I came across a interesting quiz
function bar() {
return foo;
foo = 10;
function foo() {}
var foo = '11';
}
alert(typeof bar());
My interpretation is like this (Which is wrong according to console :) ):
var foo; // global variable
function bar(){
function foo(){}
var foo; // Here variable foo should override foo function
return foo; // (according to me foo should be variable with undefined value) What is going on here, How JavaScript resolve naming order ?
foo = 10;
foo = "11";
}
Here is a reference which I am reading this
In JavaScript, a name enters a scope in one of four basic ways:
1.Language-defined: All scopes are, by default, given the names this and arguments.
2. Formal parameters: Functions can have named formal parameters, which are scoped to the body of that function.
3. Function declarations: These are of the form function foo() {}.
4. Variable declarations: These take the form var foo;
He later quoted :
The most important special case to keep in mind is name resolution order. Remember that there are four ways for names to enter a given scope. The order I listed them above is the order they are resolved in. In general, if a name has already been defined, it is never overridden by another property of the same name. This means that a function declaration takes priority over a variable declaration. This does not mean that an assignment to that name will not work, just that the declaration portion will be ignored.
Which is confusing for me, Can anyone simplify this with reference to above example ?
Main point I want to know :
How variables without var inside a function hoisted ?
Does variable overriding occurs during hoisting ?
Lets leave the function foo for the moment. Within a function, if a variable is declared anywhere inside that function, the declaration will be moved to the top of the function. So, it is actually evaluated like this
function bar() {
var foo;
return foo;
foo = 10;
foo = '11';
}
But when you have a function declared by the same name, it will take precedence and it will be evaluated similar to this
function bar() {
var foo = function() {};
return foo;
foo = 10;
foo = '11';
}
That is why you are getting function in the alert box.
Any variable without var inside a function becomes a global variable by default.
when you have a function declaration inside another function(like in your example), it gets hoisted first followed by the variable declarations.
examples to demonstrate variable overriding.
function bar() {
var foo = 10;
function foo() {}
return foo;
}
bar(); //--> returns 10;
function bar() {
var foo;
function foo() {}
return foo;
}
bar(); //--> returns the function object foo.
Related
Original Question:
JSHint complains when my JavaScript calls a function that is defined further down the page than the call to it. However, my page is for a game, and no functions are called until the whole thing has downloaded. So why does the order functions appear in my code matter?
EDIT: I think I may have found the answer.
http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting
I am groaning inside. Looks like I need to spend ANOTHER day re-ordering six thousand lines of code. The learning curve with javascript is not steep at all, but it is very loooooong.
tl;dr If you're not calling anything until everything loads, you should be fine.
Edit: For an overview which also covers some ES6 declarations (let, const): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Scope_Cheatsheet
This weird behavior depends on
How you define the functions and
When you call them.
Here's some examples.
bar(); //This won't throw an error
function bar() {}
foo(); //This will throw an error
var foo = function() {}
bar();
function bar() {
foo(); //This will throw an error
}
var foo = function() {}
bar();
function bar() {
foo(); //This _won't_ throw an error
}
function foo() {}
function bar() {
foo(); //no error
}
var foo = function() {}
bar();
This is because of something called hoisting!
There are two ways to define functions: Function declaration and function expression. The difference is annoying and minute, so let's just say this slightly wrong thing: If you're writing it like function name() {}, it's a declaration, and when you write it like var name = function() {} (or an anonymous function assigned to a return, things like that), it's a function expression.
First, let's look at how variables are handled:
var foo = 42;
//the interpreter turns it into this:
var foo;
foo = 42;
Now, how function declarations are handled:
var foo = 42;
function bar() {}
//turns into
var foo; //Insanity! It's now at the top
function bar() {}
foo = 42;
The var statements "throws" the creation of foo to the very top, but doesn't assign the value to it yet. The function declaration comes next in line, and finally a value is assigned to foo.
And what about this?
bar();
var foo = 42;
function bar() {}
//=>
var foo;
function bar() {}
bar();
foo = 42;
Only the declaration of foo is moved to the top. The assignment comes only after the call to bar is made, where it was before all the hoisting occurred.
