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Parse() Version Number with a groovy script?

First time posting here.
I would like to ask if there's a way to parse a version number using a groovy script.
I extract from Ariba a payload, the issue comes with a specific field called ItemNumber. At first it was working, but this month I started to retrieve a version instead of a float.
This is the part of the script that needs to be changed, but I can't find a way to do it.
if (ItemNumber?.trim()){
list.ItemNumber = Double.parseDouble(ItemNumber.toString());
}
Any help is greatly appreciated,
Thank you,
Kostas

To parse a version number, you will need to tokenize the string. You should create a Version class to hold the version information. This can help when sorting a list of versions. After tokenizing, you can call the Version constructor, and pass the tokens as integers.
Once you have a Version object, you can access fields like major, minor, and patch.
class Version {
int major
Integer minor, patch
#Override String toString() {
return [major, minor, patch].findAll().join('.')
}
}
def parseVersion(String versionString) {
if (!versionString) return null
int[] tokens = versionString.split(/\./).collect { it as int }
return new Version(
major: tokens[0],
minor: tokens.length > 1 ? tokens[1] : null,
patch: tokens.length > 2 ? tokens[2] : null,
)
}
class Payload {
String ItemNumber
}
Payload payload = new Payload(ItemNumber: "2.4")
Version version = parseVersion(payload.ItemNumber?.trim())
printf("Major version : %d%n", version.major)
printf("Minor version : %s%n", version.minor ?: "<UNSET>")
printf("Patch version : %s%n", version.patch ?: "<UNSET>")
printf("Full version : %s%n", version)
In later versions of Groovy, you can call the Versions constructor like this:
new Version(major: tokens[0], minor: tokens?[1], patch: tokens?[2])

Capitalise first letter in the one line code [duplicate]

How do I make the first character of a string uppercase if it's a letter, but not change the case of any of the other letters?
For example:
"this is a test" → "This is a test"
"the Eiffel Tower" → "The Eiffel Tower"
"/index.html" → "/index.html"

function capitalizeFirstLetter(string) {
return string.charAt(0).toUpperCase() + string.slice(1);
}
Some other answers modify String.prototype (this answer used to as well), but I would advise against this now due to maintainability (hard to find out where the function is being added to the prototype and could cause conflicts if other code uses the same name/a browser adds a native function with that same name in future).

Edited to add this DISCLAIMER: please read the comments to understand the risks of editing JS basic types.
Here's a more object-oriented approach:
Object.defineProperty(String.prototype, 'capitalize', {
value: function() {
return this.charAt(0).toUpperCase() + this.slice(1);
},
enumerable: false
});
You'd call the function, like this:
"hello, world!".capitalize();
With the expected output being:
"Hello, world!"

In CSS:
p::first-letter {
text-transform:capitalize;
}

Here is a shortened version of the popular answer that gets the first letter by treating the string as an array:
function capitalize(s)
{
return s[0].toUpperCase() + s.slice(1);
}
Update
According to the comments below this doesn't work in IE 7 or below.
Update 2:
To avoid undefined for empty strings (see #njzk2's comment below), you can check for an empty string:
function capitalize(s)
{
return s && s[0].toUpperCase() + s.slice(1);
}
ES version
const capitalize = s => s && s[0].toUpperCase() + s.slice(1)
// to always return type string event when s may be falsy other than empty-string
const capitalize = s => (s && s[0].toUpperCase() + s.slice(1)) || ""

If you're interested in the performance of a few different methods posted:
Here are the fastest methods based on this jsperf test (ordered from fastest to slowest).
As you can see, the first two methods are essentially comparable in terms of performance, whereas altering the String.prototype is by far the slowest in terms of performance.
// 10,889,187 operations/sec
function capitalizeFirstLetter(string) {
return string[0].toUpperCase() + string.slice(1);
}
// 10,875,535 operations/sec
function capitalizeFirstLetter(string) {
return string.charAt(0).toUpperCase() + string.slice(1);
}
// 4,632,536 operations/sec
function capitalizeFirstLetter(string) {
return string.replace(/^./, string[0].toUpperCase());
}
// 1,977,828 operations/sec
String.prototype.capitalizeFirstLetter = function() {
return this.charAt(0).toUpperCase() + this.slice(1);
}

