While looking at code on github, I found the following:
(function() {
}).call(this);
This is clearly a self invoking anonymous function. But why is it written this way? I'm used to seeing the canonical variant (function() {})().
Is there any particular advantage to using .call(this) for a self invoking anonymous function?
Edit: It looks like some commonjs environments set this to a non-global value at the top level of a module. Which ones, and what do they set this to that you might want to preserve?
By default, invoking a function like (function(){/*...*/})() will set the value of this in the function to window (in a browser) irrespective of whatever the value of this may be in the enclosing context where the function was created.
Using call allows you to manually set the value of this to whatever you want. In this case, it is setting it to whatever the value of this is in the enclosing context.
Take this example:
var obj = {
foo:'bar'
};
(function() {
alert( this.foo ); // "bar"
}).call( obj );
http://jsfiddle.net/LWFAp/
You can see that we were able to manually set the value of this to the object referenced by the obj variable.
.call(this) (was actually just () until I changed it) ensures your top level this to be consistent through strict mode, --bare option and/or the running environment (where top level this doesn't point to global object).
By using:
> (function() {
> ...
> }).call(this);`
then this in the scope of the code (probaby the global object) is set as the function's this object. As far as I can tell, it's equivalent to:
(function(global) {
// global references the object passed in as *this*
// probably the global object
})(this);
In a browser, usually window is (or behaves as if it is) an alias for the global object.
C={
descript: "I'm C!<br>",
F: function() {
//set this to the caller context's 'this'
(function() {
document.write(this.descript);
}).call(this);
//set this to 'window' or 'undefined' depend the mode
(function() {
document.write(this.descript);
})();
//member function's 'this' is the object self
document.write(this.descript);
}
}
window.descript="I'm window!<br>";
C.F();
(function() {}).call(this); could set the this in the anonymous to the caller context this, in above is C.(function() {})(); will set this to window or undefined depend the mode.
Self-invoking function are useful to execute its content immediately when the script is loaded. This is convenient to initialize global scope elements.
Related
I have a JavaScript file which is loaded by require.
// loaded by require()
var a = this; // "this" is an empty object
this.anObject = {name:"An object"};
var aFunction = function() {
var innerThis = this; // "this" is node global object
};
aFunction();
(function(anyParameter){
console.log(anyParameter.anObject);
})(
this // "this" is same having anObject. Not "global"
);
My question is: this in var a = this; is an empty object whereas this statements in functions are shadows of node.js global object. I know this keyword is different in functions but I could not understand why first this is not equal to global and this in functions equals to global.
How does node.js inject global to this in function scopes, and why it does not inject it to the module scope?
Here's a few fundamental facts you must understand to clarify the situation:
In the top-level code in a Node module, this is equivalent to module.exports. That's the empty object you see.
When you use this inside of a function, the value of this is determined anew before each and every execution of the function, and its value is determined by how the function is executed. This means that two invocations of the exact same function object could have different this values if the invocation mechanisms are different (e.g. aFunction() vs. aFunction.call(newThis) vs. emitter.addEventListener("someEvent", aFunction);, etc.) In your case, aFunction() in non-strict mode runs the function with this set to the global object.
When JavaScript files are required as Node modules, the Node engine runs the module code inside of a wrapper function. That module-wrapping function is invoked with a this set to module.exports. (Recall, above, a function may be run with an abitrary this value.)
Thus, you get different this values because each this resides inside a different function: the first is inside of the Node-created module-wrapper function and the second is inside of aFunction.
To understand this, you need to understand that Node.js actually wraps your module code in to a function, like this
(function (exports, require, module, __filename, __dirname) {
var test = function(){
console.log('From test: ' + this);
};
console.log(this);
test();
});
Detailed explanation can be found in this answer.
Now, this wrapped function is actually invoked like this
var args = [self.exports, require, self, filename, dirname];
return compiledWrapper.apply(self.exports, args);
So, this, at the module level, is actually the exports object.
You can confirm that like this
console.log(this, this === module.exports);
// {} true
Summary:
In Javascript the value of this is determined when a function is called. Not when a function is created. In nodeJS in the outermost scope of a module the value of this is the current module.exports object. When a function is called as a property of an object the value of this changes to the object it was called. You can remember this simply by the left-of-the-dot rule:
When a function is called you can determine the value of this by looking at the place of the function invocation. The object left of the dot is the value of this. If there is no object left of the dot the value of this is the module.exports object (window in browsers).
caveats:
This rule does not apply for es2015 arrow function which don't have their own binding of this.
The functions call, apply, and bind can bend the rules regarding the this value.