And finally, for conciseness:
bar();
function bar() {}
//turns to
function bar() {}
bar();
Now, what about function expressions?
var foo = function() {}
foo();
//=>
var foo;
foo = function() {}
foo();
Just like regular variables, first foo is declared at the highest point of the scope, then it is assigned a value.
Let's see why the second example throws an error.
bar();
function bar() {
foo();
}
var foo = function() {}
//=>
var foo;
function bar() {
foo();
}
bar();
foo = function() {}
As we've seen before, only the creating of foo is hoisted, the assignment comes where it appeared in the "original" (un-hoisted) code. When bar is called, it is before foo is assigned a value, so foo === undefined. Now in the function-body of bar, it's as if you're doing undefined(), which throws an error.
The main reason is probably that JSLint does only one pass on the file so it doesn't know you will define such a function.
If you used functions statement syntax
function foo(){ ... }
There is actually no difference at all where you declare the function (it always behaves as if the declaration is on the beginning).
On the other hand, if your function was set like a regular variable
var foo = function() { ... };
You have to guarantee you wont call it before the initialization (this can actually be a source of bugs).
Since reordering tons of code is complicated and can be a source of bugs in itself, I would suggest you search for a workaround. I'm pretty sure you can tell JSLint the name of global variables beforehand so it doesn't complain about undeclared stuff.
Put a comment on the beggining of the file
/*globals foo1 foo2 foo3*/
Or you can use a text box there for that. (I also think you can pass this in the arguments to the inner jslint function if you can meddle with it.)
There are way too many people pushing arbitrary rules about how JavaScript should be written. Most rules are utter rubbish.
Function hoisting is a feature in JavaScript because it is a good idea.
When you have an internal function which is often the utility of inner functions, adding it to the beginning of the outer function is an acceptable style of writing code, but it does have the drawback that you have to read through the details to get to what the outer function does.
You should stick to one principle throughout your codebase either put private functions first or last in your module or function. JSHint is good for enforcing consistency, but you should ABSOLUTELY adjust the .jshintrc to fit your needs, NOT adjust your source code to other peoples wacky coding concepts.
One coding style that you might see in the wild you should avoid because it gives you no advantages and only possible refactoring pain:
function bigProcess() {
var step1,step2;
step1();
step2();
step1 = function() {...};
step2 = function() {...};
}
This is exactly what function hoisting is there to avoid. Just learn the language and exploit its strengths.
Only function declaration are hoisted not function expression (assignment).
I'm not really a javascript noob at all, although in my whole life I've never came across this, but am I right in assuming that javascript must assign functions before running anything or something?
In all my experience, I expected this to return 'undefined', but obviously it returns 'function'.
function bar() {
return foo;
foo = 10;
function foo() {}
var foo = '11';
}
alert(typeof bar());
Is someone able to explain this for me?
This behaviour of JavaScript is called hoisting. There is a good explanation on the MDN (https://developer.mozilla.org/en-US/docs/Glossary/Hoisting)
In JavaScript, functions and variables are hoisted. Hoisting is JavaScript's behavior of moving declarations to the top of a scope (the global scope or the current function scope).
That means that you are able to use a function or a variable before it has been declared, or in other words: a function or variable can be declared after it has been used already.
Basically, if you declare a variable like this:
console.log(s); // s === undefined
var s = 'some string';
s declaration will be "hoisted" to the beginning of the scope (i.e. there will be no ReferenceError on the line with console.log). The variable's value will not be defined at that moment though.
The same goes with assigning an anonymous function to a variable, so:
console.log(f); // f === undefined
f(); // TypeError: f is not a function
var f = function () {}; // assigning an anonymous function as a value
f(); // ok, now it is a function ;)
The variable will be hoisted and thus visible in the entire scope, but it's value, even if it's a function, will still be undefined - hence the error if you try to execute it.
On the other hand if you declare a named function:
console.log(f); // f === function f()
f(); // we can already run it
function f() {}; // named function declaration
It's definition will also be hoisted so you can run it even in the first line of the scope you've declared it.
