let's dive deeper into the differences between let
and var
in the context of the code snippet you provided and explore the underlying concepts of JavaScript.
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Variable Declaration and Scope:
-
var
: Variables declared withvar
are function-scoped or globally scoped, but they are not block-scoped. This means that a variable declared withvar
is available throughout the entire function or globally if declared outside of any function. -
let
: Variables declared withlet
are block-scoped. This means that they are only accessible within the block (curly braces) in which they are declared. Block scope can be a loop, an if statement, a function, or any other block of code.
-
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Hoisting:
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var
: Variables declared withvar
are hoisted to the top of their containing function or global scope. However, only the declaration is hoisted, not the initialization. This means you can access avar
variable before it's declared, but it will have an initial value ofundefined
. -
let
: Variables declared withlet
are also hoisted, but they are not initialized. If you try to access alet
variable before it's declared, you'll get a ReferenceError. This behavior helps catch potential issues early.
-
-
Closures:
-
var
: Variables declared withvar
in a loop often lead to unintended closure-related issues, as we saw in your initial code snippet. This is because they capture the variable by reference and not by value, causing all closures to share the same reference. -
let
: Variables declared withlet
in a loop create a new variable with each iteration, effectively capturing the value of the variable at that iteration. This behavior is whylet
is often preferred in situations where you want to avoid closure-related problems.
-
Global declaration example:
var color = 'red'
let age = 30
//note here color variable is attached to the window object so
console.log(window.color) // red
console.log(windiw.age)// undefined
So here you can see when defined as global variables the var attaches that variable to the window object, now this is unwanted in our code because the window contains many properties and if any of variable with the same name created it will changes it.
Block declaration example
const test = ()=>{
for (let i = 0; i < 3; i++) // here defined with let
console.log(i)
console.log(i) // reference error i is not defined
}
test()
here as we can see the let has the block scope so the value of i is not accessible to the outside of the block of the for loop
However if we change the declaration of let with the var variable we can see that this value will be accessible outside of the loop as in the following example
const test = ()=>{
for (var i = 0; i < 3; i++) // here defined with var
console.log(i)
console.log(i) // 1 2 3
}
test()
here the value of i runs till 2 while the loop condition was true, then i updates again to 3 and the condition is then false, so it exits the block of the for loop and then prints the value outside of the block which is inside the function i.e, 4
Here is another clear example
const test = ()=>{
for (var i = 0; i < 3; i++) // here defined with let
console.log('inside for loop' + i)
console.log('inside test function'+i) // reference error i is not defined
if(i === 3){
console.log('I reached in if block')
}
}
test()
//inside for loop0
//inside for loop1
//inside for loop2
//inside test function3
//I reached in if block
Variable declaration, nuances of var declaration
So, when we declare the variable with var, it is hoisted to the global scope, note only the declaration is hoisted and not the variable initializing
for example consider this cheeky code
console.log(x); // undefined
var x = 5;
so here can be seen that the variable x is accessed before it is declared, let us see how this code is interpreted behind the scenes
var x; // Declaration is hoisted
console.log(x); // undefined
x = 5; // Initialization happens here
These are the simple examples on what are the basic differences between let and var. In order to deep dive into JavaScript it is still a good practice to learn the how the language behaves on practical use-case.
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