Example (NodeJS):
console.log(this); // {} , this === module.exports which is an empty object for now
module.exports.foo = 5;
console.log(this); // { foo:5 }
let obj = {
func1: function () { console.log(this); },
func2: () => { console.log(this); }
}
obj.func1(); // obj is left of the dot, so this is obj
obj.func2(); // arrow function don't have their own this
// binding, so this is module.exports, which is{ foo:5 }
Output:
It's because the default global object in a Node.js module is the exports object, and you are calling test() which doesn't specify this. In traditional JS, this points to the global object, with use strict, this will be null.
this can point to anything, it just depends on how you call it.
test(): Uses the global object (exports) as this, unless in strict mode, where this will be null;
test.call({}) or test.apply({}): You are specifying what to use as this (the first parameter)
var obj = {testRef: test}; obj.testRef(): this is set to the left of the ., that is, obj
Countering thefourtheye's answer
It is true that this in the top level of the module is exports, but that doesn't necessarily mean that this inside test() will also point to same thing as where it was called from.
Attempting to prove that this and the global object both point to exports
myGLobal = 5;
this.myGlobal; // 5
I have a JavaScript file which is loaded by require.
// loaded by require()
var a = this; // "this" is an empty object
this.anObject = {name:"An object"};
var aFunction = function() {
var innerThis = this; // "this" is node global object
};
aFunction();
(function(anyParameter){
console.log(anyParameter.anObject);
})(
this // "this" is same having anObject. Not "global"
);
My question is: this in var a = this; is an empty object whereas this statements in functions are shadows of node.js global object. I know this keyword is different in functions but I could not understand why first this is not equal to global and this in functions equals to global.
How does node.js inject global to this in function scopes, and why it does not inject it to the module scope?
Here's a few fundamental facts you must understand to clarify the situation:
In the top-level code in a Node module, this is equivalent to module.exports. That's the empty object you see.
When you use this inside of a function, the value of this is determined anew before each and every execution of the function, and its value is determined by how the function is executed. This means that two invocations of the exact same function object could have different this values if the invocation mechanisms are different (e.g. aFunction() vs. aFunction.call(newThis) vs. emitter.addEventListener("someEvent", aFunction);, etc.) In your case, aFunction() in non-strict mode runs the function with this set to the global object.
When JavaScript files are required as Node modules, the Node engine runs the module code inside of a wrapper function. That module-wrapping function is invoked with a this set to module.exports. (Recall, above, a function may be run with an abitrary this value.)
Thus, you get different this values because each this resides inside a different function: the first is inside of the Node-created module-wrapper function and the second is inside of aFunction.
To understand this, you need to understand that Node.js actually wraps your module code in to a function, like this
(function (exports, require, module, __filename, __dirname) {
var test = function(){
console.log('From test: ' + this);
};
console.log(this);
test();
});
Detailed explanation can be found in this answer.
Now, this wrapped function is actually invoked like this
var args = [self.exports, require, self, filename, dirname];
return compiledWrapper.apply(self.exports, args);
So, this, at the module level, is actually the exports object.
You can confirm that like this
console.log(this, this === module.exports);
// {} true
Summary:
In Javascript the value of this is determined when a function is called. Not when a function is created. In nodeJS in the outermost scope of a module the value of this is the current module.exports object. When a function is called as a property of an object the value of this changes to the object it was called. You can remember this simply by the left-of-the-dot rule:
When a function is called you can determine the value of this by looking at the place of the function invocation. The object left of the dot is the value of this. If there is no object left of the dot the value of this is the module.exports object (window in browsers).
caveats:
This rule does not apply for es2015 arrow function which don't have their own binding of this.
The functions call, apply, and bind can bend the rules regarding the this value.
Example (NodeJS):
console.log(this); // {} , this === module.exports which is an empty object for now
module.exports.foo = 5;
console.log(this); // { foo:5 }
let obj = {
func1: function () { console.log(this); },
func2: () => { console.log(this); }
}
obj.func1(); // obj is left of the dot, so this is obj
obj.func2(); // arrow function don't have their own this
// binding, so this is module.exports, which is{ foo:5 }
Output:
It's because the default global object in a Node.js module is the exports object, and you are calling test() which doesn't specify this. In traditional JS, this points to the global object, with use strict, this will be null.
this can point to anything, it just depends on how you call it.
test(): Uses the global object (exports) as this, unless in strict mode, where this will be null;
test.call({}) or test.apply({}): You are specifying what to use as this (the first parameter)
var obj = {testRef: test}; obj.testRef(): this is set to the left of the ., that is, obj
Countering thefourtheye's answer
It is true that this in the top level of the module is exports, but that doesn't necessarily mean that this inside test() will also point to same thing as where it was called from.
Attempting to prove that this and the global object both point to exports
myGLobal = 5;
this.myGlobal; // 5
var abc=(function(){
var self=this;
return {
self:self
}
})();
When doing abc.self I get undefined what does this happen to be in this context.
What you have can be simplified for the purpose of the explanation in
(function(){ console.log(this) })();
Your expression (in the first set of parenthesis) defines a function. You then call this function without context (the this). This construct is called an IIFE. As you don't pass a context, the behavior depends whether it is called in strict mode or not :
In non strict mode, you would have the global object (window in a browser, global in node).
In strict mode, a missing context of a function call isn't replaced, it's undefined.
