I have a concern with setTimeout function in javascript. when we call setTimeout function without return anything, it is okay for me. like
setTimeout(function() {
console.log("ok function called")
},2000);
here in the above example it just simply call that function after 2000ms,
And if I write this like
setTimeout(function(params) {
console.log("passed value is"+params)
},2000,55);
now it will call this function with 55 as an argument, right?
But problem is that when I call to write this like
setTimeout(function(params) {
console.log("passed value is"+params)
}(55),2000);
here function is calling with 55 as params but it is now waiting for 2000ms
And when I wrote like
setTimeout(function(params) {
console.log("passed value is "+params);
return function(){
console.log(params)
};
}(55),2000);
in this only return function is calling with 2000ms delay, the line console.log("passed value is "+params); is executing instantly
please help me get out of this problem.
One is a function. Another is a function call.
First, let's forget javascript for now. If you know any other programming language, what do you expect the two pieces of code below to do?
function a () { return 1 }
x = a;
y = a();
What do you expect x
to be? 1
or a pointer to function a
?
What do you expect y
to be? 1
or a pointer to function a
?
A function is not a function call. When you call a function it returns a value.
Now let's switch back to javascript. Whenever I get confused by a piece of code, I try to make the syntax simpler so that I can understand what's going on:
setTimeout(function() {console.log("ok function called")}, 2000);
Now, that's a compact piece of code, let's make the syntax simpler. The above code is the same as:
var a = function() {console.log("ok function called")};
setTimeout(a, 2000);
So what does that do? It will call the function a
after 2 seconds.
Now let's take a look at:
setTimeout(function() {console.log("ok function called")}(), 2000);
// Note this ----------^^
That's the same as:
var b = function() {console.log("ok function called")}();
setTimeout(b, 2000);
which can further be simplified to:
var a = function() {console.log("ok function called")};
var b = a();
setTimeout(b, 2000);
So I hope you see what you're really passing to setTimeout. You're passing the return value of the function, not the function.
When you write
setTimeout(function (params) { return something; }(55), 2000);
what actually happens is something like this:
var _temp_func = function (params) { return something; };
var _temp = _temp_func(55);
setTimeout(_temp, 2000);
The anonymous function you have as a parameter to setTimeout
is evaluated immediately, even before the call to setTimeout
itself. In contrast to that, the actual parameter that ends up in _temp
here is called with a delay. This is what happens in your last example.
setTimeout takes only function name without parenthesis.
correct syntax : setTimeout(Helloworld) - here you are setting function
incorrect syntax : setTimeout(HelloWorld()) - here you are calling function
or non IIFE function. It's an IIFE that you are passing hence it is getting called immediately. setTimeout(function (params) { return something; }(55), 2000);
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