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Javascript: converting string to an array

This is one of my function for a calculator project. First I needed to convert the input string into an array, and do the operation later. (assuming that input has only numbers and '+' sign for now.
My question here is, how do I improve this code? What are the other ways to deal with this problem? (Time complexity, cleanness, shorter code.......whatever)
function convertArray(input) {
let array = [];
let num = "";
for (let i = 0; i < input.length; i++) {
if (input.charAt(i) == '+') {
array.push(input.charAt(i));
} else {
do {
num += input.charAt(i);
i++;
} while (i < input.length && input.charAt(i) !== '+');
array.push(num);
num = "";
i--;
}
}
return array;
}
console.log(convertArray("10+2+3000+70+1"));

You could split with a group. this add the group as well to the array.
For other calculation signs, you could add them to the brackets.
const convertArray = string => string.split(/([+])/);
console.log(convertArray("10+2+3000+70+1"));

const q = prompt('Sum?');
alert('Answer: ' + eval(q));
Would not recommend using eval, but if all you need is a quick and dirty trick, it works.
Personally, I'd recommend a library such as Math.js, but any will do.
If you really need to do this by yourself for a project, I'd recommend checking out the answers here: Evaluating a string as a mathematical expression in JavaScript.
Hope you succeed in whatever you're planning on doing.

It seems the complexity must have something to do with your wish to determing operators. In your code you just push them all into the array. To do that is like
const re = /((\d+)|([^\d]+))/g
const convertArray = str => {
let match, arr=[];
while (match = re.exec(str)) {
arr.push(match[1]) // here you can determine if you have an operator
console.log(match[1],"Operator?",!/^\d+$/.test(match[1]))
}
return arr
}
const str = "10+2+3000+70+1";
console.log(convertArray(str));

Split string by first white space [duplicate]

This question already has answers here:
Split string on the first white space occurrence
(16 answers)
Closed 5 years ago.
I want to split string in array based just on first white space in string.
Like this:
var name = "Jone Doe Doone";
var res = ["Jone", "Doe Doone"];

Here is an example where I have used indexOf() to find the first space and then return with a splitted array with substring()
function splitAtFirstSpace(str) {
if (! str) return [];
var i = str.indexOf(' ');
if (i > 0) {
return [str.substring(0, i), str.substring(i + 1)];
}
else return [str];
}
console.log(splitAtFirstSpace("Jone Doe Doone"));
console.log(splitAtFirstSpace("BooDoone"));
console.log(splitAtFirstSpace("Doe Doone"));

Simple.
function splitFirst(s) {
var firstW = s.indexOf(" ");
if (firstW < 0) {
return s;
}
var array = [];
array.push(s.substring(0, firstW));
array.push(s.substring(firstW, s.length - 1));
return array;
}
Note: Javascript coding convention says variables have to start with lowercase.

Try the following answer.
var Name = "Jone Doe Doone";
var result = [Name.split(" ")[0]];
result.push(Name.substr(Name.split(" ")[0].length).trim());
console.log(result);

get all possible combinations of some characters javascript [duplicate]

This question already has answers here:
Is there any pre-built method for finding all permutations of a given string in JavaScript?
(8 answers)
Closed 8 years ago.
So If a have A, B, C ,
I want to create some strings whith length 4
so the output will be
AAAA
AAAB
AAAC
AABA
AABB
AABC
ABAB
....
CCCC

There are some comments, so you can understand this. Thanks to http://www.walmik.com/2013/03/rearrange-letters-of-a-word/
function permutate(theWord){
//Array to store the generated words
var words = [];
/**
* Recursive function to split a string and rearrange
* it's characters and then join the results
*/
function rearrange(str, prefix) {
var i, singleChar, balanceStr, word;
//The first time round, prefix will be empty
prefix = prefix || '';
//Loop over the str to separate each single character
for(i = 0; i < str.length; i++) {
singleChar = str[i];
balanceStr = str.slice(0, i) + str.slice(i+1);
//join the prefix with each of the combinations
word = prefix + singleChar + balanceStr;
//Inject this word only if it does not exist
if(words.indexOf(word) < 0) words.push(word);
//Recursively call this function in case there are balance characters
if(balanceStr.length > 1) rearrange(balanceStr, prefix + singleChar);
}
}
//kick start recursion
rearrange(theWord);
return words;
}
var permutatedWord = permutate('goal');
console.log(permutatedWords);

Random alpha-numeric string in JavaScript?

