Here is the behaviour I'm looking for:
function one(func){
func(5);
}
function two(arg1, arg2){
console.log(arg1);
console.log(arg2);
}
one(two(3)) //prints 3, 5
Can this behaviour or something similar be accomplished in javascript?
You can always use the bind() function to pass some arguments to your function. It'll create a new function with the first argument - arg1 - equal to the value of 3 in this example:
function one(func){
func(5);
}
function two(arg1, arg2){
console.log(arg1);
console.log(arg2);
}
one(two.bind(null, 3))
You can read more about the bind() function here: MDN - Bind
Some workaround is possible
function one() {
var args = Array.prototype.slice.call(arguments);
var func = args[0];
args.splice(0, 1);
args.push(5);
func.apply(this, args);
}
function two(arg1, arg2) {
console.log(arg1);
console.log(arg2);
}
one(two, 3)
There's a problem with your syntax: function one is expecting its single argument to be a function. Then, below, when you invoke it, you are not passing the function two, but whatever two returns when it's passed a single argument, probably undefined. I don't know what specifically you're trying to accomplish but I'd recommend a little research into closures.
function one(arg){
two(arg, 5); // func here is two so it requires two params...
}
function two(arg1, arg2){
console.log(arg1);
console.log(arg2);
}
one(3)// one expect function so can't execute function here!
as soon as one expects function as argument two(3) should return function.
this condition is required
so in order to achieve it your two function should be
function two(arg1){
console.log(arg1);
return function(arg2) {
console.log(arg2);
};
}
so two(3) function call gets passed as argument to one
so before assigning value to variable engine executes it. And execution of two(3) call logs 3 to console and returns function
function(arg2) {
console.log(arg2);
};
and then engine assigns executed value(returned function) to func variable.
so func parameter of one function now looks like
func = function(arg2) {
console.log(arg2);
};
one calls func with 5 passed in as argument.
so 5 gets logged to console.
Basically you can't specify the parameter in the function or it'll run. You need to specify the function aka one(two), but that obviously wouldn't work.
However if you dynamically create a function you should be able to accomplish the task like so :
function one(func){
func(5);
}
function two(arg1, arg2){
console.log(arg1);
console.log(arg2);
}
one(function(val) { two(3, val) }) //prints 3, 5
well somehow this things work for me
function one(func){
console.log("one is running");
func;
}
function two(args1, args2){
console.log("two is running");
console.log("args1 -> " + args1);
console.log("args2 -> " + args2);
}
//to call it
args1 = 6;
args2 = 12
one(two(args1,args2));
Related
Here are two callback function:
function callback_a(){
alert('a');
}
function callback_b(p){
alert('b says'+ p)'
}
If I want use callback_a
function test(callback){
if(condition){
callback();
}
}
test(callback_a);
But the function test isn't applicable to callback_b, So how to implement a common function that you can passing some callbacks function with multiple possible parameter lists.
There are three options:
The easiest way is to use spread operator:
function test(callback, ...callback_args) {
callback(...callback_args);
}
in this case the invocation of test for function callback_b would be like this:
test(callback_b,"b")
The second way is using arguments which are scoped to any function in JavaScript:
function test(callback) {
callback.apply(null, arguments.slice(1));
}
the invocation of test for function callback_b would be the same:
test(callback_b,"b")
Another options is to use partially applied functions. In this case you should define b_callback like this (ES6 syntax):
let callback_b = (p) => () => void{
alert('b says'+ p)'
}
or without ES6:
function callback_b(p) {
return function(){
alert('b says'+ p)'
}
}
and invoke it like this:
test(callback_b("b"))
There is a special object called arguments that gets created when a function is invoked. It's an array-like object that represents the arguments passed in to a function:
It can be used like this:
test();
// no arguments passed, but it still gets created:
// arguments.length = 0
// arguments >> []
test(a);
// ONE argument passed:
// arguments.length = 1
// arguments >> [a]
test(a,b,c,d);
// FOUR arguments passed:
// arguments.length = 4
// arguments >> [a,b,c,d]
Knowing this, one can call a callback with the rest of the arguments passed in from the parent function using apply like this:
function test(callback) {
callback.apply(null, Array.prototype.slice.call(arguments, 1));
}
// arguments passed into test are available in the function scope when
// .slice is used here to only pass the portion of the arguments
// array relevant to the callback (i.e. any arguments minus the
// first argument which is the callback itself.)
