I want to implement a function that takes another function as an argument, and returns a new version of that function that can only be called once.
The first function works, but the 2nd one doesn't work.
Why doesn't the 2nd function work, but can somehow still access the word without a function inside it like the first one?
var logOnce = once(console.log)
function once(fn) {
var call = true;
return function(word) {
if(call) {
call = false;
return fn(word);
}
}
}
function once(fn) {
var call = true;
if (call === true) {
call = false;
return fn;
}
}
logOnce("foo"); ----> "foo"
logOnce("blue"); ----> "blue"
Your second approach doesn't work because it returns the same fn. In fact, it is equivalent to
function once(fn) {
return fn;
}
Therefore, once(console.log) is just console.log, and you can call it as many times as you want.
The first approach works because you return a different function, which will call the original one or not depending on a variable.
Related
Hello Everyone hope you all doing great ,
this is my code , function with name and callback
taking the name to callback function to make return the name and console.log it
if i
function doSomething(name,callback) {
callback(name);
}
function foo(n) {
return n;
}
var val = doSomething("TEST",foo);
console.log(val);
i got undefined .
if i
function doSomething(name,callback) {
callback(name);
}
function foo(n) {
console.log(n);
}
var val = doSomething("TEST",foo);
that works fine and console.log the TEST name .
So why the return not working ?
thanks
Your doSomething() function doesn't return anything, which means an assignment using it will be undefined. But, that's not really the problem here.
The underlying problem is that you seem to be mixing two different data processing patterns here: if you're writing purely synchronous code, then use returning functions (which immediately return some value). If you need asynchronous code, then use a callback (which will "eventually" do something). Mixing those two patterns is a recipe for problems and frustration:
Either:
don't name your function a "callback", and have it return its processed value, or
make the callback responsible for doing whatever it is you were going to do with val.
Case 1:
function doSomething(data, processor) {
return processor(data);
}
function passThrough(v) { return v; }
var val = doSomething("test", passThrough);
// immediately use "val" here in for whatever thing you need to do.
Case 2:
function doSomething(data, callback) {
// _eventually_ a callback happens - for instance, this
// function pulls some data from a database, which is one
// of those inherently asynchronous tasks. Let's fake that
// with a timeout for demonstration purposes:
setTimemout(() => callback(data), 500);
}
function handleData(val) {
// use "val" here in for whatever thing you need to do. Eventually.
}
doSomething("test", handleData);
And if you want to go with case 2, you really want to have a look at "Promises" and async/await in modern Javascript, which are highly improved approaches based on the idea of "calling back once there is something to call back about".
2021 edit: a third option since original writing this answer is to use the async/await pattern, which is syntactic sugar around Promises.
Case 3:
async function doSomething(input) {
// we're still _eventually_ returning something,
// but we're now exploiting `async` to wrap a promise,
// which lets us write normal-looking code, even if what
// we're really doing is returning a Promise object,
// with the "await" keyword auto-unpacking that for us.
return someModernAsyncAPI.getThing(input);
}
function handleData(val) {
// ...
}
async function run() {
const data = await doSomething("test");
handleData(data);
}
run();
function doSomething(name,callback) {
callback(name);
}
function foo(n) {
console.log(n);
return n;
}
var val = doSomething("TEST",foo);
Take a look at above code. When you call doSomething, which internally executes foo it prints on the console because thats what console.log is for. However, after this statement it returns n as well which then is received in doSomething. But its not being returned. To put it simply, what you are mainly doing is
function doSomething(name,callback) {
const returnValue = callback(name);
}
If you call the above method, it will return undefined. To make it return correct value, you have to call "return returnValue". Similary you have to say
return callback(name)
Hope this helps.
Happy Learning
Inorder to assign the returning value/object of a function(in this case doSomething, it should have a return statement. Else the function returns nothing, so when you assign that to val, it is still undefined.
