I cannot understand why this code is behaving as it is:
for (var i = 1; i < 3; i++) {
var j;
if (!j) {
j = 1;
} else {
alert("why are we here? j shouldn't be defined but it's " + j);
}
}
(jsFiddle)
If I set j to null and check for null, it works the way I think it should.
This isn't how Java, C#, C++, etc. work, hence, the confusion.
This is because variables in JavaScript are scoped to functions (or the global scope if not within a function). A var is not scoped inside of a loop, and curly braces do not defined a new closure. Basically, the value of j persists after the loop body, and the var does not re-define j as undefined. This is why explicitly setting var j = null; has the expected effect.
Example 1:
A good way to think about this is, any time you declare a variable with var, like this.
function someFunc() {
for(var i = 0; i < 3; i++){
var j;
}
}
The interpreter hoist the variable declarations like so.
function someFunc() {
var i;
var j;
for(i = 0; i < 3; i++){
}
}
Notice that since the var j declaration is hoisted to the top of the function, the declaration actually does nothing within the loop.
Example 2:
However, if you were to initialize the variable with null like this.
function someFunc() {
for(var i = 0; i < 3; i++){
var j = null;
}
}
It would be interpreted like this.
function someFunc() {
var i;
var j;
for(i = 0; i < 3; i++){
j = null;
}
}
Notice how for every loop, j is set to null.
ES6 let keyword:
There is a keyword in ES6 which will create a scope in a loop like this, it is the let keyword. Keep in mind that browser support for the let keyword is poor at this point.
for (var i = 1; i < 3; i++) {
let j;
if (!j) {
j = 1;
} else {
alert("why are we here? j shouldn't be defined but it's "+ j);
}
}
The first time your for loop executes the body of the loop, j is undefined and thus your code sets j=1. On the subsequent iterations of the loop, j is already defined and set to 1 so it goes into your else clause as would be expected.
This occurs because variables defined with var in Javascript are function scoped, not block scoped and if not inside a function, then they are global. So, there is only one variable j in your jsFiddle and each iteration of the for loop uses the same variable (thus inheriting the value from the previous iteration).
It will work if you initialize j = null; inside the body of the for loop because then you're reinitializing it for each iteration rather than using the value from the previous iteration.
ES6 proposes to add the let declaration which would scope to the nearest block. See What's the difference between using "let" and "var" to declare a variable? for more info on let.
There isn't a block scope in JavaScript, only global and function scopes. Although, JavaScript variable statements are so flexible that they will give the programmer the illusion that a block scope could exist, but the truth is that variables declared in JavaScript functions are later hoisted by the interpreter. This means that their declarations are moved to the top of the most-recently declared function. Take this for example:
function test() {
console.log('a test');
for (var i = 0; i < 100; i++) {
var k = i + Math.random();
console.log(k)
}
}
The JavaScript interpreter, internally, will "transform" the code to the following:
function test() {
var i, k; // var declarations are now here!
console.log('a test');
for (i = 0; i < 100; i++) {
k = i + Math.random();
console.log(k)
}
}
It will move all the var declarations to the begining of the most recent function declaration. In your code, the hoisting will create the following code:
// A anonymous function created by jsFiddle to run your script
function () {
var i, j;
for (i = 1; i < 3; i++) {
if (!j) {
j = 1;
} else {
alert("Why are we here? j shouldn't be defined, but it's " + j);
}
}
}
The variable is undefined in the first time, then it gets assigned 1 and then your message is printed.
Related
Quoting from the book JavaScript: The Definitive Guide
Unlike variables declared with let, it is legal to declare the same variable multiple times with var. And because var variables have function scope instead of block scope, it is actually common to do this kind of redeclaration. The variable i is frequently used for integer values, and especially as the index variable of for loops. In a function with multiple for loops, it is typical for each one to begin for(var i = 0; .... Because var does not scope these variables to the loop body, each of these loops is (harmlessly) re-declaring and re-initializing the same variable.
