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Did I use recursion correctly?

I'm working through some practice JavaScript problems, and solved a problem involving recursion. Although I got it right, my implementation is different from the "official" solution, so I was wondering if anyone had any insight on whether the official answer is better and if so, why.
Question
Implement a function that takes a function as its first argument, a number num as its second argument, then executes the passed in function num times.
It's ok to use a loop in your implementation, bonus points if you use recursion instead.
My Solution
function repeat(operation, num) {
if (num > 0) {
operation();
repeat(operation, num - 1);
};
};
Given Solution
function repeat(operation, num) {
if (num <= 0)
return;
operation();
return repeat(operation, --num);
};
Is there anything about the given solution that makes it better than mine?

There is a specific reason to directly return the result of a recursive call. This is called "tail recursion", and a clever run-time environment can optimise for that case and execute without using any additional stack space for the recursive calls (it simply reuses the stack space of the current function). In the latest ECMAScipt 6 specification (Javascript to you and me) it specifically calls out that the runtime environment should optimise tail-recursive calls.
In practice, your code does in fact not do anything after the recursive call, so it is properly tail-recursive. Putting the recursive call with a return statement makes it clear that it should be a tail-recursive call, and a bit more likely that the run-time environment will correctly optimise it.

Just bear in the mind. There is something that every JavaScript developer MUST know. If recursion continues very long time the browser can kill your script or alert error to user. Often JavaScript developers uses approaches like this:
var inputArray = [1,2,3,4,5,6];
(function handleArray() {
var currentElement = inputArray.shift();
// do something with currentElement
if (inputArray.length > 0 ) {
setTimeout(handleArray, 0);
}
}());
In this way you will break long time operation on small parts.

The solutions are identical, but in practice the given solution is easier to understand. There is a clear "base case," and the recursion is done in the return statement. Both of these properties lead to better code quality and readability when dealing with recursive functions.

Javascript recursive function inside a for loop

var f_drum_min = function myself(a){
alert(a);
$f_min_node.push(a);
for (i=0;i<=$m;i++){
if ($f_leg[i][1]==a){
myself($f_leg[i][0]);
}
}
};
myself($f_leg[i][0]); breaks the for loop , how can I make it run multiple times in loop?

Your function is riddled with bad habits
There's no way for me to improve this function because I have no idea what all of those external states do. Nor is it immediately apparent what their data types are.
These are bad habits because there's no possible way to know the effect of your function. Its only input is a, yet the function depends on $f_min_node, $f_leg, and $m.
What is the value of those variables at the time you call your function?
What other functions change those values?
I assigned $f_min_node to some value and then called f_drum_min. How was I supposed to know that $f_min_node was going to get changed?
Every time you call your function, it's a big surprise what happens as a result. These are the woes of writing non-deterministic ("impure") functions.
Until you can fix these problems, recursion in a for-loop the least of your concerns
I have annotated your code with comments here
// bad function naming. what??
var f_drum_min = function myself(a){
// side effect
alert(a);
// external state: $f_min_node
// mutation: $f_min_node
$f_min_node.push(a);
// leaked global: i
// external state: $m
for (i=0;i<=$m;i++){
// external state: $f_leg
// loose equality operator: ==
if ($f_leg[i][1]==a){
myself($f_leg[i][0]);
}
}
};
I can help you write a deterministic recursive function that uses a linear iterative process though. Most importantly, it doesn't depend on any external state and it never mutates an input.
// Number -> Number
var fibonacci = function(n) {
function iter(i, a, b) {
if (i === 0)
return a;
else
return iter(i-1, b, a+b);
}
return iter(n, 0, 1);
}
fibonacci(6); // 8
for loops are pretty primitive; Imperative programmers will reach for it almost immediately thinking it's the only way to solve an iteration problem.
I could've used a for loop in this function, but thinking about the problem in a different way allows me to express it differently and avoid the for loop altogether.