So you modify your code like this:
function doSomething(name,callback) {
return callback(name);
}
function foo(n) {
return n;
}
var val = doSomething("TEST",foo);
console.log(val);
undefined is implicitly returned if you don't have a return in your function.
when you call var val = doSomething("TEST",foo), you are aligning the return value of doSomething to val, which is undefined.
function doSomething(name,callback) {
return callback(name);
}
function foo(n) {
return n;
}
var val = doSomething("TEST",foo);
console.log(val);
This question already has answers here:
Implementing a 'once' function in JavaScript
(3 answers)
Closed 8 years ago.
I have a code below,
var a = 0;
var addByOne = doOnce(function() { a += 1; });
// I need to define a doOnce function here
// Run addByOne two times
addByOne();
addByOne();
This will result the variable a holds 2 as its value. My question is, how do I make the doOnce function so that it will result in running the function inside doOnce (in the case above, function () { a += 1; } ) just one time. So no matter how many times addByOne is called, variable a will be incremented just once.
Thanks
This can be achieved by creating a doOnce function which returns a wrapper for calling the passed function if it has not already been run. This may look something like this;
doOnce = function(fn) {
var hasRun = false,
result;
return function() {
if (hasRun === false) {
result = fn.apply(this, arguments);
hasRun = true;
}
return result;
}
}
Try this:
function doOnce(fn) {
// Keep track of whether the function has already been called
var hasBeenCalled = false;
// Returns a new function
return function() {
// If it has already been called, no need to call it again
// Return (undefined)
if (hasBeenCalled) return;
// Set hasBeenCalled to true
hasBeenCalled = true;
return fn.apply(this, arguments);
}
}
If you want, you can keep track of the return value and return that instead of undefined.
I'm relatively new to coding in JavaScript, and I've came across a problem. I like to nest functions to keep things orderly, but how would I exit from a parent function from inside a child function?
example:
function foo1() {
function foo2() {
//return foo1() and foo2()?
}
foo2();
}
See update under the fold
You can't. You can only return from the child function, and then return from the parent function.
I should note that in your example, nothing ever calls foo2 (As of your edit, something does). Let's look at a more real example (and one that comes up a lot): Let's say we want know if an array contains an entry matching some criterion. A first stab might be:
function doesArrayContainEntry(someArray) {
someArray.forEach(function(entry) {
if (entryMatchesCondition(entry)) {
return true; // Yes it does <-- This is wrong
}
});
return false; // No it doesn't
}
You can't directly do that. Instead, you have to return from your anonymous iterator function in a way to stop the forEach loop. Since forEach doesn't offer a way to do that, you use some, which does:
function doesArrayContainEntry(someArray) {
return someArray.some(function(entry) {
if (entryMatchesCondition(entry)) {
return true; // Yes it does
}
});
}
some returns true (and stops looping) if any call to the iterator function returns true; it returns false if no call to the iterator returned true.
Again, that's just one common example.
You've referred to setInterval below, which tells me that you're almost certainly doing this in a browser environment.
If so, your play function almost certainly has already returned by the time you want to do what you're talking about, assuming the game has any interaction with the user other than alert and confirm. This is because of the asynchronous nature of the environment.
For example:
function play() {
var health = 100;
function handleEvent() {
// Handle the event, impacting health
if (health < 0 {
// Here's where you probably wanted to call die()
}
}
hookUpSomeEvent(handleEvent);
}
The thing is, that play will run and return almost immediately. Then the browser waits for the event you hooked up to occur, and if it does, it triggers the code in handleEvent. But play has long-since returned.