I didn’t see any mention in the existing answers of issues related to astral plane code points or internationalization. “Uppercase” doesn’t mean the same thing in every language using a given script.
Initially I didn’t see any answers addressing issues related to astral plane code points. There is one, but it’s a bit buried (like this one will be, I guess!)
Overview of the hidden problem and various approaches to it
Most of the proposed functions look like this:
function capitalizeFirstLetter(str) {
return str[0].toUpperCase() + str.slice(1);
}
However, some cased characters fall outside the BMP (basic multilingual plane, code points U+0 to U+FFFF). For example take this Deseret text:
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉"); // "𐐶𐐲𐑌𐐼𐐲𐑉"
The first character here fails to capitalize because the array-indexed properties of strings don’t access “characters” or code points*. They access UTF-16 code units. This is true also when slicing — the index values point at code units.
It happens to be that UTF-16 code units are 1:1 with USV code points within two ranges, U+0 to U+D7FF and U+E000 to U+FFFF inclusive. Most cased characters fall into those two ranges, but not all of them.
From ES2015 on, dealing with this became a bit easier. String.prototype[##iterator] yields strings corresponding to code points**. So for example, we can do this:
function capitalizeFirstLetter([ first='', ...rest ]) {
return [ first.toUpperCase(), ...rest ].join('');
}
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉") // "𐐎𐐲𐑌𐐼𐐲𐑉"
For longer strings, this is probably not terribly efficient*** — we don’t really need to iterate the remainder. We could use String.prototype.codePointAt to get at that first (possible) letter, but we’d still need to determine where the slice should begin. One way to avoid iterating the remainder would be to test whether the first codepoint is outside the BMP; if it isn’t, the slice begins at 1, and if it is, the slice begins at 2.
function capitalizeFirstLetter(str) {
if (!str) return '';
const firstCP = str.codePointAt(0);
const index = firstCP > 0xFFFF ? 2 : 1;
return String.fromCodePoint(firstCP).toUpperCase() + str.slice(index);
}
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉") // "𐐎𐐲𐑌𐐼𐐲𐑉"
You could use bitwise math instead of > 0xFFFF there, but it’s probably easier to understand this way and either would achieve the same thing.
We can also make this work in ES5 and below by taking that logic a bit further if necessary. There are no intrinsic methods in ES5 for working with codepoints, so we have to manually test whether the first code unit is a surrogate****:
function capitalizeFirstLetter(str) {
if (!str) return '';
var firstCodeUnit = str[0];
if (firstCodeUnit < '\uD800' || firstCodeUnit > '\uDFFF') {
return str[0].toUpperCase() + str.slice(1);
}
return str.slice(0, 2).toUpperCase() + str.slice(2);
}
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉") // "𐐎𐐲𐑌𐐼𐐲𐑉"
Deeper into internationalization (whose capitalization?)
At the start I also mentioned internationalization considerations. Some of these are very difficult to account for because they require knowledge not only of what language is being used, but also may require specific knowledge of the words in the language. For example, the Irish digraph "mb" capitalizes as "mB" at the start of a word. Another example, the German eszett, never begins a word (afaik), but still helps illustrate the problem. The lowercase eszett (“ß”) capitalizes to “SS,” but “SS” could lowercase to either “ß” or “ss” — you require out-of-band knowledge of the German language to know which is correct!
The most famous example of these kinds of issues, probably, is Turkish. In Turkish Latin, the capital form of i is İ, while the lowercase form of I is ı — they’re two different letters. Fortunately we do have a way to account for this:
function capitalizeFirstLetter([ first='', ...rest ], locale) {
return [ first.toLocaleUpperCase(locale), ...rest ].join('');
}
capitalizeFirstLetter("italy", "en") // "Italy"
capitalizeFirstLetter("italya", "tr") // "İtalya"
In a browser, the user’s most-preferred language tag is indicated by navigator.language, a list in order of preference is found at navigator.languages, and a given DOM element’s language can be obtained (usually) with Object(element.closest('[lang]')).lang || YOUR_DEFAULT_HERE in multilanguage documents.
In agents which support Unicode property character classes in RegExp, which were introduced in ES2018, we can clean stuff up further by directly expressing what characters we’re interested in:
function capitalizeFirstLetter(str, locale=navigator.language) {
return str.replace(/^\p{CWU}/u, char => char.toLocaleUpperCase(locale));
}
This could be tweaked a bit to also handle capitalizing multiple words in a string with fairly good accuracy for at least some languages, though outlying cases will be hard to avoid completely if doing so no matter what the primary language is.
The CWU or Changes_When_Uppercased character property matches all code points which change when uppercased in the generic case where specific locale data is absent. There are other interesting case-related Unicode character properties that you may wish to play around with. It’s a cool zone to explore but we’d go on all day if we enumerated em all here. Here’s something to get your curiosity going if you’re unfamiliar, though: \p{Lower} is a larger group than \p{LowercaseLetter} (aka \p{Ll}) — conveniently illustrated by the default character set comparison in this tool provided by Unicode. (NB: not everything you can reference there is also available in ES regular expressions, but most of the stuff you’re likely to want is).
Alternatives to case-mapping in JS (Firefox & CSS love the Dutch!)
If digraphs with unique locale/language/orthography capitalization rules happen to have a single-codepoint “composed” representation in Unicode, these might be used to make one’s capitalization expectations explicit even in the absence of locale data. For example, we could prefer the composed i-j digraph, ij / U+133, associated with Dutch, to ensure a case-mapping to uppercase IJ / U+132:
capitalizeFirstLetter('ijsselmeer'); // "IJsselmeer"
On the other hand, precomposed digraphs and similar are sometimes deprecated (like that one, it seems!) and may be undesirable in interchanged text regardless due to the potential copypaste nuisance if that’s not the normal way folks type the sequence in practice. Unfortunately, in the absence of the precomposition “hint,” an explicit locale won’t help here (at least as far as I know). If we spell ijsselmeer with an ordinary i + j, capitalizeFirstLetter will produce the wrong result even if we explicitly indicate nl as the locale:
capitalizeFirstLetter('ijsselmeer', 'nl'); // "Ijsselmeer" :(
(I’m not entirely sure whether there are some such cases where the behavior comes down to ICU data availability — perhaps someone else could say.)
If the point of the transformation is to display textual content in a web browser, though, you have an entirely different option available that will likely be your best bet: leveraging features of the web platform’s other core languages, HTML and CSS. Armed with HTML’s lang=... and CSS’s text-transform:..., you’ve got a (pseudo-)declarative solution that leaves extra room for the user agent to be “smart.” A JS API needs to have predictable outcomes across all browsers (generally) and isn’t free to experiment with heuristics. The user-agent itself is obligated only to its user, though, and heuristic solutions are fair game when the output is for a human being. If we tell it “this text is Dutch, but please display it capitalized,” the particular outcome might now vary between browsers, but it’s likely going to be the best each of them could do. Let’s see:
<!DOCTYPE html>
<dl>
<dt>Untransformed
<dd>ijsselmeer
<dt>Capitalized with CSS and <code>lang=en</code>
<dd lang="en" style="text-transform: capitalize">ijsselmeer
<dt>Capitalized with CSS and <code>lang=nl</code>
<dd lang="nl" style="text-transform: capitalize">ijsselmeer
In Chromium at the time of writing, both the English and Dutch lines come out as Ijsselmeer — so it does no better than JS. But try it in current Firefox! The element that we told the browser contains Dutch will be correctly rendered as IJsselmeer there.
This solution is purpose-specific (it’s not gonna help you in Node, anyway) but it was silly of me not to draw attention to it previously given some folks might not realize they’re googling the wrong question. Thanks #paul23 for clarifying more about the nature of the IJ digraph in practice and prompting further investigation!
As of January 2021, all major engines have implemented the Unicode property character class feature, but depending on your target support range you may not be able to use it safely yet. The last browser to introduce support was Firefox (78; June 30, 2020). You can check for support of this feature with the Kangax compat table. Babel can be used to compile RegExp literals with property references to equivalent patterns without them, but be aware that the resulting code can sometimes be enormous. You probably would not want to do this unless you’re certain the tradeoff is justified for your use case.
In all likelihood, people asking this question will not be concerned with Deseret capitalization or internationalization. But it’s good to be aware of these issues because there’s a good chance you’ll encounter them eventually even if they aren’t concerns presently. They’re not “edge” cases, or rather, they’re not by-definition edge cases — there’s a whole country where most people speak Turkish, anyway, and conflating code units with codepoints is a fairly common source of bugs (especially with regard to emoji). Both strings and language are pretty complicated!
* The code units of UTF-16 / UCS2 are also Unicode code points in the sense that e.g. U+D800 is technically a code point, but that’s not what it “means” here ... sort of ... though it gets pretty fuzzy. What the surrogates definitely are not, though, is USVs (Unicode scalar values).
** Though if a surrogate code unit is “orphaned” — i.e., not part of a logical pair — you could still get surrogates here, too.
*** maybe. I haven’t tested it. Unless you have determined capitalization is a meaningful bottleneck, I probably wouldn’t sweat it — choose whatever you believe is most clear and readable.
**** such a function might wish to test both the first and second code units instead of just the first, since it’s possible that the first unit is an orphaned surrogate. For example the input "\uD800x" would capitalize the X as-is, which may or may not be expected.