What's the shortest way (within reason) to generate a random alpha-numeric (uppercase, lowercase, and numbers) string in JavaScript to use as a probably-unique identifier?

I just came across this as a really nice and elegant solution:
Math.random().toString(36).slice(2)
Notes on this implementation:
This will produce a string anywhere between zero and 12 characters long, usually 11 characters, due to the fact that floating point stringification removes trailing zeros.
It won't generate capital letters, only lower-case and numbers.
Because the randomness comes from Math.random(), the output may be predictable and therefore not necessarily unique.
Even assuming an ideal implementation, the output has at most 52 bits of entropy, which means you can expect a duplicate after around 70M strings generated.

If you only want to allow specific characters, you could also do it like this:
function randomString(length, chars) {
var result = '';
for (var i = length; i > 0; --i) result += chars[Math.floor(Math.random() * chars.length)];
return result;
}
var rString = randomString(32, '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ');
Here's a jsfiddle to demonstrate: http://jsfiddle.net/wSQBx/
Another way to do it could be to use a special string that tells the function what types of characters to use. You could do that like this:
function randomString(length, chars) {
var mask = '';
if (chars.indexOf('a') > -1) mask += 'abcdefghijklmnopqrstuvwxyz';
if (chars.indexOf('A') > -1) mask += 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
if (chars.indexOf('#') > -1) mask += '0123456789';
if (chars.indexOf('!') > -1) mask += '~`!##$%^&*()_+-={}[]:";\'<>?,./|\\';
var result = '';
for (var i = length; i > 0; --i) result += mask[Math.floor(Math.random() * mask.length)];
return result;
}
console.log(randomString(16, 'aA'));
console.log(randomString(32, '#aA'));
console.log(randomString(64, '#A!'));
Fiddle: http://jsfiddle.net/wSQBx/2/
Alternatively, to use the base36 method as described below you could do something like this:
function randomString(length) {
return Math.round((Math.pow(36, length + 1) - Math.random() * Math.pow(36, length))).toString(36).slice(1);
}

UPDATED:
One-liner solution, for random 20 characters (alphanumeric lowercase):
Array.from(Array(20), () => Math.floor(Math.random() * 36).toString(36)).join('');
Or shorter with lodash:
_.times(20, () => _.random(35).toString(36)).join('');

Another variation of answer suggested by JAR.JAR.beans
(Math.random()*1e32).toString(36)
By changing multiplicator 1e32 you can change length of random string.

Or to build upon what Jar Jar suggested, this is what I used on a recent project (to overcome length restrictions):
var randomString = function (len, bits)
{
bits = bits || 36;
var outStr = "", newStr;
while (outStr.length < len)
{
newStr = Math.random().toString(bits).slice(2);
outStr += newStr.slice(0, Math.min(newStr.length, (len - outStr.length)));
}
return outStr.toUpperCase();
};
Use:
randomString(12, 16); // 12 hexadecimal characters
randomString(200); // 200 alphanumeric characters

This is cleaner
Math.random().toString(36).substr(2, length)
Example
Math.random().toString(36).substr(2, 5)

function randomString(len) {
var p = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
return [...Array(len)].reduce(a=>a+p[~~(Math.random()*p.length)],'');
}
Summary:
Create an array of the size we want (because there's no range(len) equivalent in javascript.
For each element in the array: pick a random character from p and add it to a string
Return the generated string.
Some explanation:
[...Array(len)]
Array(len) or new Array(len) creates an array with undefined pointer(s). One-liners are going to be harder to pull off. The Spread syntax conveniently defines the pointers (now they point to undefined objects!).
.reduce(
Reduce the array to, in this case, a single string. The reduce functionality is common in most languages and worth learning.
a=>a+...
We're using an arrow function.
a is the accumulator. In this case it's the end-result string we're going to return when we're done (you know it's a string because the second argument to the reduce function, the initialValue is an empty string: ''). So basically: convert each element in the array with p[~~(Math.random()*p.length)], append the result to the a string and give me a when you're done.
p[...]
p is the string of characters we're selecting from. You can access chars in a string like an index (E.g., "abcdefg"[3] gives us "d")
~~(Math.random()*p.length)
Math.random() returns a floating point between [0, 1) Math.floor(Math.random()*max) is the de facto standard for getting a random integer in javascript. ~ is the bitwise NOT operator in javascript.
~~ is a shorter, arguably sometimes faster, and definitely funner way to say Math.floor( Here's some info

