The information in this article is useful for understanding old scripts.
That’s not how we write new code.
In the very first chapter about variables, we mentioned three ways of variable declaration:
let
const
var
The var
declaration is similar to let
. Most of the time we can replace let
by var
or vice-versa and expect things to work:
var message = "Hi";
alert(message); // Hi
But internally var
is a very different beast, that originates from very old times. It’s generally not used in modern scripts, but still lurks in the old ones.
If you don’t plan on meeting such scripts you may even skip this chapter or postpone it.
On the other hand, it’s important to understand differences when migrating old scripts from var
to let
, to avoid odd errors.
“var” has no block scope
Variables, declared with var
, are either function-scoped or global-scoped. They are visible through blocks.
For instance:
if (true) {
var test = true; // use "var" instead of "let"
}
alert(test); // true, the variable lives after if
As var
ignores code blocks, we’ve got a global variable test
.
If we used let test
instead of var test
, then the variable would only be visible inside if
:
if (true) {
let test = true; // use "let"
}
alert(test); // ReferenceError: test is not defined
The same thing for loops: var
cannot be block- or loop-local:
for (var i = 0; i < 10; i++) {
var one = 1;
// ...
}
alert(i); // 10, "i" is visible after loop, it's a global variable
alert(one); // 1, "one" is visible after loop, it's a global variable
If a code block is inside a function, then var
becomes a function-level variable:
function sayHi() {
if (true) {
var phrase = "Hello";
}
alert(phrase); // works
}
sayHi();
alert(phrase); // ReferenceError: phrase is not defined
As we can see, var
pierces through if
, for
or other code blocks. That’s because a long time ago in JavaScript, blocks had no Lexical Environments, and var
is a remnant of that.
“var” tolerates redeclarations
If we declare the same variable with let
twice in the same scope, that’s an error:
let user;
let user; // SyntaxError: 'user' has already been declared
With var
, we can redeclare a variable any number of times. If we use var
with an already-declared variable, it’s just ignored:
var user = "Pete";
var user = "John"; // this "var" does nothing (already declared)
// ...it doesn't trigger an error
alert(user); // John
“var” variables can be declared below their use
var
declarations are processed when the function starts (or script starts for globals).
In other words, var
variables are defined from the beginning of the function, no matter where the definition is (assuming that the definition is not in the nested function).
So this code:
function sayHi() {
phrase = "Hello";
alert(phrase);
var phrase;
}
sayHi();
…Is technically the same as this (moved var phrase
above):
function sayHi() {
var phrase;
phrase = "Hello";
alert(phrase);
}
sayHi();
…Or even as this (remember, code blocks are ignored):
function sayHi() {
phrase = "Hello"; // (*)
if (false) {
var phrase;
}
alert(phrase);
}
sayHi();
People also call such behavior “hoisting” (raising), because all var
are “hoisted” (raised) to the top of the function.
So in the example above, if (false)
branch never executes, but that doesn’t matter. The var
inside it is processed in the beginning of the function, so at the moment of (*)
the variable exists.
Declarations are hoisted, but assignments are not.
That’s best demonstrated with an example:
function sayHi() {
alert(phrase);
var phrase = "Hello";
}
sayHi();
The line var phrase = "Hello"
has two actions in it:
- Variable declaration
var
- Variable assignment
=
.
The declaration is processed at the start of function execution (“hoisted”), but the assignment always works at the place where it appears. So the code works essentially like this:
function sayHi() {
var phrase; // declaration works at the start...
alert(phrase); // undefined
phrase = "Hello"; // ...assignment - when the execution reaches it.
}
sayHi();
Because all var
declarations are processed at the function start, we can reference them at any place. But variables are undefined until the assignments.
In both examples above, alert
runs without an error, because the variable phrase
exists. But its value is not yet assigned, so it shows undefined
.
IIFE
In the past, as there was only var
, and it has no block-level visibility, programmers invented a way to emulate it. What they did was called “immediately-invoked function expressions” (abbreviated as IIFE).
That’s not something we should use nowadays, but you can find them in old scripts.
An IIFE looks like this:
(function() {
var message = "Hello";
alert(message); // Hello
})();
Here, a Function Expression is created and immediately called. So the code executes right away and has its own private variables.
The Function Expression is wrapped with parenthesis (function {...})
, because when JavaScript engine encounters "function"
in the main code, it understands it as the start of a Function Declaration. But a Function Declaration must have a name, so this kind of code will give an error:
// Tries to declare and immediately call a function
function() { // <-- SyntaxError: Function statements require a function name
var message = "Hello";
alert(message); // Hello
}();
Even if we say: “okay, let’s add a name”, that won’t work, as JavaScript does not allow Function Declarations to be called immediately:
// syntax error because of parentheses below
function go() {
}(); // <-- can't call Function Declaration immediately
So, the parentheses around the function is a trick to show JavaScript that the function is created in the context of another expression, and hence it’s a Function Expression: it needs no name and can be called immediately.
There exist other ways besides parentheses to tell JavaScript that we mean a Function Expression:
// Ways to create IIFE
(function() {
alert("Parentheses around the function");
})();
(function() {
alert("Parentheses around the whole thing");
}());
!function() {
alert("Bitwise NOT operator starts the expression");
}();
+function() {
alert("Unary plus starts the expression");
}();
In all the above cases we declare a Function Expression and run it immediately. Let’s note again: nowadays there’s no reason to write such code.
Summary
There are two main differences of var
compared to let/const
:
var
variables have no block scope, their visibility is scoped to current function, or global, if declared outside function.var
declarations are processed at function start (script start for globals).
There’s one more very minor difference related to the global object, that we’ll cover in the next chapter.
These differences make var
worse than let
most of the time. Block-level variables is such a great thing. That’s why let
was introduced in the standard long ago, and is now a major way (along with const
) to declare a variable.
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