This question already has answers here:
Why use named function expressions?
(5 answers)
What's the point of naming function expressions if you can't really reference them by the names you give them? [duplicate]
(5 answers)
Closed 5 years ago.
var boo= function foo(){
console.log("I am foo");
}
boo(); // output: I am foo
foo(); // output: Uncaught ReferenceError
I am little confused with javascript named function. Can any one please explain why in above code snippet the foo() function call is throwing the error. Thanks in advance
var boo= function foo(){
There is a clear difference between a function and function expression.
What you have is an expression resolved to a variable. The way you are expecting to work needs to be a function or a variable resolved by a function expression.
From MDN docs
Here is an example of an anonymous function expression (the name is
not used):
var myFunction = function() {
statements
}
It is also possible to provide a name inside the definition in order
to create a named function expression:
var myFunction = function namedFunction(){
statements
}
One of the benefit of creating a named function expression is that in
case we encounted an error, the stack trace will contain the name of
the function, making it easier to find the origin of the error.
As we can see, both example do not start with the function keyword.
Statements involving functions which do not start with function are
function expressions.
Related
This question already has answers here:
var functionName = function() {} vs function functionName() {}
(41 answers)
Closed 6 years ago.
When I run a function in js it seems to work fine but will this be fine?
function myfunction(){
}
VS
var myfunction = function(){
}
Because both are called the same so does it make no difference?
Sorry new to js so i'm just making sure it's good to learn this style. (The first function is what i'd prefer to use.)
This question is already asked at this link,
One of the basic difference comes from declaration, for instance :-
foo();
var foo = function(){
console.log('hi');
}
would lead to undefined, since it is called before it's declared, on the other hand,
foo();
function foo(){
console.log('hi');
}
would console 'hi' with no problem, for further insight check the above link.
As shown in the W3Schools Function guide
You can use a function declaration or a function expression.
Both are the same in general. Thought there are certain things to consider:
In a function expression the function is only defined when that line in code is reached
In a function decleration the function is defined as soon as the code is run, so it can be called before it is written in your code (due to hoisting)
The first example you wrote is a function Declaration
The second example you wrote is a function Expression
The only difference between the two is how you will invoke them in your code after you defined them.
If you are wondering which is better, it all comes down to which you are more comfortable with and think the syntax for is more readable, also try not to mix up the two to keep conventions in your code.
This question already has an answer here:
Function call javascript [duplicate]
(1 answer)
Closed 6 years ago.
Can you explain this?
var guessWhat = function(){ console.log('Print this!!!'); };
function guessWhat(){ console.log('Print that???'); }
guessWhat();
// output: Print this!!!
Both are declared on the global scope. Why is the the second line not overriding the first? Is the second function lost in limbo?
function guessWhat(){ console.log('Print that???'); } // declaration
This is a function declaration, it is defined before any code is executed.
var guessWhat = function(){ console.log('Print this!!!'); }; // literal
This is a function literal, it is defined at run-time.
so, the function definition gets loaded first (before any code), and the function literal afterwards, which overrides the first definition, hence this behaviour.
Read more here.
This question already has answers here:
Why JavaScript function declaration (and expression)?
(5 answers)
Closed 7 years ago.
can anybody tell me whats the difference between this two examples:
//difference with this:
var x = function withName(a,b,c) {} //why i should declare a name here?
//and this
var y = function (a,b,c) {}
According to the ECMA specification, a function can be written either through declaration or through expression,
Declaration is writing function as below,
function withName(a,b,c) {
}
With declaration, it is required to write an identifier which in this case is withName
Both the example that you have given are of function expression where it is not required to write the identifier i.e. withName as you can still call this function with the variable x or y
var x = function withName(a,b,c) {}
var y = function (a,b,c) {}
However, the only difference here is that if you don't specify the identifier you are creating a anonymous funtion.
You can see this link for detailed explanation.
On this particular case, there is no difference. But in general, the fact that the function assigned to a variable has a name, makes it possible to invoke the function from inside the same function (recursion).
This question already has answers here:
var functionName = function() {} vs function functionName() {}
(41 answers)
Why use named function expressions?
(5 answers)
Closed 9 years ago.
I'm going through this blog about the difference between function declarations and function expressions.
It gives these two examples. They call the first an "anonymous function expression" and the second a "named function expression."
// anonymous function expression
var a = function(){
return 3;
}
// named function expression
var b = function bar(){
return 3;
}
I tested these two in Chrome's JS console and I see the following:
a()
=> 3
b()
=> 3
bar()
=> bar is not defined
My question is: In the second function expression declaration, what is the point of "bar"? In general, why does one ever use a named function expression?
Some people prefer to do it like this because if errors occur, your functions have names. It's mostly a matter of preference and how often you have trouble with unnamed functions.
You don't normally see it used in a var declaration, but instead when declaring callbacks:
callbackFunction(function success() { ... }, function fail() { ... })
That way you know which argument is which, they're labelled, and if one of them fails you get a precise indication of which one broke.
var b = function bar(){
return 3;
}
bar()
=> bar is not defined
The identifier bar is only available inside of the function. Try
var b = function bar() {
console.log(bar);
}
b();
why does one ever use a named function expression?
To allow referencing a function expression that was not assigned to a reachable or constant variable, e.g. for recursion in an IEFE.
Also, named functions show up different during debugging, e.g. in call stack (trace)s or breakpoint listings. Often you can use a (named) function declaration instead of a function expression, see also http://blog.niftysnippets.org/2010/03/anonymouses-anonymous.html.
This question already has answers here:
var functionName = function() {} vs function functionName() {}
(41 answers)
Are named functions preferred over anonymous functions in JavaScript? [duplicate]
(4 answers)
Closed 9 years ago.
I saw the following JavaScript functions works exactly same, Then what is the difference between them other than syntax.
The function are:
var functionName=function(){
//some code here
};
function functionName(){
//some code here
}
I call them in the same way as:
functionName();
Please dont' tell me there syntax is different, Other than that is there any difference like
1)speed of execution
2)Memory utilization etc.
Thanks in advance!
This has been answered many times in StackOverflow. It is just the way of naming. So taking up some points from the answers, I would say:
Function declarations and variable declarations are always moved ("hoisted") invisibly to the top of their containing scope by the JavaScript interpreter. Function parameters and language-defined names are, obviously, already there.
Advantages & Disadvantages:
There are few advantages to naming functions:
names for meta analysis. functionInstance.name will show you the name.
Far more importantly, the name will be printed in stack traces.
names also help write self documenting or literate code.
There is a single disadvantage to named functions expressions
IE has memory leaks for NFE
Another main difference
The difference is that functionTwo is defined at parse-time for a script block, whereas functionOne is defined at run-time. For example:
<script>
// Error
functionOne();
var functionOne = function() {
}
</script>
<script>
// No error
functionTwo();
function functionTwo() {
}
</script>
References
var functionName = function() {} vs function functionName() {}
Are named functions or anonymous functions preferred in JavaScript?
Named function expressions demystified
Function Declarations vs. Function Expressions.
var functionName = function() {} vs function functionName() {}
1st one is Named Function Expressions, which should return some value to the caller.
2nd one is just a function, it's upto you whether you return value or not