What is the difference between below 2 code snippets ?
i could not understand any differences between them.
Both are using closures(as per my knowledge and correct me if i am wrong)
and prints the same output that is 0,1,2,3,4.
for(var i = 0; i < 5; i++) {
(function(){
var tmp = i;
setTimeout(function(){
console.log(tmp);
}, 0)
})();
}
and
for(var i = 0; i < 5; i++) {
setTimeout((function(tmp){
return function() {
console.log(tmp);
}
})(i), 0);
}
If you forget about the identifier resolution done by
var tmp = i;
and rewrite the first snippet to
for(var i = 0; i < 5; i++) {
(function(tmp){
setTimeout(function(){
console.log(tmp);
}, 0)
})(i);
}
then the IIFE here creates a scope and sets delayed execution, while the IIFE in the second snippet creates a scope and returns a function with that scope.
Related
I understand the behavior of using var and let in for loop in typescript/javascript but can someone explain why and how a const variable as a loop variable behaves ?
for (const i = 0; i < 5; i++) {
setTimeout(function() {
console.log(i)
}, 100 * i);
}
From what i understand , when you declare a variable as const and initialize its value , the value cannot be changed
Yet you can see the value being changed in the console.log() .An error has to be thrown while compilation right ?What am i missing here ?
I have created 2 examples for this behavior .
Loop variable as a const
Const variable re assignment
Can someone help me understand this ?
It works in Stackblitz because it is running traspiled code:
AppComponent.prototype.test = function () {
var _loop_1 = function (i) {
setTimeout(function () {
console.log(i);
}, 100 * i);
};
for (var i = 0; i < 5; i++) {
_loop_1(i);
}
};
It won't work if you add a snippet here because it is not transpiled
for (const i = 0; i < 5; i++) {
setTimeout(function() {
console.log(i)
}, 100 * i);
}
Answering your question,
test(){
for(const i =0 ; i< 5; i++){
setTimeout(function(){
console.log(i)
},100*i);
}
}
This code essentially becomes,
test(){
// can be only initialized once
const i;
for(i = 0 ; i< 5; i++){
setTimeout(function(){
console.log(i)
},100*i);
}
}
Because every JavaScript variable is hoisted at the top of its scope, in this case the test() as its const variable that's why its hoisted in that block and not accessible outside of it.
To correct the piece of the code:
test(){
// can be only multiple times in that block
for(let i = 0 ; i< 5; i++){
setTimeout(function(){
console.log(i)
},100*i);
}
}
Which becomes,
test(){
let i;
// can be only multiple times in that block
for(i = 0 ; i< 5; i++){
setTimeout(function(){
console.log(i)
},100*i);
}
}
As both const and let have block scope and is hoisted at the top of the block its defined in, the only difference between const and let is variables declared const cannot be reinitialized.
I wrote a simple javascript code. My for loop iterates a "let" declared variable, i between 0 and 2. A function gets declared within the loop only when i == 2. The function has to return the value of i variable. When I call this function from outside the loop, the function returns the value of i = 2 (which is natural for a block scope variable i. However, when I rewrite the loop code as its non-loop equivalent code-block, the function (still called from outside the block) returns the vale of i = 3. What is going on?
"use strict";
var printNumTwo;
for (let i = 0; i < 3; i++) {
if (i === 2) {
printNumTwo = function() {
return i;
};
}
}
console.log(printNumTwo()); //returns 2
// loop equivalent
{
let i = 0;
i = 1;
i = 2;
printNumTwo = function() {
return i;
}
i = 3;
}
console.log(printNumTwo()); // returns 3
Your example is bad because your loop is not counting after 2. So If your loop looks like i <= 3:
for (let i = 0; i <= 3; i++) {
if (i === 2) {
printNumTwo = function() {
return i;
};
}
}
You would get exactly same result as your non-loop example and that's because of closure in javascript but return breaks for loop. Your function is saving reference to that variable from outside scope.
