 to have the same name since the first occurrence is the name in the outer scope and the second is the name in the function body.
Or simply do function crazy() { crazy(); } which will define crazy in both scopes.
how can I get the function to refer to
itself?
The idea of 'itself' does not exist with functions. What you need is an object and not just a function. An object has knowledge of itself available through the keyword 'this'. Within a function, 'this' points to the global object - in this case the window object. But if you use your function as a constructor function to create an object (using the new operator) then the object's 'this' pointer will point to the object itself.
i.e this points to the object if you write:
var anObject = new crazy();
So you can re-write your code as follows:
var crazy = function() {
this.printMe = function(){
console.log(this);
console.log(this.isCrazy);
}
}
var anObject = new crazy(); //create an object
anObject.isCrazy = 'totally'; //add a new property to the object
anObject.printMe(); //now print
In case you wish to add the property before the object is created, then you have to add the property to the function's prototype as follows:
var crazy = function() {
console.log(this);
console.log(this.isCrazy);
}
crazy.prototype.isCrazy = 'totally'; //add the property to the function's prototype
var anObject = new crazy(); //invoke the constructor
See more on my blog for a detailed explanation of these concepts with code-samples.
Are you actually trying to create an object 'class'?
function crazy(crazyState) {
this.isCrazy = crazyState;
console.log(this);
console.log(this.isCrazy);
}
crazy.prototype.alertMe = function() { alert('I am '+ this.isCrazy +' crazy.'); }
var crazyObj = new crazy('totally');
crazyObj.alertMe();
crazyObj.isCrazy = 'not';
crazyObj.alertMe();
Funny that you should ask, mate. I just went through this same issue for a different purpose. The quick version of the final code is:
$a = function() {};
$ = function() {
if (!(this instanceof $)) {
return new $();
}
this.name = "levi";
return this;
};
//helper function
var log = function(message) {
document.write((message ? message : '') + "<br/>");
};
log("$().name == window.name: " + ($().name == window.name)); //false
log("$().name: " + $().name); //levi
log("window.name: " + window.name); //result
log();
log("$a instanceof $: " + ($a instanceof $)); //false
log("typeof $a: " + (typeof $a)); //function
log("typeof $: " + (typeof $)); //function
The critical piece:
if (!(this instanceof $)) {
return new $();
}
If this isn't pointing to an object of the right type, then it makes a new one, which will properly scope this. The rest of the code is just there for verification that it does indeed work as intended.
In order to make you code to work follow below
function crazy_object (crazy) {
this.isCrazy = crazy
}
var create_crazy = new crazy_object('hello') //creating object
console.log(create_crazy); //=> { isCrazy = 'hello' }
var crazy = function() {
console.log(this); //=> { isCrazy = 'totally' }
console.log(this.isCrazy); //=> 'totally'
}
create_crazy.isCrazy = 'totally'; //=> isCrazy = 'totally'
//below we pass the created object in function crazy.
//And doing that we can use the keywork `this` and refer to the object
crazy.call(create_crazy, null);
Using the call and apply method we can pass to a function a
property,and in that function we can use the property with the keyword this
For example:
function speak (message) {
console.log(`A person with name ${this.name} say ${message}`);
}
speak.call({ name: 'Roland' }, 'Javascript is awesome');
To use it with property:
function speak (message) {
console.log(`A person with name ${this.name} say ${message}`);
}
var name = 'Roland'
speak.call({ name }, 'Javascript is awesome');
Related
I need the current function name as a string to log to our log facility. But arguments.callee.name only works in loose mode. How to get the function name under "use strict"?