For another case I need it to capitalize the first letter and lowercase the rest. The following cases made me change this function:
//es5
function capitalize(string) {
return string.charAt(0).toUpperCase() + string.slice(1).toLowerCase();
}
capitalize("alfredo") // => "Alfredo"
capitalize("Alejandro")// => "Alejandro
capitalize("ALBERTO") // => "Alberto"
capitalize("ArMaNdO") // => "Armando"
// es6 using destructuring
const capitalize = ([first,...rest]) => first.toUpperCase() + rest.join('').toLowerCase();

This is the 2018 ECMAScript 6+ Solution:
const str = 'the Eiffel Tower';
const newStr = `${str[0].toUpperCase()}${str.slice(1)}`;
console.log('Original String:', str); // the Eiffel Tower
console.log('New String:', newStr); // The Eiffel Tower

If you're already (or considering) using Lodash, the solution is easy:
_.upperFirst('fred');
// => 'Fred'
_.upperFirst('FRED');
// => 'FRED'
_.capitalize('fred') //=> 'Fred'
See their documentation: https://lodash.com/docs#capitalize
_.camelCase('Foo Bar'); //=> 'fooBar'
https://lodash.com/docs/4.15.0#camelCase
_.lowerFirst('Fred');
// => 'fred'
_.lowerFirst('FRED');
// => 'fRED'
_.snakeCase('Foo Bar');
// => 'foo_bar'
Vanilla JavaScript for first upper case:
function upperCaseFirst(str){
return str.charAt(0).toUpperCase() + str.substring(1);
}

There is a very simple way to implement it by replace. For ECMAScript 6:
'foo'.replace(/^./, str => str.toUpperCase())
Result:
'Foo'

Capitalize the first letter of all words in a string:
function ucFirstAllWords( str )
{
var pieces = str.split(" ");
for ( var i = 0; i < pieces.length; i++ )
{
var j = pieces[i].charAt(0).toUpperCase();
pieces[i] = j + pieces[i].substr(1);
}
return pieces.join(" ");
}

CSS only
If the transformation is needed only for displaying on a web page:
p::first-letter {
text-transform: uppercase;
}
Despite being called "::first-letter", it applies to the first character, i.e. in case of string %a, this selector would apply to % and as such a would not be capitalized.
In IE9+ or IE5.5+ it's supported in legacy notation with only one colon (:first-letter).
ES2015 one-liner
const capitalizeFirstChar = str => str.charAt(0).toUpperCase() + str.substring(1);
Remarks
In the benchmark I performed, there was no significant difference between string.charAt(0) and string[0]. Note however, that string[0] would be undefined for an empty string, so the function would have to be rewritten to use "string && string[0]", which is way too verbose, compared to the alternative.
string.substring(1) is faster than string.slice(1).
Benchmark between substring() and slice()
The difference is rather minuscule nowadays (run the test yourself):
21,580,613.15 ops/s ±1.6% for substring(),
21,096,394.34 ops/s ±1.8% (2.24% slower) for slice().

It's always better to handle these kinds of stuff using CSS first, in general, if you can solve something using CSS, go for that first, then try JavaScript to solve your problems, so in this case try using :first-letter in CSS and apply text-transform:capitalize;
So try creating a class for that, so you can use it globally, for example: .first-letter-uppercase and add something like below in your CSS:
.first-letter-uppercase:first-letter {
text-transform:capitalize;
}
Also the alternative option is JavaScript, so the best gonna be something like this:
function capitalizeTxt(txt) {
return txt.charAt(0).toUpperCase() + txt.slice(1); //or if you want lowercase the rest txt.slice(1).toLowerCase();
}
and call it like:
capitalizeTxt('this is a test'); // return 'This is a test'
capitalizeTxt('the Eiffel Tower'); // return 'The Eiffel Tower'
capitalizeTxt('/index.html'); // return '/index.html'
capitalizeTxt('alireza'); // return 'Alireza'
capitalizeTxt('dezfoolian'); // return 'Dezfoolian'
If you want to reuse it over and over, it's better attach it to javascript native String, so something like below:
String.prototype.capitalizeTxt = String.prototype.capitalizeTxt || function() {
return this.charAt(0).toUpperCase() + this.slice(1);
}
and call it as below:
'this is a test'.capitalizeTxt(); // return 'This is a test'
'the Eiffel Tower'.capitalizeTxt(); // return 'The Eiffel Tower'
'/index.html'.capitalizeTxt(); // return '/index.html'
'alireza'.capitalizeTxt(); // return 'Alireza'

String.prototype.capitalize = function(allWords) {
return (allWords) ? // If all words
this.split(' ').map(word => word.capitalize()).join(' ') : // Break down the phrase to words and then recursive
// calls until capitalizing all words
this.charAt(0).toUpperCase() + this.slice(1); // If allWords is undefined, capitalize only the first word,
// meaning the first character of the whole string
}
And then:
"capitalize just the first word".capitalize(); ==> "Capitalize just the first word"
"capitalize all words".capitalize(true); ==> "Capitalize All Words"
Update November 2016 (ES6), just for fun:
const capitalize = (string = '') => [...string].map( // Convert to array with each item is a char of
// string by using spread operator (...)
(char, index) => index ? char : char.toUpperCase() // Index true means not equal 0, so (!index) is
// the first character which is capitalized by
// the `toUpperCase()` method
).join('') // Return back to string
then capitalize("hello") // Hello

SHORTEST 3 solutions, 1 and 2 handle cases when s string is "", null and undefined:
s&&s[0].toUpperCase()+s.slice(1) // 32 char
s&&s.replace(/./,s[0].toUpperCase()) // 36 char - using regexp
'foo'.replace(/./,x=>x.toUpperCase()) // 31 char - direct on string, ES6
let s='foo bar';
console.log( s&&s[0].toUpperCase()+s.slice(1) );
console.log( s&&s.replace(/./,s[0].toUpperCase()) );
console.log( 'foo bar'.replace(/./,x=>x.toUpperCase()) );

We could get the first character with one of my favorite RegExp, looks like a cute smiley: /^./
String.prototype.capitalize = function () {
return this.replace(/^./, function (match) {
return match.toUpperCase();
});
};
And for all coffee-junkies:
String::capitalize = ->
#replace /^./, (match) ->
match.toUpperCase()
...and for all guys who think that there's a better way of doing this, without extending native prototypes:
var capitalize = function (input) {
return input.replace(/^./, function (match) {
return match.toUpperCase();
});
};

Here is a function called ucfirst()(short for "upper case first letter"):
function ucfirst(str) {
var firstLetter = str.substr(0, 1);
return firstLetter.toUpperCase() + str.substr(1);
}
You can capitalise a string by calling ucfirst("some string") -- for example,
ucfirst("this is a test") --> "This is a test"
It works by splitting the string into two pieces. On the first line it pulls out firstLetter and then on the second line it capitalises firstLetter by calling firstLetter.toUpperCase() and joins it with the rest of the string, which is found by calling str.substr(1).
You might think this would fail for an empty string, and indeed in a language like C you would have to cater for this. However in JavaScript, when you take a substring of an empty string, you just get an empty string back.

Use:
var str = "ruby java";
console.log(str.charAt(0).toUpperCase() + str.substring(1));
It will output "Ruby java" to the console.

