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.
Basic JavaScript - Find the Length of a String
How does String.length work in JavaScript? - Stack Overflow
javascript - Create a string of variable length, filled with a repeated character - Stack Overflow
Javascript, string length function.
Videos
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.
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.
A string is immutable in JavaScript.
a += "somestring" doesn't change the length of a string but makes a new string.
This means there is no "new length", but the length is just part of the definition of the string (more precisely it is stored in the same structure in implementations).
Regarding
for(i=0;i<a.length;i++){ // did you forget the 'var' keyword ?
a not so uncommon practice (if you don't change a) was to optimize it as
for (var i=0, l=a.length; i<l; i++)
in order to avoid the reading of the length but if you compare the performances with modern engines, you'll see this doesn't make the code any faster now.
What you must remember : querying the length of a string is fast because there is no computation. What's a little less fast is building strings (for example with concatenation).
Strings are a primitive type. At least that's what the documentation says. But we can access the length of the string as if we are accessing the property of an object(with the dot notation). Which indicates it's an object, Right?
Turns out, whenever we make a call from the string primitive to some property using the dot notation (for example, say length), the Js engine will take this primitive string and wrap it into an equivalent wrapper object, which is a String object. And then, the .length on that String object returns the length.
Interesting thing to note here is, that when we do something like this, our string still stays the same primitive string during all of this. And a temporary object is created to make our string operation work. Once the required property is fetched, this temporary object is deleted from the memory.
Hope this gives some high level understanding.
The best way to do this (that I've seen) is
var str = new Array(len + 1).join( character );
That creates an array with the given length, and then joins it with the given string to repeat. The .join() function honors the array length regardless of whether the elements have values assigned, and undefined values are rendered as empty strings.
You have to add 1 to the desired length because the separator string goes between the array elements.
Give this a try :P
s = '#'.repeat(10)
document.body.innerHTML = s