I think the following is the simplest solution which allows for a given length:
Array(myLength).fill(0).map(x => Math.random().toString(36).charAt(2)).join('')
It depends on the arrow function syntax.

for 32 characters:
for(var c = ''; c.length < 32;) c += Math.random().toString(36).substr(2, 1)

Random character:
String.fromCharCode(i); //where is an int
Random int:
Math.floor(Math.random()*100);
Put it all together:
function randomNum(hi){
return Math.floor(Math.random()*hi);
}
function randomChar(){
return String.fromCharCode(randomNum(100));
}
function randomString(length){
var str = "";
for(var i = 0; i < length; ++i){
str += randomChar();
}
return str;
}
var RandomString = randomString(32); //32 length string
Fiddle: http://jsfiddle.net/maniator/QZ9J2/

Using lodash:
function createRandomString(length) {
var chars = "abcdefghijklmnopqrstufwxyzABCDEFGHIJKLMNOPQRSTUFWXYZ1234567890"
var pwd = _.sampleSize(chars, length || 12) // lodash v4: use _.sampleSize
return pwd.join("")
}
document.write(createRandomString(8))
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>

Random Key Generator
keyLength argument is the character length you want for the key
function keyGen(keyLength) {
var i, key = "", characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var charactersLength = characters.length;
for (i = 0; i < keyLength; i++) {
key += characters.substr(Math.floor((Math.random() * charactersLength) + 1), 1);
}
return key;
}
keyGen(12)
"QEt9mYBiTpYD"

var randomString = function(length) {
var str = '';
var chars ='0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz'.split(
'');
var charsLen = chars.length;
if (!length) {
length = ~~(Math.random() * charsLen);
}
for (var i = 0; i < length; i++) {
str += chars[~~(Math.random() * charsLen)];
}
return str;
};

When I saw this question I thought of when I had to generate UUIDs. I can't take credit for the code, as I am sure I found it here on stackoverflow. If you dont want the dashes in your string then take out the dashes. Here is the function:
function generateUUID() {
var d = new Date().getTime();
var uuid = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g,function(c) {
var r = (d + Math.random()*16)%16 | 0;
d = Math.floor(d/16);
return (c=='x' ? r : (r&0x7|0x8)).toString(16);
});
return uuid.toUpperCase();
}
Fiddle: http://jsfiddle.net/nlviands/fNPvf/11227/

This function should give a random string in any length.
function randString(length) {
var l = length > 25 ? 25 : length;
var str = Math.random().toString(36).substr(2, l);
if(str.length >= length){
return str;
}
return str.concat(this.randString(length - str.length));
}
I've tested it with the following test that succeeded.
function test(){
for(var x = 0; x < 300000; x++){
if(randString(x).length != x){
throw new Error('invalid result for len ' + x);
}
}
}
The reason i have chosen 25 is since that in practice the length of the string returned from Math.random().toString(36).substr(2, 25) has length 25. This number can be changed as you wish.
This function is recursive and hence calling the function with very large values can result with Maximum call stack size exceeded. From my testing i was able to get string in the length of 300,000 characters.
This function can be converted to a tail recursion by sending the string to the function as a second parameter. I'm not sure if JS uses Tail call optimization

A simple function that takes the length
getRandomToken(len: number): string {
return Math.random().toString(36).substr(2, len);
}
Ff you pass 6 it will generate 6 digit alphanumeric number

Nice and simple, and not limited to a certain number of characters:
let len = 20, str = "";
while(str.length < len) str += Math.random().toString(36).substr(2);
str = str.substr(0, len);

One could just use lodash uniqueId:
_.uniqueId([prefix=''])
Generates a unique ID. If prefix is given, the ID is appended to it.