This is quite easily tested;
foo(1);
function foo(i) {
if (bar()) {
alert("foo called, bar true, i = " + i);
};
}
foo(2);
function bar() {
return true;
}
foo(3);
DEMO
This shows that Javascript loads all functions before executing anything. Therefor it does not matter what order functions are defined.
Well in JavaScript function is nothing but Object.
When you say typeof bar(),in the bar function you are returning foo which is another function.You just returning name of the function,So,it return the constructor of the foo function.So,your typeof get the value of foo constructor which is type of function.So,it alert function.It still refer the foo because of closure
Again in the bar defination,you returning foo,but its definition is still not encountered.In JavaScript when while parsing the instruction,declaration of variable and function is placed on top in current function scope.
So,
your statement
function bar() {
return foo;
foo = 10;
function foo() {}
var foo = '11';
}
is equivalent to
function bar() {
function foo() {}
return foo;
foo = 10;
var foo = '11';
This is called JavaScript top hoist
}
Consider two styles of javascript function declarations (out of four, I think):
function foo() {
// ...
}
versus
var foo = function() {
// ...
}
In many circumstances these will behave the same, and I think I grok the main difference as explained in e.g. these SO questions:
difference-between-var-foo-function-and-function-foo
usual-functions-vs-function-variables-in-javascript
which both have answers linking to this very helpful explanation:
javascript-function-declaration-ambiguity
But I would like to combine both styles in one statement; one short/minifiable local variable name (because I will need to refer to it quite often) and one descriptive name (I want to get something friendly out of .name).
This is where I get confused. It is as if the act of immediately assigning the function to a variable leaves it undefined under its own name:
var f = function foo() {
// ...
};
console.log( f.name ); // "foo"
console.log( foo.name ); // !? error: foo is not defined ?!
So to get to the question: why does this not work? Or, more likely, what might I still be misunderstanding about these two declaration styles?
Note, that the following does not result in an error:
var f = foo;
function foo() {
// ...
}
console.log( f.name ); // "foo"
console.log( foo.name ); // "foo"
How, exactly, is this different?
PS: I think this is different from this SO question:
in-javascript-what-is-the-motivation-or-advantage-of-using-var-foo-function-f...
which is about a special case of my predicament, where the variable name and function name are the same, i.e.
var foo = function foo() {
// ...
};
When you write
var f = function foo () { ... }
the scope of foo is just the body of the function, not the enclosing function. This is mainly useful for creating anonymous recursive functions.
whats the difference between function foo(){} and foo = function(){}?
recommends against using that notation, because they don't work correctly in some implementations.
This is how I see it. Every function in javascript inherits from Function object and Function object has property .name.
this creates the function object with the name foo in the global space:
function foo(){}
this creates local anonymous(thus no name) function and assigns is to the variable f which lives in global space:
var f = function(){}
this creates the function object and assigns it to variable, it doesn't exist in global context, only local to f, the same as above, but assigns the name foo:
var f = function foo(){}
edit: for better picture consider following
(function foo(){console.log("executing foo")})();
(function(){console.log("executing anonymous")})();
are both function(objects) in global context - first with name, second without name. If they are created in variable it works the same just context is different.
I'm trying to learn JS on codeacademy and I can't understand/get past this thing. Can someone please provide an answer and also an explanation of why is it so? Would deeply appreciate.
// This function tries to set foo to be the
// value specified.
function setFoo(val) {
// foo is declared in a function. It is not
// accessible outside of the function.
var foo = val;
}
setFoo(10);
// Now that we are outside the function, foo is
// not defined and the program will crash! Fix this
// by moving the declaration of foo outside of the
// function. Make sure that setFoo will still update
// the value of foo.
alert(foo);
You can see scope as a term meaning what variables you can reach at a specific "level" in the code. In JavaScript, these "levels" are defined by functions. Each function introduces a new level.
