In the past I have used
var App = App || {};
to assign or instantiate a common App object in various js scripts loaded separately into the browser.
However, using let
and const
instead of var
throws a reference error:
const App = App || {}; // or let App = App || {};
Uncaught ReferenceError: App is not defined
at <anonymous>:1:11
What's going on here? If I want to continue using this pattern, do I have to stick to var
?
This is because when you are declaring a variable using let
or a constant, the variable is in the temporal dead zone before being initialized.
In other words, trying let foo = foo
will throw an error because foo
is still in the temporal dead zone, and cannot be used. The same goes for const
.
Note also that a variable defined whith let
or a constant defined with const
cannot share its name space in the same scope with another variable, be it with let
, const
, or var
.
When javascript engine sees the statement, var App = App || {}
var App = App || {}
, it breaks the statement down as follows:
var App;
App = App || {};
Due to variable hoisting, var App = App || {}
var App = App || {}
is broken down into two parts.
var App
App = App || {}
App = App || {}
The let
and const
declarations on the other hand, don't involve variable hoisting. Hence, the ReferenceError
, as no variable called App exists yet.
var
is behaving differently than let
or const
.
In case of var
, JS engine first creates a variables definitions, allocate memory space for them and initialize them with an undefined
value. Only then it start executing the code line by line.
In case of let
or const
, it initialize it to undefined
only when the declaration actually happens (and only if there is no immediately assignment).
In your example, the variable App
is still in the Temporal Dead Zone and trying to access it at that point results in a Reference Error.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.