Here's a simple code to generate random string alphabet.
Have a look how this code works.
go(lenthOfStringToPrint); - Use this function to generate the final string.
var letters = {
1: ["q","w","e","r","t","y","u","i","o","p","a","s","d","f","g","h","j","k","l","z","x","c","v","b","n","m"],
2: ["Q","W","E","R","T","Y","U","I","O","P","A","S","D","F","G","H","J","K","L","Z","X","C","V","B","N","M"]
},i,letter,final="";
random = (max,min) => {
return Math.floor(Math.random()*(max-min+1)+min);
}
function go(length) {
final="",letter="";
for (i=1; i<=length; i++){
letter = letters[random(0,3)][random(0,25)];
final+=letter;
}
return final;
}

I used #Nimphious excellent second approach and found that occasionally the string returned was numeric - not alphanumeric.
The solution I used was to test using !isNaN and use recursion to call the function again.
Why bother? I was using this function to create object keys, if all the keys are alphanumeric everything sorts properly but if you use
numbers as keys mixed with alphanumeric (strings) looping through the object will produce a different order to original order.
function newRandomString(length, chars) {
var mask = '';
if (chars.indexOf('a') > -1) mask += 'abcdefghijklmnopqrstuvwxyz';
if (chars.indexOf('A') > -1) mask += 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
if (chars.indexOf('#') > -1) mask += '0123456789';
if (chars.indexOf('$') > -1) mask += '0123456789';
var result = '';
for (var i = length; i > 0; --i) result += mask[Math.floor(Math.random() *
mask.length)];
/*
we need a string not a number !isNaN(result)) will return true if '1234' or '3E77'
because if we're looping through object keys (created by newRandomString()) and
a number is used and all the other keys are strings then the number will
be first even if it was the 2nd or third key in object
*/
//use recursion to try again
if(!isNaN(result)){
console.log('found a number....:'+result);
return newRandomString(length, chars)
}else{
return result;
}
};
var i=0;
while (i < 1000) {
var a = newRandomString(4, '#$aA');
console.log(i+' - '+a);
//now we're using recursion this won't occur
if(!isNaN(a)){
console.log('=============='+i+' - '+a);
}
i++;
}
console.log('3E77:'+!isNaN('3E77'));//true
console.log('1234:'+!isNaN('1234'));//true
console.log('ab34:'+!isNaN('ab34'));//false

After looking at solutions in answers to this question and other sources, this is the solution that is simplest while allowing for modification of the included characters and selection in the length of the returned result.
// generate random string of n characters
function randomString(length) {
const characters = '0123456789abcdefghijklmnopqrstuvwxyz'; // characters used in string
let result = ''; // initialize the result variable passed out of the function
for (let i = length; i > 0; i--) {
result += characters[Math.floor(Math.random() * characters.length)];
}
return result;
}
console.log(randomString(6));

Use md5 library: https://github.com/blueimp/JavaScript-MD5
The shortest way:
md5(Math.random())
If you want to limit the size to 5:
md5(Math.random()).substr(0, 5)

Count the number of occurrences of a character in a string in Javascript

I need to count the number of occurrences of a character in a string.
For example, suppose my string contains:
var mainStr = "str1,str2,str3,str4";
I want to find the count of comma , character, which is 3. And the count of individual strings after the split along comma, which is 4.
I also need to validate that each of the strings i.e str1 or str2 or str3 or str4 should not exceed, say, 15 characters.

