I always thought that these callbacks had their own scope. What's going on here?
Eton_file_sync.prototype.add_file_listener = function(filename){
//use chokidar to make a watcher for the filename
var watcher = this.chokidar.watch(filename, {
ignored: /[\/\\]\./,
persistent: true
});
var some_variable = 1;
watcher.on('change', function(path) {
console.log ('File', path, 'has been changed');
console.log (some_variable);
});
};
when calling it by changing the file, why does the output of some_variable actually work?
File buffercont.asc has been changed
1
They do have their own scope. If you define the value of that variable from within the event handler callback, you'd only be defining the value inside of that scope, but it would not affect the parent scope.
var some_variable = 1;
console.log(some_variable); // prints "1"
var callback = function() {
var some_variable = 5;
console.log (some_variable); // prints "5"
};
callback();
console.log(some_variable); // prints "1"
Notice in the above sample that defining the variable inside the function did not change it outside the function. Each function carries with it a scope chain, the same scope chain that it is created within. You can always access variables higher up in the chain unless they've been overridden further down the chain like in the above example.
Related
var inner = function() { console.log(x); }
// test 1
(function(cb) { var x = 123; cb(); })(inner);
// test 2
(function(cb) { var x = 123; cb.apply(this); })(inner);
// test 3
(function(cb) { var x = 123; cb.bind(this)(); })(inner);
// test 4
(function(cb) { cb.bind({x: 123})(); })(inner);
All tests result in:
ReferenceError: x is not defined
Do someone know how it is possible to access 'x' as a local variable inside the callback?
Fact: when you do var inner = function() { console.log(x); } in your first line, x is not defined. Why? Because, inside your inner function, there's no local declaration of x (which would be done with var x = something). The runtime will then look up in the next scope, that is the global scope. There isn't, also, a declaration of x, so x is also not defined there.
The only places where there is a variable called x are inside each one of your 4 IIFEs following. But inside the IIFEs, each x is a different variable, in a different scope. So, if what you want is to console.log() the x defined inside each IIFE, you are taking the wrong approach.
Keep in mind that, when you define inner, you are capturing the environment inside the function's closure. It means that, whatever value could x have there (in the declaration of the function), would be the available value to the x variable later, when the inner function would be used. The fact that your x there is not defined is only an accessory, and is not what is causing the undesired behavior.
So, what happens is that when you call your inner function inside any of your IIFEs, the x referred to inside the inner function declaration is a captured value of what x had as a value when the function was defined, not the value that x has now in the scope where the function is currently being called. This is what is called lexical scope.
To solve this, you would have to pass the value that you want to console.log() inside the inner function as a parameter to the inner function, as so:
var inner = function(x) { console.log(x); }
// test 1
(function(cb) { var x = 123; cb(x); })(inner);
The only way to access the local variable x in the callback, is to pass it as an argument:
var inner = function(some_var) { console.log(some_var); }; //logs 123
(function(cb) { var x = 123; cb(x); })(inner);
OR
var inner = function(some_var) { console.log(some_var); }; //logs 123
(function(cb) { var x = 123; cb.apply(this,[x]); })(inner);
OR
var inner = function(some_var) { console.log(some_var); }; //logs 123
(function(cb) { var x = 123; cb.call(this,x); })(inner);
FURTHER
Because JS is lexically scoped, trying to reference the local variable after the anonymous function has finished executing is impossible by any other means. If you don't pass it as an argument to make it available elsewhere, JS will see it as non-reachable and it will be eligible for garbage collection.
You could redefine the callback function in the current scope:
var inner = function() { console.log(x); }
(function(cb) { var x = 123; eval('cb = ' + cb.toString()); cb(); })(inner);
// or
(function(cb) { var x = 123; eval('(' + cb.toString() + ')')(); })(inner);
This will not work if the function relies on anything in the scope in which it was originally defined or if the Javascript file has been minified. The use of eval may introduce security, performance, and code quality issues.
Have you tried using events? Emit an event inside the anonymous function, then subscribe to it in your own function somewhere else that carries out your logic.
I ran into an issue with some javascript I was running and broke it down to the code below. I'm confused as to why the _localVar variable doesn't change after init() is called. I find that if I reference this._localVar in the revealingModule, the results are as expected. I am confused about the scope. Could someone please clarify why this is occurring. I thought that if I didn't use this, then the next scope would be the module, but I don't think that's happening.
var myRevealingModule = (function () {
var _localVar = "Default";
function init() {
console.log(_localVar);
_localVar = "Init";
}
function getTest() {
console.log(_localVar);
}
return {
init: init,
getTest: getTest,
localVar: _localVar
};
})();
myRevealingModule.getTest(); // "Default"
console.log(myRevealingModule.localVar); // "Default"
myRevealingModule.init(); // "Default"
myRevealingModule.getTest(); // "Init"
console.log(myRevealingModule.localVar); // "Default" * WHY *
myRevealingModule.localVar is not a reference to the variable's value; it simply copied the string when it's created.
When you use this.localVar, you're using the variable in the returned object. Hence, when you change that identifier, it also updates with myRevealingModule.localVar.