As you get undefined, I guess you're in strict mode. You probably have "use strict"; at the start of the file or in an enclosing function.
If you wanted to pass a context, you might for example have done
(function(){ console.log(this) }).call(someobject);
in that "root scope" this is window
and
console.log(abc.self);
results for me in
Window {top: Window, window: Window, ...}
this refers to the current object. In your condition this will be window, and as you're trying to get the value abc.self, you need to use like this:
this.self = this;
Now, only you can get the value:
var xyz = new abc();
xyz.self
But to note, you cannot use abc as the constructor like above code because you are using the closure.
I have a custom binding handler and want to modify it to IIFE. I have been reading on internet about IIFE but could not able how to change my custom handle into IIFE. So how can I change following binding handler into IIFE
define(['knockout', 'jquery'], function(ko, $) {
ko.bindingHandlers.er = {
init: function(el, va) {
console.log($(el).find('.n').text());
$(el).find('.edit').hide().end().find('.detail').show();
$(el).find('.ebtn').on('click', function() {
$(el).find('.edit, .detail').toggle();
});
$(el).find('.ubtn').on('click', function() {
$(el).find('.edit, .detail').toggle();
});
}
};
});
It's quite unclear what you're asking here :(.
Immediately-Invoked Function Expression (IIFE) is an anonymous function that executes immediately. In your case, if you can't be more specific, it's really hard to guess where do you want to place the iife.
I normally put my whole code into an iife in order to take advantage of the function scope and use it as a closure, thus minimising the global object pollution and it generally looks like this :
(function(global,undefined){
// the code goes here
// global === window
})(this);
In your case, you can do the same :
(function(global,undefined){
define(['knockout','jquery'],function(ko,$){
// ...
});
})(this);
Update :
Here are some explanations for those who need them.
The following pattern is used:
to prevent global variable leaks such as var foo = bar; (after which window.foo === foo)
to prevent the undefined value to get overwritten. Let's say you want to check if something is undefined. you have 2 ways to do it :
typeof foo === 'undefined'
foo === undefined
the second one is shorter but is often not preffered because one can simple overwrite the undefined variable's value : undefined = 13;.
By not proviiding an actual value for the undefined argument in the iife, you're actually resetting the undefined variable to undefined.
to generalize the global object
some say that the access to the global variable is faster than to the window object due to the fact that the js engine first looks into the lower scope
if you're running the code in a different environment than the browser and you have calls to the window object, your code might break because the global object is different from environment to ennvironment (adobe pdfs, node js, rhino ... ). This can be solved by remaping the global object to a variable (in our case global), by reffering it to the this object from the global scope
I've been toying with node.js lately and I ran into a weird behavior about the usage of this in the global scope of a module.
this is bound to module.exports in the global scope:
console.log(this === exports); // -> true
But this is bound to global in a method scope:
(function() { console.log(this === global); })(); // -> true
This also lead to this confusing behavior:
this.Foo = "Weird";
console.log(Foo); // -> throws undefined
(function() { this.Bar = "Weird"; })();
console.log(Bar); // -> "Weird"
I guess that the solution is to never use this in the global scope and explicitly use extends or global instead, but is there a logic behind all this or is it a bug or limitation in node.js?
The "logic" behind that is, that the value of this always depends on how a function is invoked.
In your case, you have a self-executing anonymous function, there, this always references the global object (non strict mode) or undefined (ES5 strict).
If you want to access the "outer" this value, you could either store a reference before executing that function, like
var outerScope = this;
(function() { outerScope.Bar = "Weird"; })();
console.log(Foo); // -> throws undefined
or re- .bind() the functions scope yourself, like
(function() { this.Bar = "Weird"; }).bind(this)();
In working on a simple CommonJS modules implementation, I had to think about what to do with this in the global scope of the module; it's not addressed by the spec.
I also set it up as the exports object at first, because I thought that would be useful, but later found some code I needed to "modulize" that was using this to get a handle to the global object, so I changed this back to the global object to provide as close of an environment to "normal" as possible for module code.
We can only guess at why node is set up the way it is (or ask the author), but my guess is it was done simply because it seemed like a useful idea, similar to the way you can give the module object an exports property in node and have it reflected in the module's actual exports (this behavior also isn't part of the spec, but doesn't go against it either).
As for the part of your question about this referencing global in functions, as the other answers explain, that's just the way this works; it's not a node-specific behavior, it's a weird javascript behavior.
I don't know if this is the exact intention of Node.js team, but I would be surprised if it was not. Consider this example ran in the dev console of a browser (e.g. chrome):
var x = function(){console.log(this)}
a = {}
a.x = x
a.xx = function(){x()}
a.x()
>> Object
a.xx()
>> DOMWindow
x()
>> DOMWindow
As you can see executing a method without specifying its context sets the context to be the global one. In this case the DOMWindow object.
When you are inside a module your context is the module, but executing a method in it without specifying a context with .call or .apply or obj. will use the global context, global, instead of the local one, module.exports.