If you use Underscore.js or Lodash, the underscore.string library provides string extensions, including capitalize:
_.capitalize(string) Converts first letter of the string to
uppercase.
Example:
_.capitalize("foo bar") == "Foo bar"

If you're ok with capitalizing the first letter of every word, and your usecase is in HTML, you can use the following CSS:
<style type="text/css">
p.capitalize {text-transform:capitalize;}
</style>
<p class="capitalize">This is some text.</p>
This is from CSS text-transform Property (at W3Schools).

var capitalized = yourstring[0].toUpperCase() + yourstring.substr(1);

If you are wanting to reformat all-caps text, you might want to modify the other examples as such:
function capitalize (text) {
return text.charAt(0).toUpperCase() + text.slice(1).toLowerCase();
}
This will ensure that the following text is changed:
TEST => Test
This Is A TeST => This is a test

String.prototype.capitalize = function(){
return this.replace(/(^|\s)([a-z])/g,
function(m, p1, p2) {
return p1 + p2.toUpperCase();
});
};
Usage:
capitalizedString = someString.capitalize();
This is a text string => This Is A Text String

function capitalize(s) {
// returns the first letter capitalized + the string from index 1 and out aka. the rest of the string
return s[0].toUpperCase() + s.substr(1);
}
// examples
capitalize('this is a test');
=> 'This is a test'
capitalize('the Eiffel Tower');
=> 'The Eiffel Tower'
capitalize('/index.html');
=> '/index.html'

yourString.replace(/\w/, c => c.toUpperCase())
I found this arrow function easiest. Replace matches the first letter character (\w) of your string and converts it to uppercase. Nothing fancier is necessary.

var str = "test string";
str = str.substring(0,1).toUpperCase() + str.substring(1);

𝗔 𝗦𝗼𝗹𝘂𝘁𝗶𝗼𝗻 𝗧𝗵𝗮𝘁 𝗪𝗼𝗿𝗸𝘀 𝗙𝗼𝗿 𝗔𝗹𝗹 𝗨𝗻𝗶𝗰𝗼𝗱𝗲 𝗖𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿𝘀
57 81 different answers for this question, some off-topic, and yet none of them raise the important issue that none of the solutions listed will work with Asian characters, emoji's, and other high Unicode-point-value characters in many browsers. Here is a solution that will:
const consistantCapitalizeFirstLetter = "\uD852\uDF62".length === 1 ?
function(S) {
"use-strict"; // Hooray! The browser uses UTF-32!
return S.charAt(0).toUpperCase() + S.substring(1);
} : function(S) {
"use-strict";
// The browser is using UCS16 to store UTF-16
var code = S.charCodeAt(0)|0;
return (
code >= 0xD800 && code <= 0xDBFF ? // Detect surrogate pair
S.slice(0,2).toUpperCase() + S.substring(2) :
S.charAt(0).toUpperCase() + S.substring(1)
);
};
const prettyCapitalizeFirstLetter = "\uD852\uDF62".length === 1 ?
function(S) {
"use-strict"; // Hooray! The browser uses UTF-32!
return S.charAt(0).toLocaleUpperCase() + S.substring(1);
} : function(S) {
"use-strict";
// The browser is using UCS16 to store UTF-16
var code = S.charCodeAt(0)|0;
return (
code >= 0xD800 && code <= 0xDBFF ? // Detect surrogate pair
S.slice(0,2).toLocaleUpperCase() + S.substring(2) :
S.charAt(0).toLocaleUpperCase() + S.substring(1)
);
};
Do note that the above solution tries to account for UTF-32. However, the specification officially states that browsers are required to do everything in UTF-16 mapped into UCS2. Nevertheless, if we all come together, do our part, and start preparing for UTF32, then there is a chance that the TC39 may allow browsers to start using UTF-32 (like how Python uses 24-bits for each character of the string). This must seem silly to an English speaker: no one who uses only latin-1 has ever had to deal with Mojibake because Latin-I is supported by all character encodings. But, users in other countries (such as China, Japan, Indonesia, etc.) are not so fortunate. They constantly struggle with encoding problems not just from the webpage, but also from the JavaScript: many Chinese/Japanese characters are treated as two letters by JavaScript and thus may be broken apart in the middle, resulting in � and � (two question-marks that make no sense to the end user). If we could start getting ready for UTF-32, then the TC39 might just allow browsers do what Python did many years ago which had made Python very popular for working with high Unicode characters: using UTF-32.
consistantCapitalizeFirstLetter works correctly in Internet Explorer 3+ (when the const is changed to var). prettyCapitalizeFirstLetter requires Internet Explorer 5.5+ (see the top of page 250 of this document). However, these fact are more of just jokes because it is very likely that the rest of the code on your webpage will not even work in Internet Explorer 8 - because of all the DOM and JScript bugs and lack of features in these older browsers. Further, no one uses Internet Explorer 3 or Internet Explorer 5.5 any more.

Check out this solution:
var stringVal = 'master';
stringVal.replace(/^./, stringVal[0].toUpperCase()); // Returns Master

Only because this is really a one-liner I will include this answer. It's an ES6-based interpolated string one-liner.
let setStringName = 'the Eiffel Tower';
setStringName = `${setStringName[0].toUpperCase()}${setStringName.substring(1)}`;

with arrow function
let fLCapital = s => s.replace(/./, c => c.toUpperCase())
fLCapital('this is a test') // "This is a test"
with arrow function, another solution
let fLCapital = s => s = s.charAt(0).toUpperCase() + s.slice(1);
fLCapital('this is a test') // "This is a test"
with array and map()
let namesCapital = names => names.map(name => name.replace(/./, c => c.toUpperCase()))
namesCapital(['james', 'robert', 'mary']) // ["James", "Robert", "Mary"]

Override Number.toLocaleString("fa-IR") and change its thousands separator character for Persian numbers

I need to reformat numeric values based on the locale format, and to do this I use Number.toLocaleString("LOCAL") function. In Iran, the majority of people prefer to use , as a thousands separator character, but Number.toLocaleString("fa-IR") uses ٬ (Technically٬ it is correct).
Although you can replace the character by using the following code, is there any way to override the Number.toLocaleString("fa-IR")'s functionality?
function formatNumber(number) {
let string = number.toLocaleString('fa-IR'); // ۱۲٬۳۴۵٫۶۷۹
number = number.replace(/\٬/g, ",‬");
return string;
}
formatNumber(12345.6789); // the result: ۱۲,۳۴۵٫۶۷۹
PS: I know there are plenty of similar code-snippets out there, but I would like to override the built-in function (if possible).

