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If we execute this Javascript, what will the browser's console show?

var text = 'outside'; function logIt(){ console.log(text); var text = 'inside'; }; logIt();

It's not "outside".

It's not "inside".

The script won't throw an error!

The console will log undefined.

To understand this, we need to explain a few things about Javascript.

Function-level scope. Functions create new scopes in Javascript:

function setVar(){ // inside this function we have a new scope // so this variable, declared in this function's scope, won't be available outside the function var varInFunction = 'inside a function'; } setVar(); console.log(varInFunction); // throws 'ReferenceError: varInFunction is not defined'

Blocks like if statements and for loops do not create a new scope (this is also true of Python and recent versions of Ruby, but untrue of Java and C):

if (true) { // this if statement doesn't create a new scope // so varInIf is available in the global scope var varInIf = 'inside an if statement'; } console.log(varInIf); // logs 'inside an if statement'

Declaration vs. assignment. A variable declaration simply tells the interpreter that a variable exists. By default it initializes the variable to undefined:

var unicorn; console.log(unicorn); // logs undefined (NOT a ReferenceError)

A variable assignment assigns a value to the variable:

unicorn = 'Sparkles McGiggleton';

We can both declare and assign in the same line:

var centaur = 'Horsey McPersonhead';

Hoisting. In Javascript, variable declarations are "hoisted" to the top of the current scope. Variable assignments, however, are not.

So returning to the original problem:

var text = 'outside'; function logIt(){ console.log(text); var text = 'inside'; }; logIt();

The declaration (but not the assignment) of text gets hoisted to the top of logIt. So our code gets interpreted as though it were:

var text = 'outside'; function logIt(){ var text; console.log(text); text = 'inside'; }; logIt();

So we have a new variable text inside of logIt that is initialized to undefined, which is what it holds when we hit our log statement.

Remember: when you declare a variable in JavaScript (using "var"), that variable declaration is "hoisted" to the top of the current scope—meaning the top of the current function or the top of the script if the variable isn't in a function.

Hoisting can cause unexpected behavior, so a good way to keep things clear is to always declare your variables at the top of the scope.

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