I have updated this answer. I like the idea of using a match better, but it is slower:
console.log(("str1,str2,str3,str4".match(/,/g) || []).length); //logs 3
console.log(("str1,str2,str3,str4".match(new RegExp("str", "g")) || []).length); //logs 4
Use a regular expression literal if you know what you are searching for beforehand, if not you can use the RegExp constructor, and pass in the g flag as an argument.
match returns null with no results thus the || []
The original answer I made in 2009 is below. It creates an array unnecessarily, but using a split is faster (as of September 2014). I'm ambivalent, if I really needed the speed there would be no question that I would use a split, but I would prefer to use match.
Old answer (from 2009):
If you're looking for the commas:
(mainStr.split(",").length - 1) //3
If you're looking for the str
(mainStr.split("str").length - 1) //4
Both in #Lo's answer and in my own silly performance test split comes ahead in speed, at least in Chrome, but again creating the extra array just doesn't seem sane.

There are at least five ways. The best option, which should also be the fastest (owing to the native RegEx engine) is placed at the top.
Method 1
("this is foo bar".match(/o/g)||[]).length;
// returns 2
Method 2
"this is foo bar".split("o").length - 1;
// returns 2
Split not recommended as it is resource hungry. It allocates new instances of 'Array' for each match. Don't try it for a >100MB file via FileReader. You can observe the exact resource usage using Chrome's profiler option.
Method 3
var stringsearch = "o"
,str = "this is foo bar";
for(var count=-1,index=-2; index != -1; count++,index=str.indexOf(stringsearch,index+1) );
// returns 2
Method 4
Searching for a single character
var stringsearch = "o"
,str = "this is foo bar";
for(var i=count=0; i<str.length; count+=+(stringsearch===str[i++]));
// returns 2
Method 5
Element mapping and filtering. This is not recommended due to its overall resource preallocation rather than using Pythonian 'generators':
var str = "this is foo bar"
str.split('').map( function(e,i){ if(e === 'o') return i;} )
.filter(Boolean)
//>[9, 10]
[9, 10].length
// returns 2
Share:
I made this gist, with currently 8 methods of character-counting, so we can directly pool and share our ideas - just for fun, and perhaps some interesting benchmarks :)

Add this function to sting prototype :
String.prototype.count=function(c) {
var result = 0, i = 0;
for(i;i<this.length;i++)if(this[i]==c)result++;
return result;
};
usage:
console.log("strings".count("s")); //2

Simply, use the split to find out the number of occurrences of a character in a string.
mainStr.split(',').length // gives 4 which is the number of strings after splitting using delimiter comma
mainStr.split(',').length - 1 // gives 3 which is the count of comma

A quick Google search got this (from http://www.codecodex.com/wiki/index.php?title=Count_the_number_of_occurrences_of_a_specific_character_in_a_string#JavaScript)
String.prototype.count=function(s1) {
return (this.length - this.replace(new RegExp(s1,"g"), '').length) / s1.length;
}
Use it like this:
test = 'one,two,three,four'
commas = test.count(',') // returns 3

You can also rest your string and work with it like an array of elements using
Array.prototype.filter()
const mainStr = 'str1,str2,str3,str4';
const commas = [...mainStr].filter(l => l === ',').length;
console.log(commas);
Or
Array.prototype.reduce()
const mainStr = 'str1,str2,str3,str4';
const commas = [...mainStr].reduce((a, c) => c === ',' ? ++a : a, 0);
console.log(commas);

UPDATE: This might be simple, but it is not the fastest. See benchmarks below.
It's amazing that in 13 years, this answer hasn't shown up. Intuitively, it seems like it should be fastest:
const s = "The quick brown fox jumps over the lazy dog.";
const oCount = s.length - s.replaceAll('o', '').length;
If there are only two kinds of character in the string, then this is faster still:
const s = "001101001";
const oneCount = s.replaceAll('0', '').length;
BENCHMARKS
const { performance } = require('node:perf_hooks');
const ITERATIONS = 10000000;
const TEST_STRING = "The quick brown fox jumps over the lazy dog.";
console.log(ITERATIONS, "iterations");
let sum = 0; // make sure compiler doesn't optimize code out
let start = performance.now();
for (let i = 0; i < ITERATIONS; ++i) {
sum += TEST_STRING.length - TEST_STRING.replaceAll('o', '').length;
}
let end = performance.now();
console.log(" replaceAll duration", end - start, `(sum ${sum})`);
sum = 0;
start = performance.now();
for (let i = 0; i < ITERATIONS; ++i) {
sum += TEST_STRING.split('o').length - 1
}
end = performance.now();
console.log(" split duration", end - start, `(sum ${sum})`);
10000 iterations
replaceAll duration 2.6167500019073486 (sum 40000)
split duration 2.0777920186519623 (sum 40000)
100000 iterations
replaceAll duration 17.563208997249603 (sum 400000)
split duration 8.087624996900558 (sum 400000)
1000000 iterations
replaceAll duration 128.71587499976158 (sum 4000000)
split duration 64.15841698646545 (sum 4000000)
10000000 iterations
replaceAll duration 1223.3415840268135 (sum 40000000)
split duration 629.1629169881344 (sum 40000000)