If you're happy to change the prototype of Number, then yes. It is often warned against changing the prototype of a built in class, as that could mess with other libraries and such.
Forging ahead regardless:
const old = Number.prototype.toLocaleString;
Number.prototype.toLocaleString = function(locale)
{
let result = old.call(this, locale);
if(locale === 'fa-IR')
{
result = result.replace(/\٬/g, ",‬");
}
return result;
}

Javascript function to determine if a value is an integer

I came across a problem where I needed to determine if the field being entered by the user was an integer or float. The answer would then preselect a drop down further along a form. After much digging I found lots of framework code but none that actually did the job properly. The test data I used was;
Blank answer,
Non Numeric (Alpha),
1.0,
10,
1.10,
2.4,
3.0,
0.30,
0.00
A lot of other posts were also tested with the above data and I could not find one that passed ALL of the data correctly.
So I have written the following so that it may be reviewed by your good selves and hopefully it will help someone else out should they come across the same situation.
function isInteger(value)
{
//if(isNaN(value))return Nan;//optional check
//test for decimal point
if(!( /^-?\d+$/.test(String(value))))
{
//decimal point found
//if parseInt changes value it must be a float
if(parseInt(value) / 1 != value)return false;
}
//no decimal point so must be integer
return true;
}

Testing for integer values
ECMAScript 6 standard introduces a Number.isInteger().
This function is not yet supported by all major browsers, but a polyfill is listed on the site:
Number.isInteger = Number.isInteger || function isInteger (value) {
return typeof value === 'number' &&
isFinite(value) &&
Math.floor(value) === value
}
In case of user input (which is a string, not an integer), we can use the Number function to perform a type conversion into a number:
var input = '123' // Imagine this came from user
if (Number.isInteger(Number(input)) {
// User entered a valid integer value
}
Note, however, that the type conversion returns a valid integer-like value even for hexadecimal or octal strings. If this is not desired, you would need to further validate the original string. For detailed information about how the type conversion works, see MDN.
If such strict validation is desired, MDN also provides a good implementation using Regex (see the link for example output):
function filterInt (value) {
if(/^(\-|\+)?([0-9]+|Infinity)$/.test(value))
return Number(value)
return NaN
}
Testing for floating point numbers
isFinite() in combination with Number.isInteger() can help achieve our goal.
In case of user input (which is a string, not a float), we must use the Number function to perform a type conversion into a number:
var input = '123.5'
// Perform type conversion to a number
input = Number(input)
if (Number.isFinite(input) && ! Number.isInteger(input)) {
// A finite number that is not an integer can only be a float
}
Alternatively, a stricter variant of the parseFloat() implementation may be used instead, as listed on MDN (see the link for example output):
function filterFloat (value) {
if(/^(\-|\+)?([0-9]+(\.[0-9]+)?|Infinity)$/
.test(value))
return Number(value)
return NaN
}

Since you've mentioned user inputs, your question is about strings, so Number.isInteger is not an option.
To answer this question correctly we have to define more precisely what "integer" means when applied to strings. Is it
a sequence of digits? (example: ১২৪৫)
or a sequence of arabic digits? (example: 0000001)
or any mathematically valid integer representation? (example: 989238402389402394820394802)
or a mathematically valid integer that can be represented exactly in Javascript (i.e. it's less than MAX_SAFE_INTEGER)?
My guess is that you're looking for 4), here's the code for this case:
function isValidIntegerRepresentation(str) {
return /^-?\d+$/.test(str) && String(Number(str)) === str;
}
test = ['0', '00', '123', '-123', '239482039482309820394820394'];
test.forEach(function(n) {
document.write(n + "=" + isValidIntegerRepresentation(n) + "<br>");
});
This is very similar to what you already have.

How do I make the first letter of a string uppercase in JavaScript?

How do I make the first character of a string uppercase if it's a letter, but not change the case of any of the other letters?
For example:
"this is a test" → "This is a test"
"the Eiffel Tower" → "The Eiffel Tower"
"/index.html" → "/index.html"

function capitalizeFirstLetter(string) {
return string.charAt(0).toUpperCase() + string.slice(1);
}
Some other answers modify String.prototype (this answer used to as well), but I would advise against this now due to maintainability (hard to find out where the function is being added to the prototype and could cause conflicts if other code uses the same name/a browser adds a native function with that same name in future).

Edited to add this DISCLAIMER: please read the comments to understand the risks of editing JS basic types.
Here's a more object-oriented approach:
Object.defineProperty(String.prototype, 'capitalize', {
value: function() {
return this.charAt(0).toUpperCase() + this.slice(1);
},
enumerable: false
});
You'd call the function, like this:
"hello, world!".capitalize();
With the expected output being:
"Hello, world!"

In CSS:
p::first-letter {
text-transform:capitalize;
}

Here is a shortened version of the popular answer that gets the first letter by treating the string as an array:
function capitalize(s)
{
return s[0].toUpperCase() + s.slice(1);
}
Update
According to the comments below this doesn't work in IE 7 or below.
Update 2:
To avoid undefined for empty strings (see #njzk2's comment below), you can check for an empty string:
function capitalize(s)
{
return s && s[0].toUpperCase() + s.slice(1);
}
ES version
const capitalize = s => s && s[0].toUpperCase() + s.slice(1)
// to always return type string event when s may be falsy other than empty-string
const capitalize = s => (s && s[0].toUpperCase() + s.slice(1)) || ""

If you're interested in the performance of a few different methods posted:
Here are the fastest methods based on this jsperf test (ordered from fastest to slowest).
As you can see, the first two methods are essentially comparable in terms of performance, whereas altering the String.prototype is by far the slowest in terms of performance.
// 10,889,187 operations/sec
function capitalizeFirstLetter(string) {
return string[0].toUpperCase() + string.slice(1);
}
// 10,875,535 operations/sec
function capitalizeFirstLetter(string) {
return string.charAt(0).toUpperCase() + string.slice(1);
}
// 4,632,536 operations/sec
function capitalizeFirstLetter(string) {
return string.replace(/^./, string[0].toUpperCase());
}
// 1,977,828 operations/sec
String.prototype.capitalizeFirstLetter = function() {
return this.charAt(0).toUpperCase() + this.slice(1);
}