One basic problem with the code, which would cause it to break under almost any circumstances, is that the loop variable i is a global, and is thus shared by all recursive invocations of the function.
For example, let's say the function is invoked for the first time. i is 0. Now it recurses, and let's say that the condition in the if is never true. At the end of the 2nd call, i = $m + 1. When you return to the first call, because i is global, the loop in the first call ends. I assume this is not what you want.
The fix for this is to declare i as local:
for (var i=0;i<=$m;i++){
This may or may not fix all of your problems (as pointed out in comments, we'd have to see more of your code to identify all possible issues), but it is a critical first step.

why write a function that never returns a value

I am reading a very comprehensive javascript programming book. In the "functions" section of language basics I have come to the following:
"The return statement can also be used without specifying a return value. When used in this way, the function stops executing immediately and returns undefined as its value. This is typically used in functions that don’t return a value to stop function execution early, as in the following example, where the alert won’t be displayed:"
function sayHi(name, message) {
return;
alert(“Hello “ + name + “, “ + message); //never called
}
I am trying to understand why anyone would want to do this. Why write a function that returns "undefined"? I have tried googling, and searching SO, but have not had much luck though this may be because I am not phrasing my search correctly as I am learning js.
Can anyone give me a real-world example of where this might be useful so that I can understand?

Usually it's a conditional return. Something like
function calculateSomething(obj, condition) {
if (condition !=0 ) return;
obj.data = obj.data * 42;
}
in this case if some condition fails - function exits right away. Otherwise data in a passed object is modified.

The function you provide is useless...
However, not every function needs to return something. Sometimes a function does things "elsewhere" and the value that is returned is irrelevant.

The function may have side effects, such as altering the state of the page. Then, the caller may not expect any return value.

In my opinion, learned from masters so to speak, the idea here is to only temporarily disable the function. So if you have like an x amount of function calls to sayHi() like for example in a repeater or spread out over multiple files, the idea can be useful to simply return; without a value. Useful? Yes, to avoid commenting out chunks of code, but that's it.
Leaving this in a development environment, which initially will enter the www, you should not write it like this, and always make sure that: when a function has to return "something", then "something" always counts for value, no matter what the condition. With lowest value a boolean => return false;. This counts for every code block inside that function => for if else while ...
/* INCORRECT: it only returns something conditionally */
function do(something){
if (condition){
return false;
}
}
/* CORRECT: it returns value no matter what the condition */
function do(something){
if (!condition){
return true;
}
return false;
}
/* OPTIMIZED: a lot of ideas can be optimized, especially when returning booleans */
function do(something){
return !condition;
}
The rules only apply if you want to write proper code. Using tools like jslint, jshint or re-sharper helps a lot in understanding the basic principles of writing ecmascript valid code. Since not everyone is aware of these rules, all examples will yield the same result. In the sens of "it works, but it's not valid code".

Because A JS function is an object(has:Attributs+methods)
And , An object should have a value which can be undefined .
The proof is that
function sayHi(){}
Can be handled such as object even calling it :
var returnedValue=sayHi.call() ;
If sayHi does not return anything, returnedValue will be undefined

Example if you want to use classes I am sure about it you will use setter functions to give value for an instance attribute.
Like this:
function Something(val) {
this.value = val;
}

What is the JavaScript convention for no operation?

What is the JavaScript convention for no operation? Like a Python pass command.
One option is simply an empty function: function() {}
jQuery offers $.noop(), which simply calls the empty function above.
Is it acceptable to simply enter a value of false or 0?
In context... all of these work without throwing an error in Chrome:
var a = 2;
(a === 1) ? alert(1) : function() {};
(a === 1) ? alert(1) : $.noop();
(a === 1) ? alert(1) : false;
(a === 1) ? alert(1) : 0;
EDIT: A lot of people responded with, "don't do this! Change the code structure!" This reminds me of a post where someone asked how to sniff the browser. He received a barrage of posts saying, "DON'T DO THAT! IT'S EVIL," but nobody told him how to sniff the browser. This is not a code review. Imagine that you are dealing with legacy code that can't be changed, and without some function passed in, it will toss an error. Or, simply, that's the way the customer wants it, and they're paying me. So, respectfully, please answer the question: What is the best way to specify a "no operation" function in JavaScript?
EDIT2: How about one of these?
true;
false;
0;
1;
null;