For logging/debugging purposes, you can create a new Error object in the logger and inspect its .stack property, e.g.
function logIt(message) {
var stack = new Error().stack,
caller = stack.split('\n')[2].trim();
console.log(caller + ":" + message);
}
function a(b) {
b()
}
a(function xyz() {
logIt('hello');
});
You can bind function as its context then you can access its name via this.nameproperty:
function x(){
console.log(this.name);
}
x.bind(x)();
After little research here is a good solution :
function getFnName(fn) {
var f = typeof fn == 'function';
var s = f && ((fn.name && ['', fn.name]) || fn.toString().match(/function ([^\(]+)/));
return (!f && 'not a function') || (s && s[1] || 'anonymous');
}
function test(){
console.log(getFnName(this));
}
test = test.bind(test);
test(); // 'test'
Source : https://gist.github.com/dfkaye/6384439
Building on #georg solution, this one returns just the function name. Note though that it may fail if called from an anonymous function
function getFncName() {
const stackLine = (new Error())!.stack!.split('\n')[2].trim()
const fncName = stackLine.match(/at Object.([^ ]+)/)?.[1]
return fncName
}
function Foo() {
console.log(getFncName()) // prints 'Foo'
}
A simple solution to dynamically retrieve function names [like magic variables] is the use of scoped variables, and the Function.name property.
{
function foo() {
alert (a.name);
}; let a = foo
}
{
function foo2() {
alert(a.name)
}; let a = foo2
};
foo();//logs foo
foo2();//logs foo2
Note: Nested functions cease to be source elements, and are hence not hoisted. Also, this technique cannot work with anonymous functions.
If (like me) you want to define this elsewhere and call it generically, you can store the code as a string somewhere global or import it, then eval() it wherever to access the current function name. (Using eval keeps the context at the point of invocation.)
There's gotta be a way to do this without using a string, but whatever.
SomeObject.whatFunc =
'const s = new Error().stack;' +
"const stackLine = new Error().stack.split('\\n')[2].trim();" +
'const fncName = stackLine.match(/(?<=at )(.*)(?= \\()/gm)[0];' +
'console.log(fncName);'
// Whereever you want the func name
function countBananas('weeee') {
eval(SomeObject.whatFunc)
// blah blah blah
}
countBananas() // logs 'countBananas'
just an update to get the full name :
function logIt(message) {
var stack = new Error().stack,
// update is on this line
caller = stack.split('\n')[2].trim().split(/\s+/)[1];
console.log(caller.trim().split(/\s+/)[1];);
}
function a(b) {
b()
}
a(function xyz() {
logIt('hello');
});
I thought I understood the javascript prototype object and how to use it, but now I've run into something that has me a little stumped. When trying to run the code below mpo.testFire() fires with no issues, but I get the error Uncaught TypeError: Object # has no method 'fireAlert' when trying to invoke mpo.fireAlert() which I thought was part of the prototype:
<body>
Click Me
</body>
// Click handler, create new object
// call parent method and prototype method
$("#testBtn").click(function(e) {
e.preventDefault();
var mpo = new Myobject();
mpo.testFire();
mpo.fireAlert();
});
Myobject = function () {
var testFire = function () {
alert('testFire');
};
return {
testFire:testFire
};
};
Myobject.prototype = function() {
var fireAlert = function() {
alert('made it to fireAlert');
};
return {
fireAlert:fireAlert
};
}();
If I change the code and move everything to the object's prototype like the code below everything works as expected:
$("#testBtn").click(function(e) {
e.preventDefault();
var mpo = new Myobject();
mpo.testFire();
mpo.fireAlert();
});
Myobject = function () {
// constructor logic here maybe?
};
Myobject.prototype = function() {
var fireAlert = function() {
alert('made it to fireAlert');
};
var testFire = function () {
alert('testFire');
};
return {
fireAlert:fireAlert,
testFire:testFire
};
}();
I'm guessing there is a scope issue since in the first example I return an interface from the parent object. Can anyone explain why the first example does not work?
You are returning an object literal from your first implementation of MyObject constructor which knows nothing of the prototype chain MyObject is part of. In the second implementation the empty constructor returns the new context that was created which is in fact a MyObject object which knows about the appropriate prototype chain
You're returning an object literal
return {
testFire:testFire
};
which "overrides" the result from new (because it's an object. Returning, for example, a string literal would return the created object).
This is in ES5 spec 13.2.2.
