do let and const variables get hoisted before we get to the block or when we get to the block?

Question

My question is when i declare a variable of type let or const inside a block like that

//does JS setup a memory space for let a when we are here ? { //or when we are inside the block JS setup the memory space for let a ? let a = 5; }

will it get hoisted only when we get to the block(at execution time) or will it get hoisted at the beginning ?

because when i call it outside of the block the error is different than the one inside of the block

console.log(a); ---> "Uncaught ReferenceError: a is not defined" { console.log(a); -----> "Uncaught ReferenceError: Cannot access 'a' before initialization" let a = 5; }

the error outside of the block is the same like a variable that truly doesn't exist

same error

console.log(a); ---> "Uncaught ReferenceError: a is not defined" console.log(thisVariableDoesntExists); ---> "Uncaught ReferenceError: thisVariableDoesntExists is not defined" { let a = 5; }

so again my question is will Javascript setup a memory space for the variable let a = 5 when we are inside of the block or when the execution context start the creation phase ?

because from what it seems to me javascript will only setup a memory space for let only when we enter the block

DISCLAIMER: if you didn't clearly understood my question DO NOT think that my question was about why when i console.log(a) i don't get the "undefined" like the var but instead i get an error. that is not my question !

Thanks in advance for all the helpers :)

From W3Schools: Variables defined with let and const are hoisted to the top of the block, but not initialized. Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared. — Batman
– Batman, Commented Mar 29, 2021 at 0:31
in programing language this is called scoping of variable for find more information read this link :developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — H.T
– H.T, Commented Mar 29, 2021 at 0:37
let and const are block-scoped. "let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. [...]" — ASDFGerte
– ASDFGerte, Commented Mar 29, 2021 at 0:37
That being said, when the engine allocates memory is not specified. That is too low level. It may, just to give an example, determine parts of the space probably needed, already when parsing the code, and have it ready, when executing. What's specified is, how it behaves towards the outside, and what you can access, not how it realizes that internally. — ASDFGerte
– ASDFGerte, Commented Mar 29, 2021 at 0:37
@ASDFGerte - that's the answer. The question, as written, is unaswerable because it's engine-dependent and not specified by ECMAScript. — Adam Jenkins
– Adam Jenkins, Commented Mar 29, 2021 at 1:57

machineghost · Accepted Answer · 2021-03-29 01:38:59Z

so again my question is will Javascript setup a memory space for the variable let a = 5 when we are inside of the block or when the execution context start the creation phase ?

I honestly don't know the answer to this question, but I wonder ... does it really matter? For any major web application, and likely for most Javascript applications period, it's highly unlikely that you'll even need to think about the the consequences of exactly how much memory your variables use.

Javascript is a garbage collected language, and modern computers are powerful, so the real answer is: make as many variables as you want, in or outside of closures. Make variables even if they're just for code readability, because in a practical sense they have no cost.

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