JavaScript Hoisting

Definições de variáveis e funções, são ‘hoisted’, ao topo da função onde foram definidos.

Considere o seguinte código JavaScript:

fnExpression();          // => throw TypError!
var fnExpression = function() {
    return "I'm a expression assigned to a variable";
}
fnExpression();         // => "I'm a expression assigned to a variable"

fnStatement();          // => "I'm a statement with name"
fuction fnStatement() {
    return "I'm a statement with name";
}
fnStatement();          // => "I'm a statement with name"

var scope = 'global';
function fn() {
    return scope;          
    var scope = 'local';
}
fn();                   // => undefined

O interpretador JavaScript transforma nisso:

var fnExpression, scope;
function fnStatement() {
    return "I'm a statement with name";
}
function fn() {
    var scope;
    return scope;
    scope = 'local';
}

fnExpression();
fnExpression = function() {
    return "I'm a expression assigned to a variable";
}
fnExpression();

fnStatement();
fnStatement();

scope = 'global';
fn();