I want to be able to assign a property to a function inside the function itself. I do not want to assign it to the object of invocation. So I want the equivalent of doing this:
var test = function() {
return true;
};
test.a = 'property on a function';
alert(test.a);
Instead of this, where the property is assigned to a global object:
var testAgain = function() {
this.a = "this property won't be assigned to the function";
return true;
};
testAgain();
alert(window.a);
Edit: To clarify, I'm wondering if there's something like this:
var test = function() {
function.a = 'property on a function';
};
alert(test.a); // returns 'property on a function'
Without knowing that the function is called test or having to execute it.
I know of course this isn't valid syntax
[is there a way to set a property on a function] without knowing that the function is called test or having to execute it.
Emphasis mine.
You can set a property on a function without knowing what its global variable name is necessarily going to be, however you do have to have a reference to the function in one way or another.
The module pattern is as close of a fit as I can think of:
window.test = (function () {
//the function could be named anything...
function testFn() {
...code here...
}
//...so long as the same name is used here
testFn.foo = 'bar';
return testFn;
}());
window.test.foo; //'bar'
The outer closure prevents testFn from being accessed anywhere globally, so all other references will have to use window.test.
This part of the answer is associated with the prior version of the question.
The simplest way of doing this is to use a named function:
var test = function testFn() {
testFn.foo = 'bar';
return true;
};
test.foo; //undefined
test();
test.foo; //'bar'
A better way of doing this is to use the module pattern so that you don't accidentally create issues with global leakage:
var test = (function () {
function ret() {
ret.foo = 'bar';
return true;
}
return ret;
}());
test.foo; //undefined
test();
test.foo; //'bar'
var testAgain = function() {
arguments.callee.a = "this property won't be assigned to the function";
return true;
};
testAgain();
alert(testAgain.a);
You can do this by simple using the name to assign the property like this:
var test = function () {
test.a = 'a';
return true;
};
When test is invoked, the property will be set.
Demo
You could use arguments.callee, as su- said, but that's considered really bad practice. Also, it won't work in strict mode.
var test = function() {
test.a = 'a';
};
Or you can use prototypes, read more here.
var Dog = function() {
var _instance = 'hello world';
return function() {
console.log(this._instance);
}
} (); //note that it is self invoking function
var l = new Dog(); //#> undefined
In the above case I was expecting an output of:
'hello world'
Why is this._instance not accessing the the variable which should be accessible by virtue of closure? I tested this in FF and am getting undefined.
You don't assign _instance to the object, it's just a closure variable, and should be accessed without using this:
var Dog = function() {
var _instance = 'hello world';
return function() {
console.log(_instance);
}
} (); //note that it is self invoking function
var l = new Dog();
I'd probably write it like so instead:
var Dog = (function() {
var defaults = {
name: 'Rags'
};
var Dog = function (options) {
// Take options as a constructor argument, this
// allows you to merge it with defaults to override
// options on specific instances
this.setOptions(options);
};
Dog.prototype = {
// Common methods for Dogs here
setOptions: function (options) {
// Declare all variables in the beginning of the method because
// JavaScript hoists variable declarations
var key = null;
// First assign all the defaults to this object
for ( key in defaults) {
this[key] = defaults[key];
}
// Now override with the values in options:
if (options && options.hasOwnProperty) {
for ( key in options ) {
this[key] = options[key];
}
}
}
};
return Dog; // Return the constructor method
} ()); // wrap the self-invoked function in paranthesis to visualize that
// it creates a closure
var buster = new Dog({name: 'Buster'}),
unnamed = new Dog();
alert(buster.name); // Alerts 'Buster'
alert(unnamed.name); // Alerts 'Rags'
Note that I have not tried to compile the above code, so it might contain a few mistakes. Nothing JsLint can't handle though!
You might want to consider adding filtering to the setOptions method so that it doesn't assign properties you don't want, or filter out methods etc declared in the options-parameter.
Additionally, if you use JQuery, or similar library, there are (often) utility functions for merging objects, making it trivial to write the setOptions-method:
function setOptions (options) {
// I assume JQuery here
// true as the first argument gives us a recursive merge
var mergedOptions = $.extend(true, defaults, options);
for (var key in mergedOptions ) {
if(this.checkAllowedProperty(key, typeof(mergedOptions[key])) {
this[key] = mergedOptions[key];
}
}
}
/**
* This method checks if propertyName is an allowed property on this object.