I am unable to understand what is meant in this paragraph. First I assumed the following would not have worked with second loop not iterating, because i would be function scoped:
(function foo() {
for (let i = 0; i < 2; i++) {
console.log(i);
}
for (let i = 0; i < 2; i++) {
console.log(i);
}
})();
but it prints
0
1
0
1
Then I assumed this would have printed 0 0 1 1 0 1, which is also not the case:
(function foo() {
for (var i = 0; i < 2; i++) {
console.log(i);
for (var i = 0; i < 2; i++) {
console.log(i);
}
}
})();
Can someone help me understand what is meant by
in a function of multiple for loops, var can be used harmlessly in each loop
and how it is different to let?
To me it looks like the opposite it true (which is also very confusing) where let can be used harmlessly in functions with multiple loops:
(function foo() {
for (let i = 0; i < 2; i++) {
console.log(i);
for (let i = 0; i < 2; i++) {
console.log(i);
}
}
})();
prints:
0
0
1
1
0
1
let variables are forbidden to be re-declared in the same scope, like the following:
let foo = 10;
let foo = 20;
var variables can be, though:
var foo = 10;
var foo = 20;
console.log('ok');
In your first snippet, variables declared with let in the header of a for loop only exist inside the loop; they're block scoped, and aren't scoped to the outer block or function:
(function foo() {
for (let i = 0; i < 2; i++) {
// console.log(i);
}
for (let i = 0; i < 2; i++) {
// console.log(i);
}
// doesn't exist out here:
console.log(i);
})();
The let is above create is in two different scopes, which is why it's not forbidden. Similarly:
(function foo() {
{
let i = 10;
}
{
let i = 20;
}
console.log('OK up until here');
// doesn't exist out here:
console.log(i);
})();
vars, in contrast, have function scope, not block scope - with your var version of the loop, since there's only one function, this:
(function foo() {
for (var i = 0; i < 2; i++) {
console.log(i);
for (var i = 0; i < 2; i++) {
console.log(i);
}
}
})();
is equivalent to
(function foo() {
var i; // the `i` variable exists at this level
for (i = 0; i < 2; i++) {
console.log(i);
for (i = 0; i < 2; i++) {
console.log(i);
}
}
})();
Then I assumed this would have printed 0 0 1 1 0 1, which is also not the case:
i gets initialized to zero at the beginning of each loop.
The first loop begins once, at the start of the function.
The second loop begins just after logging i, on each iteration of the outer loop.
(function foo() {
for (var i = 0; i < 2; i++) {
console.log(i);
for (var i = 0; i < 2; i++) {
console.log(i);
}
}
})();
outer loop initialization: 0 is assigned to i
0 gets logged
inner loop initialization: 0 is assigned to i
0 gets logged
inner loop increments i to 1 and starts again
inner loop: 1 gets logged
inner loop increments i to 2 and breaks, since i < 2 is no longer fulfilled
outer loop increments i to 3 and breaks, since i < 2 is no longer fulfilled
So you get 0 0 1 logged.
What the article probably means by
in a function of multiple for loops, var can be used harmlessly in each loop
is that it can work if both loops are on the same level of the function, eg
for (...) {
}
for (...) {
}
It definitely won't work with nested loops, if both loops use the same variable, because there's only ever a single i variable, rather than one for each loop.
where let can be used harmlessly in functions with multiple loops:
For nested loops, yes, since variables declared with let are unique to each loop, rather than being shared across the whole containing function.
I understand that let prevents duplicate declarations which is nice.
let x;
let x; // error!
Variables declared with let can also be used in closures which can be expected
let i = 100;
setTimeout(function () { console.log(i) }, i); // '100' after 100 ms
What I have a bit of difficulty grasping is how let applies to loops. This seems to be specific to for loops. Consider the classic problem:
// prints '10' 10 times
for (var i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }
// prints '0' through '9'
for (let i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }
Why does using let in this context work? In my imagination even though only one block is visible, for actually creates a separate block for each iteration and the let declaration is done inside of that block ... but there is only one let declaration to initialize the value. Is this just syntactic sugar for ES6? How is this working?
I understand the differences between var and let and have illustrated them above. I'm particularly interested in understanding why the different declarations result in different output using a for loop.
Is this just syntactic sugar for ES6?
