en.javascript.info/1-js/06-advanced-functions/04-var/article.md

The old "var"

In the very first chapter about variables, we mentioned three ways of variable declaration:

1. let
2. const
3. var

let and const behave exactly the same way in terms of Lexical Environments.

But 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, but then there's a chance that it bites you later.

From the first sight, var behaves similar to let. That is, declares a variable:

function sayHi() {
  var phrase = "Hello"; // local variable, "var" instead of "let"

  alert(phrase); // Hello
}

sayHi();

alert(phrase); // Error, phrase is not defined

...But here are the differences.

"var" has no block scope

Variables, declared with var, are either function-wide or global. 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); // Error: test is not defined
*/!*

The same thing for loops: var cannot be block- or loop-local:

for (var i = 0; i < 10; i++) {
  // ...
}

*!*
alert(i); // 10, "i" 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); // Error: phrase is not defined (Check the Developer Console)

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" declarations are processed at the function start

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 better to demonstrate with an example, like this:

function sayHi() {
  alert(phrase);  

*!*
  var phrase = "Hello";
*/!*
}

sayHi();

The line var phrase = "Hello" has two actions in it:

1. Variable declaration var
2. 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.

Summary

There are two main differences of var compared to let/const:

1. var variables have no block scope, they are visible minimum at the function level.
2. var declarations are processed at function start (script start for globals).

There's one more minor difference related to the global object, we'll cover that 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.