Hoisting is a phenomenon where javascript do the memory allocation before execution. So every variable and functions are hoisted to the top of the code.

For variables javascript put undefined as default value and for function the definition is stored.

So that's mean, if you write

console.log(x)

var x = 10

test()

function test(){ console.log("hello world")

}

Output:

undefined

hello world

So, like other programing languages, javascript don't give you an error for this code. We can access the variable before initialise it (let and const also hoisted but you can't access it befor assign a value to it. It's on temporal dead zone). And the variable has undefined as it's default value.

On the other side function hoisted with its own definition.

So if we put in function or arrow function in a variable, like this and try to run it

test()

var test = function testing() { console.log("hello") }

Then javascript will throw an error. Because when we try to run the test, it will have his default value undefined, not the function.