To answer the original question, the most elegant and neat implementation of a noop function in pure Javascript (as is also discussed here) is Function.prototype. This is because:
Function.prototype is a function:
typeof Function.prototype === "function" // returns true
It can be invoked as a function and essentially does nothing as shown here:
setTimeout(function() {
console.log('Start: ', Date.now());
Function.prototype();
console.log('End : ', Date.now());
}, 1000);
Although this is a "true noop" since most browsers seem to do nothing to execute the noop defined this way (and hence save CPU cycles), there might be some performance issues associated with this (as is also mentioned by others in comments or in other answers).
However, that being said, you can easily define your own noop function and, infact, many libraries and frameworks also provide noop functions. Below are some examples:
var noop = function () {}; // Define your own noop in ES3 or ES5
const noop = () => {}; // Define in ES6 as Lambda (arrow function)
setTimeout(noop, 10000); // Using the predefined noop
setTimeout(function () {} , 10000); // Using directly in ES3 or ES5
setTimeout(() => {} , 10000); // Using directly in ES6 as Lambda (arrow function)
setTimeout(angular.noop, 10000); // Using with AngularJS 1.x
setTimeout(jQuery.noop, 10000); // Using with jQuery
Here is an alphabetical list of various implementations of noop functions (or related discussions or google searches):
AngularJS 1.x, Angular 2+ (Does not seem to have a native
implementation - use your own as shown above), Ember, jQuery, Lodash, NodeJS, Ramda, React (Does not seem to have a native implementation - use your own as shown above), RxJS,
Underscore
BOTTOM LINE: Although Function.prototype is an elegant way of expressing a noop in Javascript, however, there might be some performance issues related to its use. So, you can define and use your own (as shown above) or use one defined by the library/framework that you might be using in your code.

The most concise and performant noop is an empty arrow function: ()=>{}.
Arrow functions work natively in all browsers except IE (there is a babel transform if you must):
()=>{} vs. Function.Prototype
()=>{} is 87% faster than Function.prototype in Chrome 67.
()=>{} is 25% faster than Function.prototype in Firefox 60.
()=>{} is 85% faster than Function.prototype in Edge (6/15/2018).
()=>{} is 65% less code than Function.prototype.
The test below heats up using the arrow function to give bias to Function.prototype, yet the arrow function is the clear winner:
const noop = ()=>{};
const noopProto = Function.prototype;
function test (_noop, iterations) {
const before = performance.now();
for(let i = 0; i < iterations; i++) _noop();
const after = performance.now();
const elapsed = after - before;
console.info(`${elapsed.toFixed(4)}MS\t${_noop.toString().replace('\n', '')}\tISNOOP? ${_noop() === undefined}`);
return elapsed;
}
const iterations = 10000000
console.info(`noop time for ${iterations.toLocaleString()} iterations`)
const timings = {
noop: test(noop, iterations),
noopProto: test(noopProto, iterations)
}
const percentFaster = ((timings.noopProto - timings.noop)/timings.noopProto).toLocaleString("en-us", { style: "percent" });
console.info(`()=>{} is ${percentFaster} faster than Function.prototype in the current browser!`)

