Cleop
commented
5 years ago https://www.w3schools.com/js/js_hoisting.asp
Hoisting is JavaScript's default behaviour of moving var declarations to the top of the current scope when your code is run. What this means is that whatever line your write a var declaration on e.g. var x;, when your code is run it will automatically be lifted and read as if it were written on the top lines of your current scope (to the top of the current script or the current function).
This means that you can use var's higher up in the code before the line you've declared them on. So this example:
x = "hello";
console.log(x) // logs 'hello'
var x; // declaring `x` which is hoisted as if it were written at the top on compilationIs read by the computer like this:
var x; // declaring `x` which has been hoisted to the top line on compilation
x = "hello";
console.log(x) // logs 'hello'Can all declarations be hoisted?
- No, only
vardeclarations notletorconstdeclarations.
Can initialisations be hoisted?
- No.
This example shows how it's only the declaration of a variable which is hoisted, not the initialisation / value of the variable. So when
xis logged it isundefinedand doesn't have the value of'hello'yet.var x; // this line is declaring `x` which happens to already be at the top console.log(x) // logs 'undefined' x = "hello";
Other words to define:
- scope
- declarations in js are:
var,const,let - declare a variable: defining a variable (giving it a name) but not a value e.g.
var x;<- this consists of saying what declaration you are usingvar/const/letand then giving that variable a name, in this case you are calling your variablex. Once you have done this the variable x has been declared. Declarations are different to initialisations e.g.var x = 1;, see separate definition for intialisations. - initialisation: defining a variable and providing it with an initial value: e.g.
var x = 1;.
nelsonic
Thanks for adding this one @Brymon 👍