How to check string length with JavaScript

As for the question which event you should use for this: use the input event, and fall back to keyup/keydown in older browsers.

Here’s an example, DOM0-style:

someElement.oninput = function() {
  this.onkeydown = null;
  // Your code goes here
};
someElement.onkeydown = function() {
  // Your code goes here
};

The other question is how to count the number of characters in the string. Depending on your definition of “character”, all answers posted so far are incorrect. The string.length answer is only reliable when you’re certain that only BMP Unicode symbols will be entered. For example, 'a'.length == 1, as you’d expect.

However, for supplementary (non-BMP) symbols, things are a bit different. For example, '𝌆'.length == 2, even though there’s only one Unicode symbol there. This is because JavaScript exposes UCS-2 code units as “characters”.

Luckily, it’s still possible to count the number of Unicode symbols in a JavaScript string through some hackery. You could use Punycode.js’s utility functions to convert between UCS-2 strings and Unicode code points for this:

// `String.length` replacement that only counts full Unicode characters
punycode.ucs2.decode('a').length; // 1
punycode.ucs2.decode('𝌆').length; // 1 (note that `'𝌆'.length == 2`!)

P.S. I just noticed the counter script that Stack Overflow uses gets this wrong. Try entering 𝌆, and you’ll see that it (incorrectly) counts as two characters.

You should bind a function to keyup event

textarea.keyup = function(){
   textarea.value.length....
} 

with jquery

$('textarea').keyup(function(){
   var length = $(this).val().length;
});

UPDATE: Since I wrote this, the input event has gotten a decent level of support. It is still not 100% in IE9, so you will have to wait a bit until IE9 is fully phased out. In light of my answer to this question, however, input is more than a decent replacement for the method I've presented, so I recommend switching.

Use keyup event

var inp = document.getElementById('myinput');
var chars = document.getElementById('chars');
inp.onkeyup = function() {
  chars.innerHTML = inp.value.length;
}

<input id="myinput"><span id="chars">0</span>

EDIT:

Just a note for those that suggest keydown. That won't work. The keydown fires before character is added to the input box or textarea, so the length of the value would be wrong (one step behind). Therefore, the only solution that works is keyup, which fires after the character is added.