No, it's more than syntactic sugar. The gory details are buried in §13.6.3.9
CreatePerIterationEnvironment.
How is this working?
If you use that let keyword in the for statement, it will check what names it does bind and then
create a new lexical environment with those names for a) the initialiser expression b) each iteration (previosly to evaluating the increment expression)
copy the values from all variables with those names from one to the next environment
Your loop statement for (var i = 0; i < 10; i++) process.nextTick(_ => console.log(i)); desugars to a simple
// omitting braces when they don't introduce a block
var i;
i = 0;
if (i < 10)
process.nextTick(_ => console.log(i))
i++;
if (i < 10)
process.nextTick(_ => console.log(i))
i++;
…
while for (let i = 0; i < 10; i++) process.nextTick(_ => console.log(i)); does "desugar" to the much more complicated
// using braces to explicitly denote block scopes,
// using indentation for control flow
{ let i;
i = 0;
__status = {i};
}
{ let {i} = __status;
if (i < 10)
process.nextTick(_ => console.log(i))
__status = {i};
} { let {i} = __status;
i++;
if (i < 10)
process.nextTick(_ => console.log(i))
__status = {i};
} { let {i} = __status;
i++;
…
I found this explanation from Exploring ES6 book the best:
var-declaring a variable in the head of a for loop creates a single
binding (storage space) for that variable:
const arr = [];
for (var i=0; i < 3; i++) {
arr.push(() => i);
}
arr.map(x => x()); // [3,3,3]
Every i in the bodies of the three arrow functions refers to the same
binding, which is why they all return the same value.
If you let-declare a variable, a new binding is created for each loop
iteration:
const arr = [];
for (let i=0; i < 3; i++) {
arr.push(() => i);
}
arr.map(x => x()); // [0,1,2]
This time, each i refers to the binding of one specific iteration and
preserves the value that was current at that time. Therefore, each
arrow function returns a different value.
let introduces block scoping and equivalent binding, much like functions create a scope with closure. I believe the relevant section of the spec is 13.2.1, where the note mentions that let declarations are part of a LexicalBinding and both live within a Lexical Environment. Section 13.2.2 states that var declarations are attached to a VariableEnvironment, rather than a LexicalBinding.
The MDN explanation supports this as well, stating that:
It works by binding zero or more variables in the lexical scope of a single block of code
suggesting that the variables are bound to the block, which varies each iteration requiring a new LexicalBinding (I believe, not 100% on that point), rather than the surrounding Lexical Environment or VariableEnvironment which would be constant for the duration of the call.
In short, when using let, the closure is at the loop body and the variable is different each time, so it must be captured again. When using var, the variable is at the surrounding function, so there is no requirement to reclose and the same reference is passed to each iteration.
Adapting your example to run in the browser:
// prints '10' 10 times
for (var i = 0; i < 10; i++) {
setTimeout(_ => console.log('var', i), 0);
}
// prints '0' through '9'
for (let i = 0; i < 10; i++) {
setTimeout(_ => console.log('let', i), 0);
}
certainly shows the latter printing each value. If you look at how Babel transpiles this, it produces:
for (var i = 0; i < 10; i++) {
setTimeout(function(_) {
return console.log(i);
}, 0);
}
var _loop = function(_i) {
setTimeout(function(_) {
return console.log(_i);
}, 0);
};
// prints '0' through '9'
for (var _i = 0; _i < 10; _i++) {
_loop(_i);
}
Assuming that Babel is fairly conformant, that matches up with my interpretation of the spec.
Recently I got confused about this problem too. According to the above answers, here is my understanding:
for (let i=0;i<n;i++)
{
//loop code
}
is equivalent to
// initial
{
let i=0
}
// loop
{
// Sugar: For-Let help you to redefine i for binding it into current block scope
let i=__i_value_from_last_loop__
if (i<=n){
//loop code
}
i++
}
Let us see “let” and “var” with the setTimeout majorly asked in the interview.
(function timer() {
for (var i=0; i<=2; i++)
{ setTimeout(function clog() {console.log(i)}, i*1000); }
})();
(function timer() {
for (let i=0; i<=2; i++)
{ setTimeout(function clog() {console.log(i)}, i*1000); }
})();
Let's see in detail how this code executes in the javascript compiler.
