JavaScript’s function scope means that all variables declared within a function are visi-
ble throughout the body of the function. Curiously, this means that variables are even
visible before they are declared. This feature of JavaScript is informally known as hoist-
ing: JavaScript code behaves as if all variable declarations in a function (but not any
associated assignments) are “hoisted” to the top of the function. Consider the following
code:
var scope = "global"; function f() { console.log(scope); // Prints "undefined", not "global" var scope = "local"; // Variable initialized here, but defined everywhere console.log(scope); // Prints "local" }
You might think that the first line of the function would print “global”, because the
var statement declaring the local variable has not yet been executed. Because of the
rules of function scope, however, this is not what happens. The local variable is defined
throughout the body of the function, which means the global variable by the same name
is hidden throughout the function. Although the local variable is defined throughout,
it is not actually initialized until the var statement is executed. Thus, the function above
is equivalent to the following, in which the variable declaration is “hoisted” to the top
and the variable initialization is left where it is:
function f() { var scope; //Local variable is declared at the top of the function console.log(scope);//It exists here, but still has "undefined" value scope = "local";//Now we initialize it and give it a value console.log(scope);//And here it has the value we expect }