Note that your module uses a self-invoking function. Therefore the value of myRevealingModule.localVar is determined right after the definition and built-in invocation of myRevealingModule. At this time the value of _localVar is "Default", which is copied to returned object's localVar property. Even if you change _localVar afterwards this has no effect on myRevealingModule.localVar anymore.
Basically the following example show the same effect:
var a = 42;
var b = a;
console.log(a); // 42
console.log(b); // 42
a = 84;
console.log(a); // 84
console.log(b); // 42
b copies the value of a. If you change a afterwards that has no impact on b.
Is there a way to print the value of temp (a closure variable) from a function defined outside the closure but referenced within the closure without passing temp as a variable to funcA?
var funcA, funcB;
funcA = function () {
console.log(temp);
}
funcB = function () {var temp, funcC;
temp = 1;
funcC = funcA;
funcC();
}
funcB(); // temp is undefined.
This works, but only because funcA is defined within funcB:
funcB = function () {var temp, funcA, funcC;
temp = 1;
funcA = function () {
console.log(temp);
}
funcC = funcA;
funcC();
}
funcB(); // 1
I'm trying to find a way to pull some function definitions out of outer functions to streamline code that's getting a little complex. Can I define funcA outside of funcB but still reference the temp variable without having to pass parameters?
I read that javascript does not have dynamic run time scoping, that it's only lexical scoping, but with referencing a function (funcA via funcC) within funcB, is there a way to meet the lexical scope requirement and provide access to the scoped variables for funcB?
Using Akinkunle Allen's comment, I came up with this which seems to solve my problem.
function funcB () {
var funcA, funcB, temp;
funcA = function () {
console.log(temp);
}
funcB = function () {var funcC;
temp = 1;
funcC = funcA;
funcC();
}
return funcB();
}
funcB(); // 1
Yes, and No.
The keyword var for declaring variables behaves different depending upon what the current scope is. When var is executed on the global scope it is optional. The variable becomes a property of the global object. When executing in the browser this global object is window.
So the following in global space has the same result.
var temp = 1;
window.temp = 1;
this.temp = 1;
All the above is just window.temp because of the global context.
When you use var inside a function then the variable is attached to the function. All functions in Javascript are objects so local var variables will live as long as the parent function is still being used somewhere.
Javascript will walk the hierarchy of executing scopes to find a variable identifier. So any inner functions can access their outer function variables (as in your example).
What you can do is play around with the this reference in functions.
The identifier this in Javascript is dynamic (meaning that you can change it). I can pass a variable to an unknown function that was declared outside the calling function. Since the this.temp is used to reference the variable the function funcA is able to display the value.
funcB = function(otherFunc)
{
this.temp = 1;
otherFunc();
}
funcA = function()
{
alert(this.temp);
}
funcB(funcA);
http://jsfiddle.net/thinkingmedia/dfLvj/
See the above jsfiddle example.
What you can do with this is change the meaning of this on the fly. Here is a better example.
funcB = function(otherFunc)
{
var temp = {
message: "Hello World!"
};
var foo = otherFunc.bind(temp);
foo();
}
funcA = function()
{
alert(this.message);
}
funcB(funcA);
http://jsfiddle.net/thinkingmedia/K5Pw6/
Dynamically changing this can have a lot of benefits by allowing a function to accept closure references that will be executed with a custom this reference.
An example might be a click event handler where this is the DOM element that triggered the event.
How do I call class methods from functions within the class? My code is:
var myExtension = {
init: function() {
// Call onPageLoad
},
onPageLoad: function() {
// Do something
},
}
I've tried ...
onPageLoad();
... from within the init method but it's saying it's not defined.
I'm not having much luck with google because I don't understand the syntax used. All the JS OOP examples I find online are in a different format. I'm using the format Mozilla use for extension development.
The object the current method was invoked on is available via the special variable this. Any time you call a method on an object, this will refer, within the method, to the object.
var myExtension = {
init: function() {
this.onPageLoad();
},
onPageLoad: function() {
// Do something
},
};
this always refers to the calling object rather than the object the function is defined on or is a property of.
value = 'global';
var ob0 = {
value: 'foo',
val: function() {
return this.value;
},
},
ob1 = {value: 'bar'},
ob2 = {value: 'baz'};
ob0.val(); // 'foo'
ob1.val = ob0.foo;
ob1.val(); // 'bar'
ob0.val.call(ob2); // 'baz'
var val = ob0.val;
val(); // 'global'
In the last case, val is executed as a free function (a function that isn't bound to an object, i.e. not a method), in which case this takes on the value of the global object (which is window in web browsers) within the execution of val. Global variables are actually properties of the global object, hence val() returns 'global' (the value of the global variable named value). Since global variables are actually properties of the global object, you can view free functions as actually being methods of the global object. From this viewpoint, the last two lines (when executed in global scope) are equivalent to:
window.val = ob0.val;
window.val();
This viewpoint doesn't exactly match the reality of scoping, though you'll only notice the difference within functions. In a function, window.val = ... will create a global while var val won't.
value = 'global';
var ob0 = {
value: 'foo',
val: function() {
return this.value;
},
};
function lcl() {
var val = ob0.val; // doesn't set a global named `val`
return val(); // 'global'
}
lcl(); // 'global'
val(); // error; nothing named 'val'
function glbl() {
window.val = ob0.val; // sets a global named `val`
return window.val(); // 'global'
}
glbl(); // 'global'
val(); // 'global'
See MDN's reference page for more on the call method used above. For more on the this variable, see "JavaScript “this” keyword" and "How does “this” keyword work within a JavaScript object literal?"