The answer for “var” is “222” due to functional scope and for “let” is “012” because it is block scope.
Now let us look at what it looks like in detail when it Compiles for "var". (It's a little hard to explain over the code than in audio or video but I am trying my best to give you.)
var i = 0;
if(i <=2){
setTimeout(() => console.log(i));
}
i++; // here the value of "i" will be 1
if(i <=2){
setTimeout(() => console.log(i));
}
i++; // here the value of "i" will be 2
if(i <=2){
setTimeout(() => console.log(i));
}
i++; // here the value of "i" will be 3
After the code is executed finally it will print all the console.log where the value of "i" is 6. So the final output is: 222
In "let i" will be declared in every scope.
The import point to be noted here is "i" will get the value from the previous scope and not from the declaration. (Below code is just an example of how it looks like in the compiler and trying it wont work)
{
//Scope 1
{
let i;
i= 0;
if(i<=2) {
setTimeout(function clog() {console.log(i)};);
}
i++; // Here "i" will be increated to 1
}
//Scope 2
// Second Interation run
{
let i;
i=0;
// Even “i” is declared here i= 0 but it will take the value from the previous scope
// Here "i" take the value from the previous scope as 1
if(i<=2) {
setTimeout(function clog() {console.log(i)}; );
}
i++; // Here “i” will be increased to 2
}
//Scope 3
// Second Interation run
{
let i;
i=0;
// Here "i" take the value from the previous scope as 2
if(i<=2) {
setTimeout(function clog() {console.log(i)}; );
}
i++; // Here "i" will be increated to 3
}
}
So, it will print "012" value as per the block scope.
This question already has answers here:
What is the difference between "let" and "var"?
(39 answers)
Closed 5 years ago.
Although i understand that let allows you to declare variables that are limited in scope to the block, I encountered a strange difference between let and var while using it with javascript closures. Here is my example:
With let:
function buildFunction() {
var arr = [];
for(var i = 0; i < 3; i++) {
let j = i; //Using let to assign j
arr.push(
function(){
console.log(j);
}
)
}
return arr;
}
var fs = buildFunction();
fs[0]();
fs[1]();
fs[2]();
The above code snippet outputs:
0
1
2
With var:
function buildFunction() {
var arr = [];
for(var i = 0; i < 3; i++) {
var j = i; //Using var to assign j
arr.push(
function(){
console.log(j);
}
)
}
return arr;
}
var fs = buildFunction();
fs[0]();
fs[1]();
fs[2]();
The above code snippet outputs the following:
2
2
2
My question is:
If U am using var inside a block and assigning it a value during execution, shouldn't it work exactly like let and store different copies of j in memory ?
Does javascript handle let and var differently within a closure ?
Any clarifications on this would be highly appreciated.
var scopes to the function; let scopes to a block of code.
In your example j is scoped to the function buildFunction() when you use var. This means your using the 'same' j in every function. When your for loop runs j is set to 0, then 1, then 2. When you then run the console logs your referencing the j that was set to 2, so you get 2 2 2.
When you use let, you scope j to that iteration of the for loop. This means every iteration has a 'different' j, and the console logs print what you expect them to print.
If you were using ES5 and needed to use var, you could replicate the let effect (have it print 0 1 2), by wrapping your original anonymous function with a self-executing function and passed j in as an argument. This would create a new scope for j in the self-executing function whose value is the value of j in the current iteration.
function buildFunction() {
var arr = [];
for(var i = 0; i < 3; i++) {
var j = i; //Using var to assign j
arr.push(
//self executing function
(function(j) { //j here is scoped to the self executing function and has the value of j when it was called in the loop. Any changes to j here will not affect the j scope outside this function, and any changes to j outside this function will not affect the j scoped inside this function.