//
// N.B. The arguments object isn't an array but an array like object so
// .slice isn't available on it directly, hence .call was used here)
Might be worth reading up on:
The arguments object
Function.prototype.apply, Function.prototype.call and Function.prototype.bind as they are way to bind a context and arguments to a function (i.e. they'll work with the arguments object to call a function where you may not know how many arguments will be passed)
So how to implement a common function that you can passing some callbacks function with multiple possible parameter lists.
Basically, you don't. The function receiving the callback is in charge of what the callback receives as arguments. When you call Array#forEach, it's Array#forEach that decides what arguments your callback gets. Similarly, String#replace defines what it will call its callback with.
Your job is to say what test will do, what it will call its callback with. Then it's the job of the person using test to write their callback appropriately. For instance: You might document test as calling the callback with no arguments. If the caller wants to use callback_b, then it's up to them to handle the fact that callback_b expects a parameter. There are several ways they can do that:
The could wrap it in another function:
test(function() {
callback_b("appropriate value here");
});
...or use Function#bind
test(callback_b.bind(null, "appropriate value here"));
...but it's their problem, not yours.
Side note: If they pass you callback_b and you call it without any arguments, you won't get an error. JavaScript allows you to call a function with fewer arguments than it expects, or more. How the function handles that is up to the author of the function.
You can pass an anonymous function as the callback that will itself return your desired callback function with parameters.
test(function() { return callback_b(' how are you'); });
see this working snippet that will first use callback_a, then callback_b (with parameter) as the callback:
function callback_a(){
alert('a');
}
function callback_b(p){
alert('b says'+ p);
}
function test(callback){
if(true){
callback();
}
}
test(callback_a);
test(function() { return callback_b(' how are you'); });
You can pass the parameter while calling the callback
function test(callback){
if(condition){
callback();
}
else if(other condition){
callback("b");
}
}
test(callback_b);
You can write your callback function like
function callback_a_b(){
if(arguments.length){
var arg = [].slice.call(arguments);
alert('b says'+ arg[0])
}
else{
alert('a');
}
}
You can pass array of parameters as second param of test function or in ES6 use spread operator read more here
function test(callback, params){
if(condition){
if (params === undefined){
callback();
} else {
callback.apply(null, params); //params must be array
//ES6: callback(...params);
}
}
}
test(callback_a);
test(callback_b, [" whatever"]);
I've just checked in my browser (ffox 51.0.1) that the following works:
function test(callback,other_args){if(condition){callback(other_args);}}
results:
condition=true
test(callback_a)
=> shows the alert with 'a'
condition=false
test(callback_a)
=> doesn't show anything
condition=true
test(callback_b,"pepe")
=> shows the alert with 'b sayspepe'
condition=false
test(callback_b,"pepe")
=> doesn't show anything
<script>
window.something = (function(){
return function(x){
console.log(x);
};
})();
something("hello");
</script>
Just wondering, why is the parameter "hello" passed to the function inside the something function which has no parameter? Also, why doesn't the something function execute immediately? Its a self-invoking function but its strange that I have to call it first before it can execute.
You are directly calling the function, also known as the module pattern.
So when having this code
something = (function(){
return function(myparam) {
// ...
};
})();
it is equivalent to this code:
something = function(myparam) {
// ...
};
something is the result of an immediately-invoked function expression that returns the internal function. You then call that internal function with "hello".
This is how closure works. You can a function and then again call the function with parameter for return function. Take a look at this example:-
function multiply(num1) {
return function(num2) {
return num1 * num2;
};
}
var multi = multiply(2);
console.log(multi(5));
Now , as you can see we stored the first function call in a variable called multi (which you can say is the function from the first return statement of the function multiply on line 2) and the using multi as a function we are passing it an argument for num2 .
I have a custom object that implements a function that'll be executed later. Here's how someone would call it:
customObject.onSomething(function(e) {
// do something with e
console.log('foobar');
});
Here's how onSomething is getting created:
var CustomObject = function() {
this.onSomething = function(callback) {
// If the user passes in parameter(s), how can I modify them before calling?
callback.apply(this);
}
}
How can I modify the argument(s) the user passed in before performing apply or call on the function?
apply takes a second parameter which is a list of arguments to pass to the function. call does the same, except it passes its own argument-list (everything after the first parameter which is used as this).
So, if you know which parameters you expect, you can just add them to the invoking function as the second parameter to apply (or as a list of parameters to call):
this.onSomething = function(arg1, arg2) {
// reverse the first and second arguments
callback.apply(this, [arg2, arg1]);
// equivalent:
callback.call(this, arg2, arg1);
};
If you don't know what kind of arguments to expect, but you still want to do something with them, you can do so with the builtin arguments pseudo-array which holds the arguments given to the current function (even when you don't declare them explicitly).