I didn’t see any mention in the existing answers of issues related to astral plane code points or internationalization. “Uppercase” doesn’t mean the same thing in every language using a given script.
Initially I didn’t see any answers addressing issues related to astral plane code points. There is one, but it’s a bit buried (like this one will be, I guess!)
Overview of the hidden problem and various approaches to it
Most of the proposed functions look like this:
function capitalizeFirstLetter(str) {
return str[0].toUpperCase() + str.slice(1);
}
However, some cased characters fall outside the BMP (basic multilingual plane, code points U+0 to U+FFFF). For example take this Deseret text:
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉"); // "𐐶𐐲𐑌𐐼𐐲𐑉"
The first character here fails to capitalize because the array-indexed properties of strings don’t access “characters” or code points*. They access UTF-16 code units. This is true also when slicing — the index values point at code units.
It happens to be that UTF-16 code units are 1:1 with USV code points within two ranges, U+0 to U+D7FF and U+E000 to U+FFFF inclusive. Most cased characters fall into those two ranges, but not all of them.
From ES2015 on, dealing with this became a bit easier. String.prototype[##iterator] yields strings corresponding to code points**. So for example, we can do this:
function capitalizeFirstLetter([ first='', ...rest ]) {
return [ first.toUpperCase(), ...rest ].join('');
}
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉") // "𐐎𐐲𐑌𐐼𐐲𐑉"
For longer strings, this is probably not terribly efficient*** — we don’t really need to iterate the remainder. We could use String.prototype.codePointAt to get at that first (possible) letter, but we’d still need to determine where the slice should begin. One way to avoid iterating the remainder would be to test whether the first codepoint is outside the BMP; if it isn’t, the slice begins at 1, and if it is, the slice begins at 2.
function capitalizeFirstLetter(str) {
if (!str) return '';
const firstCP = str.codePointAt(0);
const index = firstCP > 0xFFFF ? 2 : 1;
return String.fromCodePoint(firstCP).toUpperCase() + str.slice(index);
}
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉") // "𐐎𐐲𐑌𐐼𐐲𐑉"
You could use bitwise math instead of > 0xFFFF there, but it’s probably easier to understand this way and either would achieve the same thing.
We can also make this work in ES5 and below by taking that logic a bit further if necessary. There are no intrinsic methods in ES5 for working with codepoints, so we have to manually test whether the first code unit is a surrogate****:
function capitalizeFirstLetter(str) {
if (!str) return '';
var firstCodeUnit = str[0];
if (firstCodeUnit < '\uD800' || firstCodeUnit > '\uDFFF') {
return str[0].toUpperCase() + str.slice(1);
}
return str.slice(0, 2).toUpperCase() + str.slice(2);
}
capitalizeFirstLetter("𐐶𐐲𐑌𐐼𐐲𐑉") // "𐐎𐐲𐑌𐐼𐐲𐑉"
Deeper into internationalization (whose capitalization?)
At the start I also mentioned internationalization considerations. Some of these are very difficult to account for because they require knowledge not only of what language is being used, but also may require specific knowledge of the words in the language. For example, the Irish digraph "mb" capitalizes as "mB" at the start of a word. Another example, the German eszett, never begins a word (afaik), but still helps illustrate the problem. The lowercase eszett (“ß”) capitalizes to “SS,” but “SS” could lowercase to either “ß” or “ss” — you require out-of-band knowledge of the German language to know which is correct!
The most famous example of these kinds of issues, probably, is Turkish. In Turkish Latin, the capital form of i is İ, while the lowercase form of I is ı — they’re two different letters. Fortunately we do have a way to account for this:
function capitalizeFirstLetter([ first='', ...rest ], locale) {
return [ first.toLocaleUpperCase(locale), ...rest ].join('');
}
capitalizeFirstLetter("italy", "en") // "Italy"
capitalizeFirstLetter("italya", "tr") // "İtalya"
In a browser, the user’s most-preferred language tag is indicated by navigator.language, a list in order of preference is found at navigator.languages, and a given DOM element’s language can be obtained (usually) with Object(element.closest('[lang]')).lang || YOUR_DEFAULT_HERE in multilanguage documents.
In agents which support Unicode property character classes in RegExp, which were introduced in ES2018, we can clean stuff up further by directly expressing what characters we’re interested in:
function capitalizeFirstLetter(str, locale=navigator.language) {
return str.replace(/^\p{CWU}/u, char => char.toLocaleUpperCase(locale));
}
This could be tweaked a bit to also handle capitalizing multiple words in a string with fairly good accuracy for at least some languages, though outlying cases will be hard to avoid completely if doing so no matter what the primary language is.
The CWU or Changes_When_Uppercased character property matches all code points which change when uppercased in the generic case where specific locale data is absent. There are other interesting case-related Unicode character properties that you may wish to play around with. It’s a cool zone to explore but we’d go on all day if we enumerated em all here. Here’s something to get your curiosity going if you’re unfamiliar, though: \p{Lower} is a larger group than \p{LowercaseLetter} (aka \p{Ll}) — conveniently illustrated by the default character set comparison in this tool provided by Unicode. (NB: not everything you can reference there is also available in ES regular expressions, but most of the stuff you’re likely to want is).
Alternatives to case-mapping in JS (Firefox & CSS love the Dutch!)
If digraphs with unique locale/language/orthography capitalization rules happen to have a single-codepoint “composed” representation in Unicode, these might be used to make one’s capitalization expectations explicit even in the absence of locale data. For example, we could prefer the composed i-j digraph, ij / U+133, associated with Dutch, to ensure a case-mapping to uppercase IJ / U+132:
capitalizeFirstLetter('ijsselmeer'); // "IJsselmeer"
On the other hand, precomposed digraphs and similar are sometimes deprecated (like that one, it seems!) and may be undesirable in interchanged text regardless due to the potential copypaste nuisance if that’s not the normal way folks type the sequence in practice. Unfortunately, in the absence of the precomposition “hint,” an explicit locale won’t help here (at least as far as I know). If we spell ijsselmeer with an ordinary i + j, capitalizeFirstLetter will produce the wrong result even if we explicitly indicate nl as the locale:
capitalizeFirstLetter('ijsselmeer', 'nl'); // "Ijsselmeer" :(
(I’m not entirely sure whether there are some such cases where the behavior comes down to ICU data availability — perhaps someone else could say.)
If the point of the transformation is to display textual content in a web browser, though, you have an entirely different option available that will likely be your best bet: leveraging features of the web platform’s other core languages, HTML and CSS. Armed with HTML’s lang=... and CSS’s text-transform:..., you’ve got a (pseudo-)declarative solution that leaves extra room for the user agent to be “smart.” A JS API needs to have predictable outcomes across all browsers (generally) and isn’t free to experiment with heuristics. The user-agent itself is obligated only to its user, though, and heuristic solutions are fair game when the output is for a human being. If we tell it “this text is Dutch, but please display it capitalized,” the particular outcome might now vary between browsers, but it’s likely going to be the best each of them could do. Let’s see:
<!DOCTYPE html>
<dl>
<dt>Untransformed
<dd>ijsselmeer
<dt>Capitalized with CSS and <code>lang=en</code>
<dd lang="en" style="text-transform: capitalize">ijsselmeer
<dt>Capitalized with CSS and <code>lang=nl</code>
<dd lang="nl" style="text-transform: capitalize">ijsselmeer
In Chromium at the time of writing, both the English and Dutch lines come out as Ijsselmeer — so it does no better than JS. But try it in current Firefox! The element that we told the browser contains Dutch will be correctly rendered as IJsselmeer there.
This solution is purpose-specific (it’s not gonna help you in Node, anyway) but it was silly of me not to draw attention to it previously given some folks might not realize they’re googling the wrong question. Thanks #paul23 for clarifying more about the nature of the IJ digraph in practice and prompting further investigation!
As of January 2021, all major engines have implemented the Unicode property character class feature, but depending on your target support range you may not be able to use it safely yet. The last browser to introduce support was Firefox (78; June 30, 2020). You can check for support of this feature with the Kangax compat table. Babel can be used to compile RegExp literals with property references to equivalent patterns without them, but be aware that the resulting code can sometimes be enormous. You probably would not want to do this unless you’re certain the tradeoff is justified for your use case.
In all likelihood, people asking this question will not be concerned with Deseret capitalization or internationalization. But it’s good to be aware of these issues because there’s a good chance you’ll encounter them eventually even if they aren’t concerns presently. They’re not “edge” cases, or rather, they’re not by-definition edge cases — there’s a whole country where most people speak Turkish, anyway, and conflating code units with codepoints is a fairly common source of bugs (especially with regard to emoji). Both strings and language are pretty complicated!
* The code units of UTF-16 / UCS2 are also Unicode code points in the sense that e.g. U+D800 is technically a code point, but that’s not what it “means” here ... sort of ... though it gets pretty fuzzy. What the surrogates definitely are not, though, is USVs (Unicode scalar values).
** Though if a surrogate code unit is “orphaned” — i.e., not part of a logical pair — you could still get surrogates here, too.
*** maybe. I haven’t tested it. Unless you have determined capitalization is a meaningful bottleneck, I probably wouldn’t sweat it — choose whatever you believe is most clear and readable.
**** such a function might wish to test both the first and second code units instead of just the first, since it’s possible that the first unit is an orphaned surrogate. For example the input "\uD800x" would capitalize the X as-is, which may or may not be expected.