It's because you're actually setting the function to return the value 3 because of the non-loop environment. You should change the loop a little, adding another variable, but first make your function look like this:
printNumTwo = function() {
return num;
}
And in your simulated loop:
i = 2;
num = i;
printNumTwo = function() {
return num;
}
i = 3;
In your non loop based code, printNumTwo is not executed at the same point of its declaration and so the value of i is updated before it is executed so the value 3 is returned.
{
let i = 0;
i = 1;
i = 2;
printNumTwo = function () {
return i;
}
i = 3;
}
console.log(printNumTwo());
but if you run the following code, it should print 2 since it is executed before value if i is set to 3
{
let i = 0;
i = 1;
i = 2;
printNumTwo = (function() {
console.log(i);
})()
i = 3;
}
Note: return in for loop breaks the further execution of the loop, so even if your first code had i <= 3 as its breaking condition, it will return 2.
for (let i = 0; i <= 3; i++) {
if (i === 2) {
printNumTwo = function() {
return i;
};
}
}
console.log(printNumTwo())
"use strict";
var printNumTwo;
for (let i = 0; i < 3; i++) {
printNumTwo = function (i) {
// when references 'i' in this function, 'i' goes to the global scope.
return i;
};
// set the value 3 for 'i' in the global scope
i = 3;
}
console.log(printNumTwo()); // return 3;
try this
"use strict";
var printNumTwo;
for (let i = 0; i < 3; i++) {
printNumTwo = function (i) {
return i;
}.bind(null, i); // you set the current value as parameter = 0
i = 3; // i = 3 and break loop
}
console.log(printNumTwo()); // return 0;
try this
"use strict";
var printNumTwo;
for (let i = 0; i < 3; i++) {
let i = 0;
i = 1;
i = 2;
printNumTwo = function (i) {
return i;
}.bind(null, i); // you set the current value as parameter = 2
i = 3; // i = 3 and break loop
}
console.log(printNumTwo()); // return 2;
I appreciate all the answers I got to my question. All pointing to the case of how a function, when called, handles the environments in which it was both called and created. I read this useful explanation in the book "Eloquent JavaScript" and think it would be good to share it,
"A good mental model is to think of function values as containing both the code in their body and the environment in which they are created. When called, the function body sees the environment in which it was created, not the environment in which it is called."
~ Eloquent_JavaScript/Closure
I'm trying to add functions into an array. These have to be named 'opdracht1' through to 'opdracht10'.
Though, I cannot figure out how to give it a name.
var opdrachtArray = [];
for (i = 0; i < 10; i++) {
opdrachtArray.push(function() {func(i); });
}
It adds the functions but as I said earlier I cannot find out how to add a name.
Also, am I later just able to define the functions and call them when I need them?
Name your functions by placing them on the window object:
for (i = 0; i < 10; i++) {
f = function() { func(i); }); // but see below
window['opdracht' + i] = f
opdrachtArray.push(f);
}
However you have a more basic problem. All your functions close over i and therefore func is always going to be called with the value of i after the loop finishes, in other words, 10. One solution is:
function make_func(i) {
return function() {
return func(i);
};
}
then
for (i = 0; i < 10; i++) {
f = make_func(i);
window['opdracht' + i] = f;
opdrachtArray.push(f);
}
Or
for (i = 0; i < 10; i++) {
(function(i) {
var f = function() { func(i); };
window['opdracht' + i] = f
opdrachtArray.push(f);
}(i));
}
or just use func.bind(null, i), which does approximately the same thing.
for (i = 0; i < 10; i++) {
f = func.bind(null, i);
window['opdracht' + i] = f;
opdrachtArray.push(f);
}
If you want each function to be assigned to a name, use dictionary, not array:
var opdrachtDict = {};
for (i = 0; i < 10; i++) {
opdrachtDict["opdracht"+i]=func.bind(null,i);
}
function func(i){
alert(i);
}
opdrachtDict["opdracht3"](); //ok, lets call it and see if you think its still a wrong answer ;)...