//original function
return function() {
console.log(j);
}
})(j) //call with the value of j in this iteration
)
}
return arr;
}
var fs = buildFunction();
fs[0]();
fs[1]();
fs[2]();
Looking at http://es6-features.org/#BlockScopedVariables
What a difference between
for (let i = 0; i < a.length; i++) { let x = a[i] … }
and
for (var i = 0; i < a.length; i++) { var x = a[i] … }
I understand in example they move declaration of variables out of block
var i, x, y
for (i = 0; i < a.length; i++)
{
x = a[i] …
}
but is there any reason for it ? why not to include declaration inside the block ? Is it a bad practice or performance hit ?
Just want to understand.
Thanks.
Block scoped variables are simply not available outside their containing block. This is easy to illustrate with an example
for (let i = 0; i < 2; i++) console.info('inner loop i', i);
try {
console.info('after loop i', i);
} catch (e) {
console.error(e.message)
}
for (var j = 0; j < 2; j++) console.info('inner loop j', j);
console.info('after loop j', j);
The primary difference is scope.
Before ES6, you were stuck using var. When using var, variables were defined in the scope of the enclosing function. If there is no function, then they are defined on the global object (typically window). This made it difficult to have two variables named the same, but in different scopes without running into conflicts.
let helps to solve this problem by allowing you to scope variables to the enclosing block, like an if or for loop.
For example, one common issue was nested for loops:
for(var i = 0; i < 4; i++){ // loop 1
console.log(i); // 0
for(var i = 0; i < 4; i++){ // loop 2
}
console.log(i); // 4 - since i was changed by loop 2, loop 1 is now unstable and will skip
}
In the example above, the problem is eliminated using let instead - each i variable is scoped to it's own block.
As for any differences in performance, I can't say. If someone else knows, I'd love to know.
I understand that let prevents duplicate declarations which is nice.
let x;
let x; // error!
Variables declared with let can also be used in closures which can be expected
let i = 100;
setTimeout(function () { console.log(i) }, i); // '100' after 100 ms
What I have a bit of difficulty grasping is how let applies to loops. This seems to be specific to for loops. Consider the classic problem:
// prints '10' 10 times
for (var i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }
// prints '0' through '9'
for (let i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }
Why does using let in this context work? In my imagination even though only one block is visible, for actually creates a separate block for each iteration and the let declaration is done inside of that block ... but there is only one let declaration to initialize the value. Is this just syntactic sugar for ES6? How is this working?
I understand the differences between var and let and have illustrated them above. I'm particularly interested in understanding why the different declarations result in different output using a for loop.
Is this just syntactic sugar for ES6?
No, it's more than syntactic sugar. The gory details are buried in §13.6.3.9
CreatePerIterationEnvironment.
How is this working?
If you use that let keyword in the for statement, it will check what names it does bind and then
create a new lexical environment with those names for a) the initialiser expression b) each iteration (previosly to evaluating the increment expression)
copy the values from all variables with those names from one to the next environment
Your loop statement for (var i = 0; i < 10; i++) process.nextTick(_ => console.log(i)); desugars to a simple
// omitting braces when they don't introduce a block
var i;
i = 0;
if (i < 10)
process.nextTick(_ => console.log(i))
i++;
if (i < 10)
process.nextTick(_ => console.log(i))
i++;
…
while for (let i = 0; i < 10; i++) process.nextTick(_ => console.log(i)); does "desugar" to the much more complicated
// using braces to explicitly denote block scopes,
// using indentation for control flow
{ let i;
i = 0;
__status = {i};
}
{ let {i} = __status;
if (i < 10)
process.nextTick(_ => console.log(i))
__status = {i};
} { let {i} = __status;
i++;
if (i < 10)
process.nextTick(_ => console.log(i))
__status = {i};
} { let {i} = __status;
i++;
…
I found this explanation from Exploring ES6 book the best:
var-declaring a variable in the head of a for loop creates a single
binding (storage space) for that variable:
const arr = [];
for (var i=0; i < 3; i++) {
arr.push(() => i);
}
arr.map(x => x()); // [3,3,3]
Every i in the bodies of the three arrow functions refers to the same
binding, which is why they all return the same value.