For another case I need it to capitalize the first letter and lowercase the rest. The following cases made me change this function:
//es5
function capitalize(string) {
return string.charAt(0).toUpperCase() + string.slice(1).toLowerCase();
}
capitalize("alfredo") // => "Alfredo"
capitalize("Alejandro")// => "Alejandro
capitalize("ALBERTO") // => "Alberto"
capitalize("ArMaNdO") // => "Armando"
// es6 using destructuring
const capitalize = ([first,...rest]) => first.toUpperCase() + rest.join('').toLowerCase();

This is the 2018 ECMAScript 6+ Solution:
const str = 'the Eiffel Tower';
const newStr = `${str[0].toUpperCase()}${str.slice(1)}`;
console.log('Original String:', str); // the Eiffel Tower
console.log('New String:', newStr); // The Eiffel Tower

If you're already (or considering) using Lodash, the solution is easy:
_.upperFirst('fred');
// => 'Fred'
_.upperFirst('FRED');
// => 'FRED'
_.capitalize('fred') //=> 'Fred'
See their documentation: https://lodash.com/docs#capitalize
_.camelCase('Foo Bar'); //=> 'fooBar'
https://lodash.com/docs/4.15.0#camelCase
_.lowerFirst('Fred');
// => 'fred'
_.lowerFirst('FRED');
// => 'fRED'
_.snakeCase('Foo Bar');
// => 'foo_bar'
Vanilla JavaScript for first upper case:
function upperCaseFirst(str){
return str.charAt(0).toUpperCase() + str.substring(1);
}

There is a very simple way to implement it by replace. For ECMAScript 6:
'foo'.replace(/^./, str => str.toUpperCase())
Result:
'Foo'

Capitalize the first letter of all words in a string:
function ucFirstAllWords( str )
{
var pieces = str.split(" ");
for ( var i = 0; i < pieces.length; i++ )
{
var j = pieces[i].charAt(0).toUpperCase();
pieces[i] = j + pieces[i].substr(1);
}
return pieces.join(" ");
}

CSS only
If the transformation is needed only for displaying on a web page:
p::first-letter {
text-transform: uppercase;
}
Despite being called "::first-letter", it applies to the first character, i.e. in case of string %a, this selector would apply to % and as such a would not be capitalized.
In IE9+ or IE5.5+ it's supported in legacy notation with only one colon (:first-letter).
ES2015 one-liner
const capitalizeFirstChar = str => str.charAt(0).toUpperCase() + str.substring(1);
Remarks
In the benchmark I performed, there was no significant difference between string.charAt(0) and string[0]. Note however, that string[0] would be undefined for an empty string, so the function would have to be rewritten to use "string && string[0]", which is way too verbose, compared to the alternative.
string.substring(1) is faster than string.slice(1).
Benchmark between substring() and slice()
The difference is rather minuscule nowadays (run the test yourself):
21,580,613.15 ops/s ±1.6% for substring(),
21,096,394.34 ops/s ±1.8% (2.24% slower) for slice().

It's always better to handle these kinds of stuff using CSS first, in general, if you can solve something using CSS, go for that first, then try JavaScript to solve your problems, so in this case try using :first-letter in CSS and apply text-transform:capitalize;
So try creating a class for that, so you can use it globally, for example: .first-letter-uppercase and add something like below in your CSS:
.first-letter-uppercase:first-letter {
text-transform:capitalize;
}
Also the alternative option is JavaScript, so the best gonna be something like this:
function capitalizeTxt(txt) {
return txt.charAt(0).toUpperCase() + txt.slice(1); //or if you want lowercase the rest txt.slice(1).toLowerCase();
}
and call it like:
capitalizeTxt('this is a test'); // return 'This is a test'
capitalizeTxt('the Eiffel Tower'); // return 'The Eiffel Tower'
capitalizeTxt('/index.html'); // return '/index.html'
capitalizeTxt('alireza'); // return 'Alireza'
capitalizeTxt('dezfoolian'); // return 'Dezfoolian'
If you want to reuse it over and over, it's better attach it to javascript native String, so something like below:
String.prototype.capitalizeTxt = String.prototype.capitalizeTxt || function() {
return this.charAt(0).toUpperCase() + this.slice(1);
}
and call it as below:
'this is a test'.capitalizeTxt(); // return 'This is a test'
'the Eiffel Tower'.capitalizeTxt(); // return 'The Eiffel Tower'
'/index.html'.capitalizeTxt(); // return '/index.html'
'alireza'.capitalizeTxt(); // return 'Alireza'

String.prototype.capitalize = function(allWords) {
return (allWords) ? // If all words
this.split(' ').map(word => word.capitalize()).join(' ') : // Break down the phrase to words and then recursive
// calls until capitalizing all words
this.charAt(0).toUpperCase() + this.slice(1); // If allWords is undefined, capitalize only the first word,
// meaning the first character of the whole string
}
And then:
"capitalize just the first word".capitalize(); ==> "Capitalize just the first word"
"capitalize all words".capitalize(true); ==> "Capitalize All Words"
Update November 2016 (ES6), just for fun:
const capitalize = (string = '') => [...string].map( // Convert to array with each item is a char of
// string by using spread operator (...)
(char, index) => index ? char : char.toUpperCase() // Index true means not equal 0, so (!index) is
// the first character which is capitalized by
// the `toUpperCase()` method
).join('') // Return back to string
then capitalize("hello") // Hello

SHORTEST 3 solutions, 1 and 2 handle cases when s string is "", null and undefined:
s&&s[0].toUpperCase()+s.slice(1) // 32 char
s&&s.replace(/./,s[0].toUpperCase()) // 36 char - using regexp
'foo'.replace(/./,x=>x.toUpperCase()) // 31 char - direct on string, ES6
let s='foo bar';
console.log( s&&s[0].toUpperCase()+s.slice(1) );
console.log( s&&s.replace(/./,s[0].toUpperCase()) );
console.log( 'foo bar'.replace(/./,x=>x.toUpperCase()) );