whatever you tend to achieve here is wrong. Ternary expressions shall not be used as a full statement, only in expression, so the answer to your question is:
none of your suggestions, instead do:
var a = 2;
if (a === 1)
alert(1)
// else do nothing!
then the code is easily understandable, readable and as much efficient as it can get.
Why make it more difficult, when it can be simple?
edit:
So then, does a "no-operation" command basically indicate an inferior code structure?
You're missing my point. All the above is about the ternary expression x ? y : z.
But, a no operation command does not makes sense in higher level languages such as Javascript.
It is usually used, in lower level languages such as assembly or C, as a way to make the processor do nothing for one instruction for timing purposes.
In JS, whether you do 0;, null;, function () {}; or an empty statement, there are great chances that it will be ignored by the interpretor when it is reading it, but before it gets interpreted, so in the end, you'll just make your program be loaded more slowly by a really tiny amount of time. Nota Bene: I'm assuming this, as I'm not involved in any widely used JS interpreter, and there are chances each interpreter has its own strategy.
In case you use something a bit more complicated, like $.noop() or var foo = function () {}; foo(), then the interpreter may do an unuseful function call that will end up spoiling a few bytes of your function stack, and a few cycles.
The only reason I see a function such as $.noop() would exist, would be to be able to still give a callback function to some event function that would throw an exception if it can't call that callback. But then, it's necessarily a function you need to give, and giving it the noop name is a good idea so you're telling your readers (and that may be you in 6 months) that you purposely give an empty function.
In the end, there's no such thing as "inferior" or "superior" code structure. You're either right or wrong in the way you use your tools.. Using a ternary for your example is like using a hammer when you want to screw. It'll work, but you're not sure you can hang something on that screw.
What could be considered either "inferior" or "superior" is the algorithm and ideas you put in your code. But that's another thing.

There is absolutely no problem or performance penalty of using Function.prototype over () => {}.
The main benefit of Function.prototype is having a singleton function rather than re-defining a new anonymous function each time. It's especially important to use a no-op like Function.prototype when defining default values and memoizing as it gives you a consistent object pointer which never changes.
The reason I'm recommending Function.prototype rather than Function is because of they're not the same:
Function() === Function()
// false
Function.prototype() === Function.prototype()
// true
Also, benchmarks from other answers are misleading. In fact, Function.prototype performs faster than () => {} depending on how you write and run the benchmark:
You can’t trust JS benchmarks << Specifically calling out benchmarks on this question.
Don't style your code from benchmarks; do whatever's maintainable and let the interpreter figure out how to optimize in the long run.

I think jQuery noop() is mostly intended to prevent code from crashing by providing a default function when the requested one is not available. For example, considering the following code sample, $.noop is chosen if fakeFunction is not defined, preventing the next call to fn from crashing:
var fn = fakeFunction || $.noop;
fn() // no crash
Then, noop() allows to save memory by avoiding to write the same empty function multiple times everywhere in your code. By the way, $.noop is a bit shorter than function(){} (6 bytes saved per token). So, there is no relationship between your code and the empty function pattern. Use null, false or 0 if you like, in your case there will be no side effect. Furthermore, it's worth noting that this code...
true/false ? alert('boo') : function(){};
... is completely useless since you'll never call the function, and this one...
true/false ? alert('boo') : $.noop();
... is even more useless since you call an empty function, which is exactly the same as...
true/false ? alert('boo') : undefined;
Let's replace the ternary expression with an if statement to see how much it's useless:
if (true/false) {
alert('boo');
} else {
$.noop(); // returns undefined which goes nowhere
}
You could simply write:
if (true/false) alert('boo');
Or even shorter:
true/false && alert('boo');
To finally answer your question, I guess a "conventional no operation" is the one which is never written.

I use:
(0); // nop
To test execution time of this run as:
console.time("mark");
(0); // nop
console.timeEnd("mark");
result: mark: 0.000ms
Using Boolean( 10 > 9) can be reduced it to simply ( 10 > 9) which returns true. Coming up with the idea to use a single operand I fully expected (0); would return false, but it simply returns the argument back as can be reviewed by performing this test at the console.
> var a = (0);
< undefined
> a
< 0

Need a succinct way of conditionally executing an expression, including function calls? (No noop necessary.)
true && expression // or `expression()`
Need a valid, callable expression with no side effects?
const noop = () => {}
if (true) noop()
Need a valid, non-callable expression with no side effects?
void 0;
false;
0;

Which of these cross-browser Javascript functions performs better?