Here is a similar solution, but it uses Array.prototype.reduce
function countCharacters(char, string) {
return string.split('').reduce((acc, ch) => ch === char ? acc + 1: acc, 0)
}
As was mentioned, String.prototype.split works much faster than String.prototype.replace.

If you are using lodash, the _.countBy method will do this:
_.countBy("abcda")['a'] //2
This method also work with array:
_.countBy(['ab', 'cd', 'ab'])['ab'] //2

ok, an other one with regexp - probably not fast, but short and better readable then others, in my case just '_' to count
key.replace(/[^_]/g,'').length
just remove everything that does not look like your char
but it does not look nice with a string as input

I have found that the best approach to search for a character in a very large string (that is 1 000 000 characters long, for example) is to use the replace() method.
window.count_replace = function (str, schar) {
return str.length - str.replace(RegExp(schar), '').length;
};
You can see yet another JSPerf suite to test this method along with other methods of finding a character in a string.

Performance of Split vs RegExp
var i = 0;
var split_start = new Date().getTime();
while (i < 30000) {
"1234,453,123,324".split(",").length -1;
i++;
}
var split_end = new Date().getTime();
var split_time = split_end - split_start;
i= 0;
var reg_start = new Date().getTime();
while (i < 30000) {
("1234,453,123,324".match(/,/g) || []).length;
i++;
}
var reg_end = new Date().getTime();
var reg_time = reg_end - reg_start;
alert ('Split Execution time: ' + split_time + "\n" + 'RegExp Execution time: ' + reg_time + "\n");

I made a slight improvement on the accepted answer, it allows to check with case-sensitive/case-insensitive matching, and is a method attached to the string object:
String.prototype.count = function(lit, cis) {
var m = this.toString().match(new RegExp(lit, ((cis) ? "gi" : "g")));
return (m != null) ? m.length : 0;
}
lit is the string to search for ( such as 'ex' ), and cis is case-insensitivity, defaulted to false, it will allow for choice of case insensitive matches.
To search the string 'I love StackOverflow.com' for the lower-case letter 'o', you would use:
var amount_of_os = 'I love StackOverflow.com'.count('o');
amount_of_os would be equal to 2.
If we were to search the same string again using case-insensitive matching, you would use:
var amount_of_os = 'I love StackOverflow.com'.count('o', true);
This time, amount_of_os would be equal to 3, since the capital O from the string gets included in the search.

Easiest way i found out...
Example-
str = 'mississippi';
function find_occurences(str, char_to_count){
return str.split(char_to_count).length - 1;
}
find_occurences(str, 'i') //outputs 4

Here is my solution. Lots of solution already posted before me. But I love to share my view here.
const mainStr = 'str1,str2,str3,str4';
const commaAndStringCounter = (str) => {
const commas = [...str].filter(letter => letter === ',').length;
const numOfStr = str.split(',').length;
return `Commas: ${commas}, String: ${numOfStr}`;
}
// Run the code
console.log(commaAndStringCounter(mainStr)); // Output: Commas: 3, String: 4
Here you find my REPL