For example, take this sample code:
var a;
// you can access a at this level
function function1() {
var b;
// you can access a, b at this level
function function2() {
var c;
// you can access a, b, c at this level
}
}
So in your case, you should declare var foo; outside the function, preferably above it. Then you can set it inside setFoo with foo = val;. foo then refers to the one you declared in the level above setFoo.
foo is accessible both in setFoo and in the alert call that way; compare it with the above sample code (function1 is setFoo, a is foo and the alert call is in the top-most level. function2, b and c are not used in your case.).
// Create globale variable
// (You should not use globale variables!)
var foo;
// set value
function setFoo(val) {
foo = val;
}
setFoo(10);
// show value
alert(foo);
Just declare foo outside any function then it will be global:
var foo = null;
function setFoo(val) {
foo = val;
}
setFoo(10);
alert(foo);
Try it !
When you declare a variable in Javascript it is only visible to code that is in the same function as it is declared, or a function inernal to that function. Because foo is originally declared in the SetFoo function nothing outside of SetFoo is able to see it, so the call to alert fails as foo does not exist in the gloabl scope.
As the comments suggest, moving the declaration of foo out of the function and into the global scope (which you can think of as a catch-all function that contains everything) would allow you to use foo when calling alert.
var foo;
function setFoo(val) {
foo = val;
}
setFoo(10);
alert(foo); // No longer crashes
Every function in Javascript has it's own scope. That means that every variable you define there with the var keyword, will only be available within that function. That means that when you call setFoo(10), you create the variable foo, give it a value of five, after which it is immediately destroyed because it went out of scope.
There are multiple ways to solve this problem. The first would be to remove the var keyword. This would put foo in the global scope, which means that it's available everywhere. However, this is discouraged, you want to keep the global scope as uncluttered as possible, so that if you have javascript code provided by multiple people on the same page, they can't overwrite other people's variables. Another way to do it would be this:
function setFoo(val){
var foo = val;
alertfoo = function(){
alert(foo)
}
}
In this example, the only thing you're putting in the global scope is the alertfoo function, because you want that to be available everywhere. The alertfoo function is defined inside the setFoo function, this means that although foo should have gone out of scope after setfoo has been executed, it is kept in memory, because alertfoo has access to it.
This makes for some nice tricks. For example, let's say you're making a javascript library that will be included on other people's pages, you'll want to create a scope inside of which you can define variables, without polluting the global scope. The most common way to do this, is by declairing a self-executing function. This is a function which is executed immediately after being defined, it looks like this:
(function(){
//set variables you want to be global in your own code
var mainpage = document.getElementById('main');
//define functions you want to make available to other people in a way that puts them in the global scope
setMainElement = function(newmain){mainpage = newmain;}
})();
You can make this even better by making only one object global, and provide your interfae through the methods of that object, this way, you create a namespace with all the functions that your library contains. The next example uses an object literal to do this. In javascript, you can create an object by putting key/value pairs petween curly braces. the key/value pairs are properties of the object. for example:
(function(){
var privatevar = 10,otherprivate=20;
publicInterface = {
'addToPrivate': function(x){privatevar+=x;},
'getPrivate': function(){return private}
};
})();
Original code:
function setFoo(val) {
var foo = val;
}
setFoo(10);
alert(foo); // Crash!
Their advice to fix the crash:
Fix this by moving the declaration of foo outside of the function
I'm guessing you're confused as to what they mean by "outside of the function".
Try this edited code:
var foo = 5; // "var" declares the variable to be in this outer scope
function setFoo(val) {
foo = val; // but we can still access it in this inner scope...
}
setFoo(10);
alert(foo); // Displays a dialog box that says "10"
Variables defined in the function is valid only in the function
function setFoo(val) {
foo = val;
}
In JavaScript, new scopes are only created by functions
Original Question:
JSHint complains when my JavaScript calls a function that is defined further down the page than the call to it. However, my page is for a game, and no functions are called until the whole thing has downloaded. So why does the order functions appear in my code matter?
EDIT: I think I may have found the answer.
http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting
I am groaning inside. Looks like I need to spend ANOTHER day re-ordering six thousand lines of code. The learning curve with javascript is not steep at all, but it is very loooooong.
tl;dr If you're not calling anything until everything loads, you should be fine.