You can use this to invoke the callback with the same arguments given to the invoking function, or some transformation of them; e.g.:
this.onSomething = function() {
// call callback with the same arguments we got
callback.apply(this, arguments);
// or, make some changes
var newArgs = ["extra argument", arguments[1], arguments[0]];
callback.apply(this, newArgs);
};
Sounds like what you're asking for is fairly simple, see below:
var CustomObject = function() {
this.onSomething = function(callback, param1, param2) {
param1 += 4;
param2 = 'Something about ' + param2 + ' is different...';
callback.apply(this, [param1, param2]);
}
}
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Is it possible to send a variable number of arguments to a JavaScript function?
I can use arguments to get a variable number of arguments within a function, but how can I pass them to another function without knowing its prototype?
function show(foo, bar) { window.alert(foo+' '+bar); }
function run(f) { f(arguments); } // not correct, what to do?
run(show, 'foo', 'bar');
Note: I cannot guarantee the number of arguments needed for the function f that is passed to run. Meaning, even though the example shown has 2 arguments, it could be 0-infinite, so the following isn't appropriate:
function run(f) { f(arguments[1], arguments[2]); }
The main way to pass a programmatically generated set of arguments to a function is by using the function's 'apply' method.
function show(foo, bar) {
window.alert(foo+' '+bar);
}
function run(f) {
// use splice to get all the arguments after 'f'
var args = Array.prototype.splice.call(arguments, 1);
f.apply(null, args);
}
run(show, 'foo', 'bar');
You can in fact do this with apply, if I understand your question correctly:
function show(foo, bar) { window.alert(foo+' '+bar); }
function run(f, args) { f.apply(null,args); }
run(show, ['foo', 'bar']);
you need to use the apply function.. here is how u do it:
function variableFunction1()
{
alert("variableFunction1 arguments length: " + arguments.length);
// calls second varargs function keeping current 'this'.
variableFunction2.apply(this, arguments);
}
function variableFunction2()
{
alert("variableFunction2 arguments length: " + arguments.length);
}
variableFunction1('a','b','c');
Demo
In your example to pass variable arguments to show this works
function show(foo, bar) { window.alert(foo+' '+bar); }
function run(f) { f.apply(null, Array().slice.call(arguments, 1)); }
run(show, 'foo', 'bar');
I understand calling function(1) but not function(1)(2), how does it work?
also possible for function(1)(2)(3)(4) too?
In this case you are supposing that function(1) returns a function, than you are calling this new, anonymous function with an argument of 2.
See this example:
function sum(a) {
return function(b) {
return a+b;
}
}
// Usage:
window.alert(sum(5)(3)); // shows 8
var add2 = sum(2);
window.alert(add2(5)); // shows 7
window.alert(typeof(add2)); // shows 'function'
Here we create a function sum that takes one argument. Inside the function sum, we create an anonymous function that takes another argument. This anonymous function is returned as the result of executing sum.
Note that this anonymous function is a great example of what we call closure. A closure is a function that keeps the context in which it was created. In this case, it will keep the value of the variable a inside it, as did the example function add2. If we create many closures, they are independent as you can see:
var add3 = sum(3);
var add4 = sum(4);
window.alert(add3(3)); // shows 6
window.alert(add4(3)); // shows 7
Furthermore, they won't get "confused" if you have similarly named local variables:
var a = "Hello, world";
function multiply(a) {
return function(b) {
return a * b;
}
}
window.alert(multiply(6)(7)); // shows 42
var twoTimes = multiply(2);
window.alert(typeof(twoTimes));
window.alert(twoTimes(5));
So, after a call to sum(2) or multiply(2) the result is not a number, nor a string, but is a function. This is a characteristic of functional languages -- languages in which functions can be passed as parameters and returned as results of other functions.
You have a function that returns a function:
function f(n) {
return function(x) {
return n + x;
};
}
When you call f(1) you get a reference to a function back. You can either store the reference in a variable and call it:
var fx = f(1);
var result = fx(2);
Or you can call it directly:
var result = f(1)(2);
To get a function that returns a function that returns a function that returns a function, you just have to repeat the process:
function f(n) {
return function(x) {
return function(y) {
return function(z) {
return n + x + y + z;
}
}
};
}
If your function returns a function, you can call that too.
x = f(1)(2)
is equivalent to:
f2 = f(1)
x = f2(2)
The parenthesis indicate invocation of a function (you "call" it). If you have
<anything>()
It means that the value of anything is a callable value. Imagine the following function:
function add(n1) {
return function add_second(n2) {
return n1+n2
}
}
You can then invoke it as add(1)(2) which would equal 3. You can naturally extend this as much as you want.