I just did a very quick and dirty test on repl.it using Node v7.4. For a single character, the standard for loop is quickest:
Some code:
// winner!
function charCount1(s, c) {
let count = 0;
c = c.charAt(0); // we save some time here
for(let i = 0; i < s.length; ++i) {
if(c === s.charAt(i)) {
++count;
}
}
return count;
}
function charCount2(s, c) {
return (s.match(new RegExp(c[0], 'g')) || []).length;
}
function charCount3(s, c) {
let count = 0;
for(ch of s) {
if(c === ch) {
++count;
}
}
return count;
}
function perfIt() {
const s = 'Hello, World!';
const c = 'o';
console.time('charCount1');
for(let i = 0; i < 10000; i++) {
charCount1(s, c);
}
console.timeEnd('charCount1');
console.time('charCount2');
for(let i = 0; i < 10000; i++) {
charCount2(s, c);
}
console.timeEnd('charCount2');
console.time('charCount3');
for(let i = 0; i < 10000; i++) {
charCount2(s, c);
}
console.timeEnd('charCount3');
}
Results from a few runs:
perfIt()
charCount1: 3.301ms
charCount2: 11.652ms
charCount3: 174.043ms
undefined
perfIt()
charCount1: 2.110ms
charCount2: 11.931ms
charCount3: 177.743ms
undefined
perfIt()
charCount1: 2.074ms
charCount2: 11.738ms
charCount3: 152.611ms
undefined
perfIt()
charCount1: 2.076ms
charCount2: 11.685ms
charCount3: 154.757ms
undefined
Update 2021-Feb-10: Fixed typo in repl.it demo
Update 2020-Oct-24: Still the case with Node.js 12 (play with it yourself here)

UPDATE 06/10/2022
So I ran various perf tests and if your use case allows it, it seems that using split is going to perform the best overall.
function countChar(char: string, string: string): number {
return string.split(char).length - 1
}
countChar('x', 'foo x bar x baz x')
I know I am late to the party here but I was rather baffled no one answered this with the most basic of approaches. A large portion of the answers provided by the community for this question are iteration based but all are moving over strings on a per-character basis which is not really efficient.
When dealing with a large string that contains thousands of characters walking over each character to get the occurance count can become rather extraneous not to mention a code-smell. The below solutions take advantage of slice, indexOf and the trusted traditional while loop. These approaches prevent us having to walk over each character and will greatly speed up the time it takes to count occurances. These follow similar logic to that you'd find in parsers and lexical analyzers that require string walks.
Using with Slice
In this approach we are leveraging slice and with every indexOf match we will move our way through the string and eliminate the previous searched potions. Each time we call indexOf the size of the string it searches will be smaller.
function countChar (char: string, search: string): number {
let num: number = 0;
let str: string = search;
let pos: number = str.indexOf(char);
while(pos > -1) {
str = str.slice(pos + 1);
pos = str.indexOf(char);
num++;
}
return num;
}
// Call the function
countChar('x', 'foo x bar x baz x') // 3
Using with IndexOf from position
Similar to the first approach using slice but instead of augmenting the string we are searching it will leverage the from parameter in indexOf method.
function countChar (char: string, str: string): number {
let num: number = 0;
let pos: number = str.indexOf(char);
while(pos > -1) {
pos = str.indexOf(char, pos + 1);
num++;
}
return num;
}
// Call the function
countChar('x', 'foo x bar x baz x') // 3
Personally, I go for the second approach over the first, but both are fine and performant when dealing with large strings but also smaller sized ones too.

s = 'dir/dir/dir/dir/'
for(i=l=0;i<s.length;i++)
if(s[i] == '/')
l++

I was working on a small project that required a sub-string counter. Searching for the wrong phrases provided me with no results, however after writing my own implementation I have stumbled upon this question. Anyway, here is my way, it is probably slower than most here but might be helpful to someone:
function count_letters() {
var counter = 0;
for (var i = 0; i < input.length; i++) {
var index_of_sub = input.indexOf(input_letter, i);
if (index_of_sub > -1) {
counter++;
i = index_of_sub;
}
}
http://jsfiddle.net/5ZzHt/1/
Please let me know if you find this implementation to fail or do not follow some standards! :)
UPDATE
You may want to substitute:
for (var i = 0; i < input.length; i++) {
With:
for (var i = 0, input_length = input.length; i < input_length; i++) {
Interesting read discussing the above:
http://www.erichynds.com/blog/javascript-length-property-is-a-stored-value