You could store the name and function in an object and push each object to the array:
var opdrachtArray = [];
for (i = 0; i < 10; i++) {
var name = 'opdracht' + (i+1);
opdrachtArray.push({name : name, callback : function() {func(i); }});
}
Well a function defined in the global scope just ends up being a property of self/window anyways. i.e.:
function mr_function(){return 5;}
self["mr_function"];
window["mr_function"];
Both (property of self / window) reference the function we defined. I guess you could name your function that way if you're careful. Or assign them to some other object's property if you'd rather not make them global.
I´m having trouble to delay a function in a for-loop.
So here is my test:
<script type="text/javascript">
var j = 0;
for(var i = 0; i <= 100; i++){
setTimeout(function(){
console.log(i);
},j*2);
j++;
}
</script>
I want that every count will appear step by step with a delay in the console. But currently only 101 appears 101 times. Why and what is a better solution?
Cheers
JavaScript doesn't have block level scoping of variables, so the value of i in any deferred functions will be the last value of i from the loop (in your case, 101). You can either use a named function or an immediately invoked function expression to create a closure that gives the value the correct scope.
<script type="text/javascript">
var j = 0;
for(var i = 0; i <= 100; i++){
(function(i) {
setTimeout(function(){
console.log(i);
},j*2);})(i);
j++;
}
</script>
Also bear in mind that the second argument passed to setTimeout is the delay in milliseconds, so that's a very short delay (a tiny fraction of a second) between each call.
function SetTimeoutLoop(i) {
setTimeout(function() { console.log(i); }, i*2);
}
for(var i = 0; i <= 100; i++){
SetTimeoutLoop(i)
}
JSFIDDLE
u may try this:
<script type="text/javascript">
var j = 0;
for(var i = 0; i <= 100; i++){
(function(i, j){
setTimeout(function(){
console.log(i);
},j*2);
})(i, j);
j++;
}
</script>
It happens as once your callback starts to fire your loop would be finished and the value will be max.
you can do like
(function me(i,j) {
if(i <= 10) {
setTimeout(function(){
alert(i++);
me(i,j);
},j++*2);
}
})(0,0);
Here is a sample fiddle.
You can't do that in for loop, instead you should use a recursive function like this:
here's a fiddle
var j = 0;
var i=0;
setTimeout(function(){
log_i(i);
},j*2);
function log_i(i){
console.log(i);
i++;
if(i<=100){
setTimeout(function(){ log_i(i); },j*2);
}
j++;
}
I'm trying to make a few functions to work one after the other with a waiting time of 1.5 seconds between them.
NOW, when i try doing so with the same Id (Inside the "NoteList(>here<)", like 1, 2, 3, or any other, it works;
for (var i = 0; i < 36; i++)
{
setTimeout(function () { OnClcRandom(NoteList[0]) }, i * 1000 + 1000);
}
BUT! when i try doing so with the var i, it doesn't work and gets the all of the functions in the page stuck. any idea why?
for (var i = 0; i < 36; i++)
{
setTimeout(function () { OnClcRandom(NoteList[i]) }, i * 1000 + 1000);
}
That would be because all of the functions refer to the same live i variable, not the value of the variable at the time you called setTimeout(). Which means by the time the timeouts actually run your function i will be 36.
Try this instead:
for (var i = 0; i < 36; i++) {
(function(x){
setTimeout(function () { OnClcRandom(NoteList[x]) }, i * 1000 + 1000);
)(i);
}
This executes an anonymous function on each iteration of the loop, with each execution getting its own x parameter for use in your original function.
Javascript doesn't create local scope for block. :)
And in your second example var i equal 36 (last value).
You need create local scope inside loop.
for (var i = 0; i < 36; i++) {
(function (i) {
setTimeout(.......);
}(i))
}
You also may fixed 'i' value assign it to function property:
for (var i = 0, f; i < 36; i++){
f = function _callback() { var i = _callback.i; .....};
f.i = i;
setTimeout(f, i * 1000);
}