Make a note whether the parent function should also return.
function foo1() {
bool shouldReturn = false;
function foo2() {
shouldReturn = true; // put some logic here to tell if foo1() should also return
return;
}
if (shouldReturn) {
return;
} else {
// continue
}
}
It only says that you can't return the parent function in the child function, but we can do a callback and make it happen.
function foo1(cb = () => null) {
function foo2() {
cb();
}
foo2();
}
foo1(() => {
// do something
});
We can use Promises for this:
const fun1 = async () => {
const shouldReturn = await new Promise((resolve, reject) => {
// in-game logic...
resolve(true)
})
if(shouldReturn) return;
}
if you wanna return from the parent function, then just resolve with true
Based on your comment, something like this might work as a main game loop.
function play() {
var stillPlaying = true;
while(stillPlaying) {
... play game ...
stillPlaying = false; // set this when some condition has determined you are done
}
}
I see different topics about the toggle function in jquery, but what is now really the best way to toggle between functions?
Is there maybe some way to do it so i don't have to garbage collect all my toggle scripts?
Some of the examples are:
var first=true;
function toggle() {
if(first) {
first= false;
// function 1
}
else {
first=true;
// function 2
}
}
And
var first=true;
function toggle() {
if(first) {
// function 1
}
else {
// function 2
}
first = !first;
}
And
var first=true;
function toggle() {
(first) ? function_1() : function_2();
first != first;
}
function function_1(){}
function function_2(){}
return an new function
var foo = (function(){
var condition
, body
body = function () {
if(condition){
//thing here
} else {
//other things here
}
}
return body
}())`
Best really depends on the criteria your application demands. This might not be the best way to this is certainly a cute way to do it:
function toggler(a, b) {
var current;
return function() {
current = current === a ? b : a;
current();
}
}
var myToggle = toggler(function_1, function_2);
myToggle(); // executes function_1
myToggle(); // executes function_2
myToggle(); // executes function_1
It's an old question but i'd like to contribute too..
Sometimes in large project i have allot of toggle scripts and use global variables to determine if it is toggled or not. So those variables needs to garbage collect for organizing variables, like if i maybe use the same variable name somehow or things like that
You could try something like this..: (using your first example)
function toggle() {
var self = arguments.callee;
if (self.first === true) {
self.first = false;
// function 1
}
else {
self.first = true;
// function 2
}
}
Without a global variable. I just added the property first to the function scope.
This way can be used the same property name for other toggle functions too.
Warning: arguments.callee is forbidden in 'strict mode'
Otherwise you may directly assign the first property to the function using directly the function name
function toggle() {
if (toggle.first === true) {
toggle.first = false;
// function 1
}
else {
toggle.first = true;
// function 2
}
}
I have a requirement where I get the anchor tags id and based on the id I determine which function to execute.. so is there anything that suites below code
function treeItemClickHandler(id)
{
a=findDisplay(id);
a();
}
You can assign a function to a variable like so:
You can also return a function pointer from a function - see the return statement of findDisplay(id).
function treeItemClickHandler(id)
{
var a= findDisplay;
var other = a(id);
other();
}
function findDisplay(id)
{
return someOtherThing;
}
function someOtherThing()
{
}
Sure, functions are first class objects in JavaScript. For example, you can create a map (an object) which holds references to the functions you want to call:
var funcs = {
'id1': function(){...},
'id2': function(){...},
...
};
function treeItemClickHandler(id) {
if(id in funcs) {
funcs[id]();
}
}
As functions are treated as any other value, you can also return them from another function:
function findDisplay(id) {
// whatever logic here
var func = function() {};
return func;
}
functions are normal javascript values, so you can pass them around, (re)assign them to variables and use them as parameter values or return values for functions. Just use them ;) Your code is correct so far.
You can map between ids and functions to call in a number of ways.
One of the simpler ones is to create an object mapping ids to functions, and find the function to call from that object (this is in essence a nicer-looking switch statement).
Example:
function treeItemClickHandler(id)
{
var idMap = {
"some-id": findDisplay,
"another-id": doSomethingElse
};
if (!idMap.hasOwnProperty(id)) {
alert("Unknown id -- how to handle it?");
return;
}
// Call the corresponding function, passing the id
// This is necessary if multiple ids get handled by the same func
(idMap[id])(id);
}
function findDisplay(id)
{
// ...
}
function doSomethingElse(id)
{
// ...
}