A lot of shiny new features came with ES2015 (ES6) and since it's 2017, it's assumed that a lot of JavaScript developers have become familiar with and have started using these features. While this assumption might be true, it's still possible that some of these features remain a mystery to some.
One of the features that came with ES6 is addition of let
and const
which can be used for variable declaration. The question now is, what makes them different from our good ol' var
which has been in use? If you are still not clear about this, this article is for you.
In this article, var
, let
and const
will be discussed with respect to their scope, use and hoisting. As you read, take note of the differences between them I'll point out.
VAR
Before the advent of ES6, var
declarations ruled as King. There are issues associated with variables declared with var
though. That is why it was necessary for new ways to declare variables to emerge. First though, let us get to understand var
more before we discuss one of such issues.
Scope of var
Scope essentially means where these variables are available for use. var
declarations are globally scoped or function/locally scoped. It is globally scoped when a var
variable is declared outside a function. This means that any variable that is declared with var
outside a function block is available for use in the whole window. var
is function scoped when it is declared within a function. This means that it is available and can be accessed only within that function.
To understand further, look at the example below.
var greeter = "hey hi";
function newFunction() {
var hello = "hello";
}
Here, greeter
is globally scoped because it exists outside a function while hello
is function scoped. So we cannot access the variable hello
outside of a function. So if we do this:
var tester = "hey hi";
function newFunction() {
var hello = "hello";
}
console.log(hello); // error: hello is not defined
We'll get an error which is as a result of hello
not being available outside the function.
var
variables can be re-declared and updated
That means that we can do this within the same scope and won't get an error.
var greeter = "hey hi";
var greeter = "say Hello instead";
and this also
var greeter = "hey hi";
greeter = "say Hello instead";
Hoisting of var
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. What this means is that if we do this:
console.log (greeter);
var greeter = "say hello"
it is interpreted as this
var greeter;
console.log(greeter); //greeter is undefined
greeter = "say hello"
So var
variables are hoisted to the top of its scope and initialized with a value of undefined.
Problem with var
There's a weakness that comes with var
. I'll use the example below to explain this.
var greeter = "hey hi";
var times = 4;
if (times > 3) {
var greeter = "say Hello instead";
}
console.log(greeter) //"say Hello instead"
So, since times > 3
returns true, greeter
is redefined to "say Hello instead"
. While this is not a problem if you knowingly want greeter
to be redefined, it becomes a problem when you do not realize that a variable greeter
has already been defined before.
If you have use greeter
in other parts of your code, you might be surprised at the output you might get. This might cause a lot of bugs in your code. This is why the let
and const
is necessary.
LET
let
is preferred for variable declaration now. It's no surprise as it comes as an improvement to the var
declarations. It also solves this problem that was raised in the last subheading. Let's consider why this is so.
let
is block scoped
A block is chunk of code bounded by {}. A block lives in curly braces. Anything within curly braces is a block. So a variable declared in a block with the let
is only available for use within that block. Let me explain this with an example.
let greeting = "say Hi";
let times = 4;
if (times > 3) {
let hello = "say Hello instead";
console.log(hello);//"say Hello instead"
}
console.log(hello) // hello is not defined
We see that using hello
outside its block(the curly braces where it was defined) returns an error. This is because let
variables are block scoped .
let
can be updated but not re-declared.
Just like var
, a variable declared with let
can be updated within its scope. Unlikevar
, a let
variable cannot be re-declared within its scope. So while this will work,
let greeting = "say Hi";
greeting = "say Hello instead";
this will return an error.
let greeting = "say Hi";
let greeting = "say Hello instead";//error: Identifier 'greeting' has already been declared
However, if the same variable is defined in different scopes, there will be no error.
let greeting = "say Hi";
if (true) {
let greeting = "say Hello instead";
console.log(greeting);//"say Hello instead"
}
console.log(greeting);//"say Hi"
Why is there no error? This is because both instances are treated as different variables since they have different scopes.
This fact makes let
a better choice than var
. When using let
, you don't have to bother if you have used a name for a variable before as a variable exists only within its scope. Also, since a variable cannot be declared more than once within a scope, then the problem discussed earlier that occurs with var
does not occur.
Hoisting of let
Just like var
, let
declarations are hoisted to the top. Unlike var
which is initialized as undefined
, the let
keyword is not initialized. So if you try to use a let
variable before declaration, you'll get a Reference Error
.
CONST
Variables declared with the const
maintain constant values. const
declarations share some similarities with let
declarations.
const
declarations are block scoped
Like let
declarations, const
declarations can only be accessed within the block it was declared.
const
cannot be updated or re-declared
This means that the value of a variable declared with const
remains the same within its scope. It cannot be updated or re-declared. So if we declare a variable with const
, we can neither do this
const greeting = "say Hi";
greeting = "say Hello instead";//error : Assignment to constant variable.
nor this
const greeting = "say Hi";
const greeting = "say Hello instead";//error : Identifier 'greeting' has already been declared
Every const declaration therefore, must be initialized at the time of declaration.