We could get the first character with one of my favorite RegExp, looks like a cute smiley: /^./
String.prototype.capitalize = function () {
return this.replace(/^./, function (match) {
return match.toUpperCase();
});
};
And for all coffee-junkies:
String::capitalize = ->
#replace /^./, (match) ->
match.toUpperCase()
...and for all guys who think that there's a better way of doing this, without extending native prototypes:
var capitalize = function (input) {
return input.replace(/^./, function (match) {
return match.toUpperCase();
});
};

Here is a function called ucfirst()(short for "upper case first letter"):
function ucfirst(str) {
var firstLetter = str.substr(0, 1);
return firstLetter.toUpperCase() + str.substr(1);
}
You can capitalise a string by calling ucfirst("some string") -- for example,
ucfirst("this is a test") --> "This is a test"
It works by splitting the string into two pieces. On the first line it pulls out firstLetter and then on the second line it capitalises firstLetter by calling firstLetter.toUpperCase() and joins it with the rest of the string, which is found by calling str.substr(1).
You might think this would fail for an empty string, and indeed in a language like C you would have to cater for this. However in JavaScript, when you take a substring of an empty string, you just get an empty string back.

Use:
var str = "ruby java";
console.log(str.charAt(0).toUpperCase() + str.substring(1));
It will output "Ruby java" to the console.

If you use Underscore.js or Lodash, the underscore.string library provides string extensions, including capitalize:
_.capitalize(string) Converts first letter of the string to
uppercase.
Example:
_.capitalize("foo bar") == "Foo bar"

If you're ok with capitalizing the first letter of every word, and your usecase is in HTML, you can use the following CSS:
<style type="text/css">
p.capitalize {text-transform:capitalize;}
</style>
<p class="capitalize">This is some text.</p>
This is from CSS text-transform Property (at W3Schools).

var capitalized = yourstring[0].toUpperCase() + yourstring.substr(1);

If you are wanting to reformat all-caps text, you might want to modify the other examples as such:
function capitalize (text) {
return text.charAt(0).toUpperCase() + text.slice(1).toLowerCase();
}
This will ensure that the following text is changed:
TEST => Test
This Is A TeST => This is a test

String.prototype.capitalize = function(){
return this.replace(/(^|\s)([a-z])/g,
function(m, p1, p2) {
return p1 + p2.toUpperCase();
});
};
Usage:
capitalizedString = someString.capitalize();
This is a text string => This Is A Text String

function capitalize(s) {
// returns the first letter capitalized + the string from index 1 and out aka. the rest of the string
return s[0].toUpperCase() + s.substr(1);
}
// examples
capitalize('this is a test');
=> 'This is a test'
capitalize('the Eiffel Tower');
=> 'The Eiffel Tower'
capitalize('/index.html');
=> '/index.html'

yourString.replace(/\w/, c => c.toUpperCase())
I found this arrow function easiest. Replace matches the first letter character (\w) of your string and converts it to uppercase. Nothing fancier is necessary.

var str = "test string";
str = str.substring(0,1).toUpperCase() + str.substring(1);

𝗔 𝗦𝗼𝗹𝘂𝘁𝗶𝗼𝗻 𝗧𝗵𝗮𝘁 𝗪𝗼𝗿𝗸𝘀 𝗙𝗼𝗿 𝗔𝗹𝗹 𝗨𝗻𝗶𝗰𝗼𝗱𝗲 𝗖𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿𝘀
57 81 different answers for this question, some off-topic, and yet none of them raise the important issue that none of the solutions listed will work with Asian characters, emoji's, and other high Unicode-point-value characters in many browsers. Here is a solution that will:
const consistantCapitalizeFirstLetter = "\uD852\uDF62".length === 1 ?
function(S) {
"use-strict"; // Hooray! The browser uses UTF-32!
return S.charAt(0).toUpperCase() + S.substring(1);
} : function(S) {
"use-strict";
// The browser is using UCS16 to store UTF-16
var code = S.charCodeAt(0)|0;
return (
code >= 0xD800 && code <= 0xDBFF ? // Detect surrogate pair
S.slice(0,2).toUpperCase() + S.substring(2) :
S.charAt(0).toUpperCase() + S.substring(1)
);
};
const prettyCapitalizeFirstLetter = "\uD852\uDF62".length === 1 ?
function(S) {
"use-strict"; // Hooray! The browser uses UTF-32!
return S.charAt(0).toLocaleUpperCase() + S.substring(1);
} : function(S) {
"use-strict";
// The browser is using UCS16 to store UTF-16
var code = S.charCodeAt(0)|0;
return (
code >= 0xD800 && code <= 0xDBFF ? // Detect surrogate pair
S.slice(0,2).toLocaleUpperCase() + S.substring(2) :
S.charAt(0).toLocaleUpperCase() + S.substring(1)
);
};
Do note that the above solution tries to account for UTF-32. However, the specification officially states that browsers are required to do everything in UTF-16 mapped into UCS2. Nevertheless, if we all come together, do our part, and start preparing for UTF32, then there is a chance that the TC39 may allow browsers to start using UTF-32 (like how Python uses 24-bits for each character of the string). This must seem silly to an English speaker: no one who uses only latin-1 has ever had to deal with Mojibake because Latin-I is supported by all character encodings. But, users in other countries (such as China, Japan, Indonesia, etc.) are not so fortunate. They constantly struggle with encoding problems not just from the webpage, but also from the JavaScript: many Chinese/Japanese characters are treated as two letters by JavaScript and thus may be broken apart in the middle, resulting in � and � (two question-marks that make no sense to the end user). If we could start getting ready for UTF-32, then the TC39 might just allow browsers do what Python did many years ago which had made Python very popular for working with high Unicode characters: using UTF-32.
consistantCapitalizeFirstLetter works correctly in Internet Explorer 3+ (when the const is changed to var). prettyCapitalizeFirstLetter requires Internet Explorer 5.5+ (see the top of page 250 of this document). However, these fact are more of just jokes because it is very likely that the rest of the code on your webpage will not even work in Internet Explorer 8 - because of all the DOM and JScript bugs and lack of features in these older browsers. Further, no one uses Internet Explorer 3 or Internet Explorer 5.5 any more.

Check out this solution:
var stringVal = 'master';
stringVal.replace(/^./, stringVal[0].toUpperCase()); // Returns Master

Only because this is really a one-liner I will include this answer. It's an ES6-based interpolated string one-liner.
let setStringName = 'the Eiffel Tower';
setStringName = `${setStringName[0].toUpperCase()}${setStringName.substring(1)}`;

with arrow function
let fLCapital = s => s.replace(/./, c => c.toUpperCase())
fLCapital('this is a test') // "This is a test"
with arrow function, another solution
let fLCapital = s => s = s.charAt(0).toUpperCase() + s.slice(1);
fLCapital('this is a test') // "This is a test"
with array and map()
let namesCapital = names => names.map(name => name.replace(/./, c => c.toUpperCase()))
namesCapital(['james', 'robert', 'mary']) // ["James", "Robert", "Mary"]