The old "var"

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

1. let
2. const
3. 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