As a rule of thumb, which of these methods of writing cross-browser Javascript functions will perform better?
Method 1
function MyFunction()
{
if (document.browserSpecificProperty)
doSomethingWith(document.browserSpecificProperty);
else
doSomethingWith(document.someOtherProperty);
}
Method 2
var MyFunction;
if(document.browserSpecificProperty) {
MyFunction = function() {
doSomethingWith(document.browserSpecificProperty);
};
} else {
MyFunction = function() {
doSomethingWith(document.someOtherProperty);
};
}
Edit: Upvote for all the fine answers so far. I've fixed the function to a more correct syntax.
Couple of points about the answers so far - whilst in the majority of cases it is a fairly pointless performance enhancement, there are a few reasons one might want to still spend some time analyzing the code:
Has to run on
slow computers, mobile devices, old
browsers etc.
Curiosity
Use the same
general principal to performance
enhance other scenarios where the
evaluation of the IF statement does
take some time.

Unless you're doing this a trillion times, it doesn't matter. Go with the one that is more readable and maintainable to you and/or your organization. The productivity gains you will get from writing clean, simple code matters way more than shaving a tenth of a microsecond off your JS execution time.
You should only even start thinking about what performs better when and only when you've written code and it is unacceptably slow. Then you should start tracking down the bottleneck, which will never be something like this. You will never get a measurable performance gain out of switching from one to the other here.

Unfortunately the code above is not actually cross-browser friendly as it relies on a mozilla quirk not present in other browsers -- namely that function statements are treated as function expressions inside branches. On browsers other that aren't built on mozilla the above code will always use the second function definition. I made a simple testcase to demonstrate this here.
Basically the ECMAScript spec says that function statements are treated similarly to var declarations, eg. they all get hoisted to the top of the current execution scope (eg. the start of a <script> tag, the start of a function, or the start of an eval block).

To clarify olliej's answer, your second method is technically a syntax error. You could rewrite it this way:
var MyFunction;
if(document.browserSpecificProperty) {
MyFunction = function() {
doSomethingWith(document.browserSpecificProperty);
};
} else {
MyFunction = function() {
doSomethingWith(document.someOtherProperty);
};
}
Which is at least correct syntax, but note that MyFunction would only be available in the scope in which that occurs. (Omit var MyFunction;, and preferably use window.MyFunction = function() ... for global.)

Technically, I would say that the second one would perform better, because the if statement is only executed once, rather than every time the function is run.
The difference, however, would be negligible to the point of being meaningless. The performance penalty of a single if statement such as this would be insignificant even compared to the performance penalty of simply calling a function. It would make a smallish difference even if if is called a million times.
The first one is easier to understand, because it doesn't have the awkwardness of defining the same function twice based on a condition, with both versions behaving differently. That seems to be a recipe for confusion later on.
I wouldn't be the first person to say that unless you are really insane about this optimization thing, you'll get more of a win out of code readability.

I generally prefer the second version, as the condition only has to be evaluated once and not on every call, but there are times when it's not really feasible because it will hamper readability.
Btw, this is a case where you might want to use the ?: operator, e.g (taken from production code):
var addEvent =
document.addEventListener ? function(type, listener) {
document.addEventListener(type, listener, false);
} :
document.attachEvent ? function(type, listener) {
document.attachEvent('on' + type, listener);
} :
throwError;

For your simplified example I would do what's below assuming that your browser property check only needs to be done once:
var MyFunction = (function() {
var rightProperty = document.browserSpecificProperty || document.someOtherProperty;
return function doSomethingWith() {
// use the rightProperty variable in your function
}
})();

The performance should be nearly equal!
Thing about using Frameworks like JQuery to get rid of the Browser compability problems!
If performance is your main goal, have a look at SlickSpeed! It is a page which benchmarks different JavaScript frameworks!