Edit: For an overview which also covers some ES6 declarations (let, const): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Scope_Cheatsheet
This weird behavior depends on
How you define the functions and
When you call them.
Here's some examples.
bar(); //This won't throw an error
function bar() {}
foo(); //This will throw an error
var foo = function() {}
bar();
function bar() {
foo(); //This will throw an error
}
var foo = function() {}
bar();
function bar() {
foo(); //This _won't_ throw an error
}
function foo() {}
function bar() {
foo(); //no error
}
var foo = function() {}
bar();
This is because of something called hoisting!
There are two ways to define functions: Function declaration and function expression. The difference is annoying and minute, so let's just say this slightly wrong thing: If you're writing it like function name() {}, it's a declaration, and when you write it like var name = function() {} (or an anonymous function assigned to a return, things like that), it's a function expression.
First, let's look at how variables are handled:
var foo = 42;
//the interpreter turns it into this:
var foo;
foo = 42;
Now, how function declarations are handled:
var foo = 42;
function bar() {}
//turns into
var foo; //Insanity! It's now at the top
function bar() {}
foo = 42;
The var statements "throws" the creation of foo to the very top, but doesn't assign the value to it yet. The function declaration comes next in line, and finally a value is assigned to foo.
And what about this?
bar();
var foo = 42;
function bar() {}
//=>
var foo;
function bar() {}
bar();
foo = 42;
Only the declaration of foo is moved to the top. The assignment comes only after the call to bar is made, where it was before all the hoisting occurred.
And finally, for conciseness:
bar();
function bar() {}
//turns to
function bar() {}
bar();
Now, what about function expressions?
var foo = function() {}
foo();
//=>
var foo;
foo = function() {}
foo();
Just like regular variables, first foo is declared at the highest point of the scope, then it is assigned a value.
Let's see why the second example throws an error.
bar();
function bar() {
foo();
}
var foo = function() {}
//=>
var foo;
function bar() {
foo();
}
bar();
foo = function() {}
As we've seen before, only the creating of foo is hoisted, the assignment comes where it appeared in the "original" (un-hoisted) code. When bar is called, it is before foo is assigned a value, so foo === undefined. Now in the function-body of bar, it's as if you're doing undefined(), which throws an error.
The main reason is probably that JSLint does only one pass on the file so it doesn't know you will define such a function.
If you used functions statement syntax
function foo(){ ... }
There is actually no difference at all where you declare the function (it always behaves as if the declaration is on the beginning).
On the other hand, if your function was set like a regular variable
var foo = function() { ... };
You have to guarantee you wont call it before the initialization (this can actually be a source of bugs).
Since reordering tons of code is complicated and can be a source of bugs in itself, I would suggest you search for a workaround. I'm pretty sure you can tell JSLint the name of global variables beforehand so it doesn't complain about undeclared stuff.
Put a comment on the beggining of the file
/*globals foo1 foo2 foo3*/
Or you can use a text box there for that. (I also think you can pass this in the arguments to the inner jslint function if you can meddle with it.)
There are way too many people pushing arbitrary rules about how JavaScript should be written. Most rules are utter rubbish.
Function hoisting is a feature in JavaScript because it is a good idea.
When you have an internal function which is often the utility of inner functions, adding it to the beginning of the outer function is an acceptable style of writing code, but it does have the drawback that you have to read through the details to get to what the outer function does.
You should stick to one principle throughout your codebase either put private functions first or last in your module or function. JSHint is good for enforcing consistency, but you should ABSOLUTELY adjust the .jshintrc to fit your needs, NOT adjust your source code to other peoples wacky coding concepts.
One coding style that you might see in the wild you should avoid because it gives you no advantages and only possible refactoring pain:
function bigProcess() {
var step1,step2;
step1();
step2();
step1 = function() {...};
step2 = function() {...};
}
This is exactly what function hoisting is there to avoid. Just learn the language and exploit its strengths.
Only function declaration are hoisted not function expression (assignment).