* If dataType is supplied it also checks if propertyName is allowed for
* dataType
* #return true if propertyName, with type dataType, is allowed on this object,
* else false
*/
function checkAllowedProperty (propertyName, dataType);
Your problem is this.
Change this._instance to _instance. You may also want to wrap your self-invoking function in parentheses like (function() { ... })(); for maximum browser compatibility.
As the others have said, you need to remove "this." from your function.
The reason for the problem is down to the binding of the "this" keyword in the two function contexts. Inside the closure, "this" refers to the function that is being returned, and not to the outer function. You could resolve this by doing the following:
var Dog = function() {
var _instance = 'hello world';
var that = this; //Assign "this" to "that"
return function() {
console.log(that._instance); //Use reference to "that"
}
} ();
var l = new Dog();
You could also probably do something closer with the function.apply() method, but I'll leave that to you.
I hope that helps.
Perhaps you are satisfied by removing "this.", but you may be interested to learn that "this" doesn't refer to what you wanted it to anyway. What it refers to really depends on how the function is called. It does not necessarily refer to an instance of an object constructed by the function you returned, or its container function, or to any other object. By default, if you merely call the function as a normal function, "this" will refer to the global window context.
What you must do to have "this" be bound to any specific object is to call the function as a method of that object, or of its prototype. e.g. foo.myMethod(). Another way is that you can use the apply or call method, passing in the object you want it to apply to. e.g. anyFunction.apply(foo).
I'm wondering if any of yall have any insight as to how one could execute a function by reference in javascript.
http://mootools.net/shell/yL93N/1/
Any discussion would be cool.
-Chase
looking at your mooshell, the way i'd handle it in mootools is this:
http://mootools.net/shell/yL93N/10/
var proxyFunction = new Class({
message: "hello",
Binds: ['passByReference','sayit'],
passByReference: function(func) {
// console.log(this, this[func]);
if (this[func] && $type(this[func]) === "function")
this[func]();
},
sayit: function() {
alert(this.message);
},
killit: function() {
document.write('we\'re dead');
}
});
$('tryit').addEvent('change',function(e){
new proxyFunction().passByReference(this.get('value'));
});
// or have a permanent proxy instance if you call methods of the class often and need it to change things.
var proxy = new proxyFunction();
$('tryit').addEvent('change',function(e){
proxy.passByReference(this.get('value'));
});
the advantage of doing so is that all your proxied functions are behind a common object, don't pollute your window namespace as global variables and can share data that relates to the event.
Not exactly sure what you mean, but you can do this:
var func = window.alert;
var args = ["hello world"]
func.apply(window, args)
Globally-defined functions (and variables) are visible as members of the global window object.
Members of an object can be fetched by name using the square bracket notation: o['k'] is the same as o.k. So, for your example:
var function_name= $(this).val();
window[function_name]();
Like this?
function blah() {
...do stuff
}
myref = blah
myref()
The best way is to do:
func.call();
Function variables in JavaScript already are references. If you have a function:
var explode = function() { alert('boom!'); };
You can pass explode around as an argument, and it's only passing a handle to that function, not the entire function body.
For proof of this, try:
explode.id = 5;
var detonate = explode;
alert(detonate.id); // => 5
explode.id = 6;
alert(detonate.id); // => 6
functions are first class objects in Java Script. Effectively this means that you can treat it very much as if it were a variable, and pass it anywhere that you would expect a variable.
e.g.
var myFn = function() { alert('inside anonymous fn'); }
function callMyFn(paramFn)
{
paramFn();
}
callMyFn(myFn); //inside anonymous fn
function MyFnHolders(argFn)
{
this.argFn = argFn;
this.fieldFn = function() {
alert('inside fn field');
}
}
var myFnHolders = new MyFnHolders(myFn);
myFnHolders.argFn(); //'inside anonymous fn'
myFnHolders.fieldFn(); //'inside fn field'
//etc
so passing a function by ref can be done simply by assigning it to a variable and passing it around.
Here's one with a closure for your arguments...
function Runner(func, args) {
return function() { return func.apply(window, args); };
}
var ref = new Runner(window.alert, ["hello world"]);
ref();