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Cover image for What is Hoisting😰😰 in JavaScript

What is Hoisting😰😰 in JavaScript

saigowthamr profile image Sai gowtham Updated on ・1 min read

Hoisting is not something functions and variables are moving to the top.
Actually, they are not moving technically anywhere.

The thing happens at the time of function declaration the complete function
is sitting in the memory.

For variable declaration, JavaScript engine initializes with the value "undefined" at the time of creation.i used debugger to stop the running code at line 6.

For example
hoisting

Have you Observed one thing in above image JavaScript is already initialized with value 'undefined'?

Whenever JavaScript engine runs the line 6 then it updates the undefined to 'hoisting'.

For functions, the full function is added to the memory space.

That is the reason we can invoke the functions anywhere in the file but not variables.for variables, we only used once it is declared first.

Have you checked hoisting for the let and var keywords?

for 'let' keyword if you try to access the variable before the declaration javascript engine hits the error.

hoisting

for 'var' it doesn't show any error.
hoisting

Hope you guys love these.

Check out My Book On JavaScript

Discussion

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josegonz321 profile image
Jose Gonzalez

Thanks for the write up Sai.

May I just share something that can help a bit more with this topic:

We can be tempted to look at var a = 2; as one statement, but the JavaScript Engine does not see it that way. It sees var a and a = 2 as two separate statements, the first one a compiler-phase task, and the second one an execution-phase task.

What this leads to is that all declarations in a scope, regardless of where they appear, are processed first before the code itself is executed. You can visualize this as declarations (variables and functions) being "moved" to the top of their respective scopes, which we call "hoisting".

Declarations themselves are hoisted, but assignments, even assignments of function expressions, are not hoisted.

From You Don't Know JS

strongly recommend the whole series

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saigowthamr profile image
Sai gowtham Author

Yeah sure I have also seen from other books(JavaScript for ninjas). Check out JavaScript weird parts good explanation about hoisting.

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nektro profile image
Meghan (she/her)

It moves the declaration for var up but not the value set

var value;
console.log(value);
console.log('hi hoisting');
value = 'hoisting';
console.log(value);
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josegonz321 profile image
Jose Gonzalez

Yup!

Variable declaration and assignment are done separately.

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rhymes profile image
rhymes

Funny, you and @genta both posted about JS hoisting recently :D

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rhymes profile image
rhymes

just a figure of speech :-) I mean it's interesting that you both were "struggling" with this behavior of JS at the same time and wrote about it.

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genta profile image
Fabio Russo

That’s something that all Javascript developer should know... so more points of view... are better 💛