This behavior is somehow different when it comes to objects declared with const
. While a const
object cannot be updated, the properties of this objects can be updated. Therefore, if we declare a const
object as this
const greeting = {
message : "say Hi",
times : 4
}
while we cannot do this
const greeting = {
words : "Hello",
number : "five"
}//error : Assignment to constant variable.
we can do this
greeting.message = "say Hello instead";
This will update the value of greeting.message
without returning errors.
Hoisting of const
Just like let
, const
declarations are hoisted to the top but are not initialized.
So just in case, you missed the differences, here they are :
var
declarations are globally scoped or function scoped whilelet
andconst
are block scoped.var
variables can be updated and re-declared within its scope;let
variables can be updated but not re-declared;const
variables can neither be updated nor re-declared.They are all hoisted to the top of their scope but while
var
variables are initialized withundefined
,let
andconst
variables are not initialized.While
var
andlet
can be declared without being initialized,const
must be initialized during declaration.
Got any question or addition? please leave a comment.
Thank you for reading :)
Top comments (99)
this is a good article..however you should update your last example
const flow = {
message : "say Hi",
times : 4
}
to
const greeting = {
message : "say Hi",
times : 4
}
don't know how anybody who's reading this article missed that!
I noticed something did not add up but I didn't pay attention to the details since I got what I was looking for. Good catch, tho.
left me in blanks... I am glad you had pointed it out! Thanks'
and btw, the article is indeed one of the best!
thank you
It's updated now. Thanks for notifying
Yeah
For a noob like me, this explained "let" declarations for me.
I'm glad it did.
Good post. I wasn't sure about these. Just knew that 'var' should be avoided.
I think you have typo here. Remove the second 'let' from the first code block. :)
So while this will work,
let greeting = "say Hi";
let greeting = "say Hello instead";
this will return an error.
let greeting = "say Hi";
let greeting = "say Hello instead";//error: Identifier 'greeting' has already been declared
Thanks a lot for pointing that out. I'll correct it right away.
Thank for a great article
Error still exists in the article..
Why should var be avoided?
'Var' allows for re-declarations in code which does not throw errors and can create unintended side-effects in your code.
'Let' allows for variable reassignment but not for duplicate declarations (block-scoped), much like strongly typed languages like C# and Java.
'Const' allows for declaration once and for assignment once, and can never be re-declared or reassigned (block-scoped). For instance, I use 'const' for inline function declarations, so that I don't accidentally redefine this function's behavior at some later point in time by mistake. Even though I am unlikely to do so, it is just safer for myself and others to work on.
To paraphrase Sarah more generically:
"While it is not a problem if you knowingly want a var to be a certain value at a certain point in code, it becomes a problem when you do not realize that this same var has already been declared/defined before or, even worse, has had other functions operate on this var without your knowledge."
So, in other words, it's a much better idea to use 'let' and 'const' in order have safer control over your variables and constants.
This was incredibly helpful, and a great TLDR!
"Variable shadowing"
const alone does not guarantee protection for your data or let me say it does not protect your data from mutation,
Example:
Though the above code makes use of const i can still update the values via the code below
But this could easily be avoided by making use of
Object.freeze()
to freeze our const variables.Just add this line of code and you will discover that the values cannot be updated.
For more clarification check: freecodecamp.org/learn/javascript-...
Hi, great article but I think you have a mistake about the hoisting,you wrote:
"They are all hoisted to the top of their scope but while var varvariables are initialized with undefined, let and const variables are not initialized."
If you will look at the JavaScript Hoisting section in w3schools.com website,
you will notice that:"Variables and constants declared with let or const are not hoisted!"
Please check in:
w3schools.com/js/js_hoisting.asp
According to MDN and ECMA 2015 her words are right
check this developer.mozilla.org/en-US/docs/W...
That's exactly I m thinking the new let and const in javascript are not hoisted..she wrote wrong things..u should correct this article..do not mislead people with this ??
Years later, this is still the most concise explanation and welcomed even by those of us who are not lightweights - but don't stray into the JS badlands much. You have great teacher chromosomes, Sarah. Ignore the nit-pickers - this is still the best explanation primarily because of its brevity.
Thanks for the kind words, I really appreciate it.
The article is well written, but the problem is with the visualization of the concepts, if only these were depicted in pictorial or tabular format, can have immense effect on readers. Thanks for the article thought.
That's a great article. Inspired me to write:
Var is function-scoped and not block-scoped which means that constructs like if statements, while loops etc do not create new scopes while using var. It also means that, as var is function-scoped, if we do not create a function, we do not create new scope while using var.
Read More here:
Var vs Let in JavaScript
Hi, you have said that variable declared as var will have global scope if declared outside a function. But the same can even be said for variables declared as let and const. They indeed are locally scoped but when done outside a function, they will have global scope like var.
Please correct me if am wrong?
Thank you for your post, I am a newbie to Nodejs and quite confusing on when to use
let
,const
orvar
. As I saw some require a module withvar
while some withconst
, could you advise which is better when requiring a module? After reading your post, I thinkconst
is better to require another module because it will never change.One more question could you please do me a favor to explain the memory usage between these three, is there any different other than
scope
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