What about string.split(desiredCharecter).length-1
Example:
var str = "hellow how is life";
var len = str.split("h").length-1; will give count 2 for character "h" in the above string;

The fastest method seems to be via the index operator:
function charOccurances (str, char)
{
for (var c = 0, i = 0, len = str.length; i < len; ++i)
{
if (str[i] == char)
{
++c;
}
}
return c;
}
console.log( charOccurances('example/path/script.js', '/') ); // 2
Or as a prototype function:
String.prototype.charOccurances = function (char)
{
for (var c = 0, i = 0, len = this.length; i < len; ++i)
{
if (this[i] == char)
{
++c;
}
}
return c;
}
console.log( 'example/path/script.js'.charOccurances('/') ); // 2

function len(text,char){
return text.innerText.split(string).length
}
console.log(len("str1,str2,str3,str4",","))
This is a very short function.

The following uses a regular expression to test the length. testex ensures you don't have 16 or greater consecutive non-comma characters. If it passes the test, then it proceeds to split the string. counting the commas is as simple as counting the tokens minus one.
var mainStr = "str1,str2,str3,str4";
var testregex = /([^,]{16,})/g;
if (testregex.test(mainStr)) {
alert("values must be separated by commas and each may not exceed 15 characters");
} else {
var strs = mainStr.split(',');
alert("mainStr contains " + strs.length + " substrings separated by commas.");
alert("mainStr contains " + (strs.length-1) + " commas.");
}

I'm using Node.js v.6.0.0 and the fastest is the one with index (the 3rd method in Lo Sauer's answer).
The second is:
function count(s, c) {
var n = 0;
for (let x of s) {
if (x == c)
n++;
}
return n;
}

And there is:
function character_count(string, char, ptr = 0, count = 0) {
while (ptr = string.indexOf(char, ptr) + 1) {count ++}
return count
}
Works with integers too!

Here's one just as fast as the split() and the replace methods, which are a tiny bit faster than the regex method (in Chrome and Firefox both).
let num = 0;
let str = "str1,str2,str3,str4";
//Note: Pre-calculating `.length` is an optimization;
//otherwise, it recalculates it every loop iteration.
let len = str.length;
//Note: Don't use a `for (... of ...)` loop, it's slow!
for (let charIndex = 0; charIndex < len; ++charIndex) {
if (str[charIndex] === ',') {
++num;
}
}

var mainStr = "str1,str2,str3,str4";
var splitStr = mainStr.split(",").length - 1; // subtracting 1 is important!
alert(splitStr);
Splitting into an array gives us a number of elements, which will always be 1 more than the number of instances of the character. This may not be the most memory efficient, but if your input is always going to be small, this is a straight-forward and easy to understand way to do it.
If you need to parse very large strings (greater than a few hundred characters), or if this is in a core loop that processes large volumes of data, I would recommend a different strategy.

String.prototype.reduce = Array.prototype.reduce;
String.prototype.count = function(c) {
return this.reduce(((n, x) => n + (x === c ? 1 : 0)), 0)
};
const n = "bugs bunny was here".count("b")
console.log(n)
Similar to the prototype based above, but does not allocate an array for the string. Allocation is the problem of nearly every version above, except the loop variants. This avoids loop code, reusing the browser implemented Array.reduce function.

My solution:
function countOcurrences(str, value){
var regExp = new RegExp(value, "gi");
return str.match(regExp) ? str.match(regExp).length : 0;
}

I know this might be an old question but I have a simple solution for low-level beginners in JavaScript.
As a beginner, I could only understand some of the solutions to this question so I used two nested FOR loops to check each character against every other character in the string, incrementing a count variable for each character found that equals that character.
I created a new blank object where each property key is a character and the value is how many times each character appeared in the string(count).
Example function:-
function countAllCharacters(str) {
var obj = {};
if(str.length!==0){
for(i=0;i<str.length;i++){
var count = 0;
for(j=0;j<str.length;j++){
if(str[i] === str[j]){
count++;
}
}
if(!obj.hasOwnProperty(str[i])){
obj[str[i]] = count;
}
}
}
return obj;
}