If you let-declare a variable, a new binding is created for each loop
iteration:
const arr = [];
for (let i=0; i < 3; i++) {
arr.push(() => i);
}
arr.map(x => x()); // [0,1,2]
This time, each i refers to the binding of one specific iteration and
preserves the value that was current at that time. Therefore, each
arrow function returns a different value.
let introduces block scoping and equivalent binding, much like functions create a scope with closure. I believe the relevant section of the spec is 13.2.1, where the note mentions that let declarations are part of a LexicalBinding and both live within a Lexical Environment. Section 13.2.2 states that var declarations are attached to a VariableEnvironment, rather than a LexicalBinding.
The MDN explanation supports this as well, stating that:
It works by binding zero or more variables in the lexical scope of a single block of code
suggesting that the variables are bound to the block, which varies each iteration requiring a new LexicalBinding (I believe, not 100% on that point), rather than the surrounding Lexical Environment or VariableEnvironment which would be constant for the duration of the call.
In short, when using let, the closure is at the loop body and the variable is different each time, so it must be captured again. When using var, the variable is at the surrounding function, so there is no requirement to reclose and the same reference is passed to each iteration.
Adapting your example to run in the browser:
// prints '10' 10 times
for (var i = 0; i < 10; i++) {
setTimeout(_ => console.log('var', i), 0);
}
// prints '0' through '9'
for (let i = 0; i < 10; i++) {
setTimeout(_ => console.log('let', i), 0);
}
certainly shows the latter printing each value. If you look at how Babel transpiles this, it produces:
for (var i = 0; i < 10; i++) {
setTimeout(function(_) {
return console.log(i);
}, 0);
}
var _loop = function(_i) {
setTimeout(function(_) {
return console.log(_i);
}, 0);
};
// prints '0' through '9'
for (var _i = 0; _i < 10; _i++) {
_loop(_i);
}
Assuming that Babel is fairly conformant, that matches up with my interpretation of the spec.
Recently I got confused about this problem too. According to the above answers, here is my understanding:
for (let i=0;i<n;i++)
{
//loop code
}
is equivalent to
// initial
{
let i=0
}
// loop
{
// Sugar: For-Let help you to redefine i for binding it into current block scope
let i=__i_value_from_last_loop__
if (i<=n){
//loop code
}
i++
}
Let us see “let” and “var” with the setTimeout majorly asked in the interview.
(function timer() {
for (var i=0; i<=2; i++)
{ setTimeout(function clog() {console.log(i)}, i*1000); }
})();
(function timer() {
for (let i=0; i<=2; i++)
{ setTimeout(function clog() {console.log(i)}, i*1000); }
})();
Let's see in detail how this code executes in the javascript compiler.
The answer for “var” is “222” due to functional scope and for “let” is “012” because it is block scope.
Now let us look at what it looks like in detail when it Compiles for "var". (It's a little hard to explain over the code than in audio or video but I am trying my best to give you.)
var i = 0;
if(i <=2){
setTimeout(() => console.log(i));
}
i++; // here the value of "i" will be 1
if(i <=2){
setTimeout(() => console.log(i));
}
i++; // here the value of "i" will be 2
if(i <=2){
setTimeout(() => console.log(i));
}
i++; // here the value of "i" will be 3
After the code is executed finally it will print all the console.log where the value of "i" is 6. So the final output is: 222
In "let i" will be declared in every scope.
The import point to be noted here is "i" will get the value from the previous scope and not from the declaration. (Below code is just an example of how it looks like in the compiler and trying it wont work)
{
//Scope 1
{
let i;
i= 0;
if(i<=2) {
setTimeout(function clog() {console.log(i)};);
}
i++; // Here "i" will be increated to 1
}
//Scope 2
// Second Interation run
{
let i;
i=0;
// Even “i” is declared here i= 0 but it will take the value from the previous scope
// Here "i" take the value from the previous scope as 1
if(i<=2) {
setTimeout(function clog() {console.log(i)}; );
}
i++; // Here “i” will be increased to 2
}
//Scope 3
// Second Interation run
{
let i;
i=0;
// Here "i" take the value from the previous scope as 2
if(i<=2) {
setTimeout(function clog() {console.log(i)}; );
}
i++; // Here "i" will be increated to 3
}
}
So, it will print "012" value as per the block scope.