The importance of var in JavaScript

September 4, 2008 code 2 min read

I would consider myself proficient at JavaScript. Not a rockstar, but I can hold my own. But I didn’t learn it out of a book; I picked it up slowly over several years. So occasionally I come across something that I probably would have learned in the first couple chapters of a decent JavaScript book.

Today, I experienced the importance of declaring variables with var.

For the unenlightened like myself, JavaScript basically has two scopes for variables: global and local. Variables assigned outside of a function are global, and variables assigned inside of a function, using the var keyword, are local (not rocket surgery). However, if you leave the var keyword off, it assigns a global variable, regardless of where it’s declared.

Here’s a bit of code that demonstrates this:

function fail() {
  date = new Date();
  console.log('Before: ' + Number(date));
  setTimeout(function() {
    console.log('After: ' + Number(date));
  }, 1000);
}

fail();
fail();

The code defines a function that sets a variable, logs the variable, then defines a callback that logs the variable again on second later. If you call the function twice in a row, you can see the effect:

Before: 1220487263486
Before: 1220487263499
After: 1220487263499 <- This should be 1220487263486
After: 1220487263499

Declaring the date variable with var gives us this output:

Before: 1220487287985
Before: 1220487287994
After: 1220487287985 <- see, this is the same as the first line
After: 1220487287994

So remember kids, declare local variables with var!

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— @bkeepers