Assuming that you have called init like this:
myExtension.init();
then it should be:
init: function() {
// before
this.onPageLoad();
// after
}
But in Javascript functions are not actually bound to objects and you can call any function on any other object, like this:
myExtension.init.call(anotherObject); // or
myExtension.init.apply(anotherObject);
In this example this within init would be anotherObject, which doesn't have onPageLoad defined. If you want to support this kind of usage you'll have to manually reference the initial object:
init: function() {
// before
myExtension.onPageLoad();
// after
}
In Javascript you need to explicitly mention the this
this.onPageLoad()
The same is also true for other member variables (remember that in Javascript methods are just member variables that happen to be functions)
this.memberVariable
Have you considered a closure, instead?
For example:
var myExtension = ( function() {
var me = {};
var private_thingy = "abcDEFG123";
var onPageLoad = function() {
// do page loading business
alert( private_thingy );
}
me.onPageLoad = onPageLoad;
var init = function() {
onPageLoad();
}
me.init = init;
return me;
})();
///////////////
myExtension.private_thingy = "DIDDLED!";
// will NOT modify the private_thingy declared within the closure
myExtension.init(); // will work fine.
Anything you define within the closure is available within the closure at all times, and when implemented carefully will yield private members not accessible to users of the object. Only members that you explicitly export - e.g., the me.xxx = xxx lines - are publicly available.
Thus, when onPageLoad executes, "abcDEFG123" will be displayed in the alert, not "DIDDLED!". Users of the object can modify properties using the dot operator until the cows come home; what's not made public by the closure, however, can never be modified: even if the user reassigns a function on the public interface, calls to the private function from within the closure will always point to the function defined within the closure.
The important part: it unties you from the constant use of this (unless you really want to use it; save your fingers for more important typing!).
Give it a shot. And have a look at Javascript: The Good Parts by Crockford.
What I want to do is to sending data between two handlers.
element.onmousedown = function() {
data = precalculate();
}
element.onmouseup = function() {
dosomething(data);
}
if the data is a global variable it works. People says global variable is evil. But I don't know how to do without it.
or I misunderstood "global variable"?
Just scope the variable if you don't want/need it to be global:
(function() {
var data;
element.onmousedown = function() {
data = precalculate();
}
element.onmouseup = function() {
dosomething(data);
}
})();
EDIT: To clarify, the only way to create a new variable scope in javascript is in a function.
Any variable declared with var inside a function is inaccessible to the outer scope.
In the code above, I created an IIFE (immediately invoked function expression), which is simply a function that is invoked as soon as it is created, and I placed your data variable (along with the handler assignments) inside of it.
Because the handlers were created in a scope that has access to the data variable, they retain their access to that variable.
To give another example:
var a = "a"; // global variable
(function() {
var b = "b"; // new variable in this scope
(function() {
var c = "c"; // new variable in this scope
// in this function, you have access to 'a', 'b' and 'c'
})();
// in this function you have access to 'a' and 'b' variables, but not 'c'
})();
// globally, you have access to the 'a' variable, but not 'b' or 'c'
In this case a global variable would make sense. Another possibility is to attach the value to the DOM element:
element.onmousedown = function() {
// 'this' should point to the element being mouse downed
this.data = precalculate();
};
element.onmouseup = function() {
// 'this' should point to the element being mouse upped
var data = this.data;
dosomething(data);
};
You misunderstood "global variable is evil".
In fact, what really happened is that someone wanted to be "part of the crowd", and so told you a sweeping generalisation, when in fact they should have said "only use global variables where appropriate".
Well, they are appropriate here, my friend.
JQuery makes this possible by using the .data() function:
http://api.jquery.com/jQuery.data/
You can get away with using global variables as long as you keep them to a minimum, for example you can place stuff in a single global namespace:
App = {};
element.onmousedown = function() {
App.data = "hello";
}
element.onmouseup = function() {
console.log(App.data);
}
Another more general solution is to create functions that cache their results using a technique that is called memoization. There's plenty of stuff if you search.
The following code does exactly that :
Function.prototype.memoize = function() {
var fn = this;
this.memory = {};
return function() {
var args = Array.prototype.slice.call(arguments);
return fn.memory[args] ? fn.memory[args] : fn.memory[args] = fn.apply(this, arguments);
};
};
e.g. You have an expensive function called exfunc..
newFunc = exfunc.memoize();
The above statement creates a new function called newFunc that caches the result of the original function so that the first time the actual code is being executed and all subsequent calls are retrieved from a local cache.
This mostly works with functions whose return value does not depend on global state.
More info :
http://osteele.com/archives/2006/04/javascript-memoization