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Bringing numbers to the front

(Javascript)
functionName(“2 plus 3 is = 5!”);
would produce following message in console:
235 plus is =
I am unable to bring the numbers to the front, I am a little stumped
function frontNum(str) {
let emptyArr = [];
let rev = str.split("");
let expression = /[\d]/g;
for(let i = 0; i < rev.length; i++) {
if (rev[i].match(expression)) {
emptyArr += rev.pop(rev[i]);
}
}
console.log(emptyArr + rev.join(''));
}
frontNum("2 plus 3 is = 5!");

Since this is your home work I won't give you the correct version, but give you some pointers instead:
your emptyArr is an array, but you are adding data to it as if it was a string.
take a look at this topic, your pop causing problems
you can use your expression to capture all the digits and remove them from the string without need converting the string into array and loop through it (it can be done with 2 lines of code)

a way todo that
'use strict'
function frontNum(str)
{
let s = ''
, n = ''
for (let x of str)
{
if (/[0-9]/.test(x)) n += x
else s += x
}
console.log( n + s.replace(/ +/g,' ') )
}
frontNum('2 plus 3 is = 5!')

Joining numbers together with a string

I know that for an array join() can be used to produce what I am trying to accomplish here, but I am working with a string. What method would work with a string?
I want my output to look like "3 then 4 then 5 then 6 then 7", etc.
I've come close to getting what I am looking for but my current code adds an extra "then" at the end, which is not what I want:
let appendString = '';
let then = ' then ';
function countUp(start) {
for(var i = 0; i < 10; i++){
appendString += (start++) + then;
}
console.log(appendString);
}
I do not want solutions, I just would appreciate being pointed in the right direction.

I will try to not spoil the fun for you.
Think of it like this. If you have an array that has all your numbers, can you join them using .join ?
Now the question will be how to initialise an array with the numbers you want.
Try looking into array initialisations.
Does this answer your question ?

You are right that with an array you can use join. The thing is that you can turn a series of sequential numbers to an array easily:
function countUp(start, end, then) {
let arr = Array.from({length: end-start+1}, (_, i) => start + i);
return arr.join(then);
}
console.log(countUp(1, 10, ' then '));

what about this?
let appendString = '';
let then = ' then ';
function countUp(start) {
for(var i = 0; i < 10; i++){
appendString += (start++)
if(i<9){
appendString+=then
}
}
console.log(appendString);
}
or
let appendArray = [];
let then = ' then ';
function countUp(start) {
for(var i = 0; i < 10; i++){
appendArray.push(start++);
}
console.log(appendArray.join(then));
}

Unable to decode/encode Url properly

I am working on a Url decoding and encoding system. But for some odd reason, it will only decode/encode certain string. Also, it seems to decode/encode a string when it has a certain piece in it. It is quite hard to explain but it is quite confusion as to what this might not work. I tried figuring out the problem of it but it just makes the whole issue seem more illogical than logical.
I hope someone can help me with this. With explanation to it. I would like it if the code is in the same style as it is as well.
I also know that there are probably some packages online to easily do that but I would rather just make my own. It's a way I can practice JS more.
// I know I don't have all the characters markek. I am doing that later
var Url = {
filterEncode : ["%2B","%3A","%3D","%3F","%2F","%26","%252F","%253A","%253D","%253F","%252B"],
filterDecode : ["+",":","=","?","/","&","%2F","%3A","%3D","%3F","%2B"],
decode : function(decodeText){
let returnString, a, b;
let filterEncode = Url.filterEncode;
let filterDecode = Url.filterDecode;
for (a = 0; a < filterEncode.length; a++){
let regexEn = new RegExp(filterEncode[a],"g");
let regexDe = new RegExp("/" + filterDecode[a],"g");
let regex = new RegExp(regexEn,"g");
let array = (decodeText.match(regex) || []).length
for (b = 0; b < array; b++){
returnString = decodeText.replace(filterEncode[a],filterDecode[a]);
decodeText = returnString;
}
}
return returnString;
},
encode : function(encodeText){
let returnString, a, b;
let filterEncode = Url.filterEncode;
let filterDecode = Url.filterDecode;
for (a = 0; a < filterEncode.length; a++){
let regexEn = new RegExp("[" + filterEncode[2] + "]","g");
let regexDe = new RegExp("[" + filterDecode[2] + "]","g");
let regex = new RegExp(regexEn,"g");
let array = (encodeText.match(regex) || []).length;
for (b = 0; b < array; b++){
returnString = encodeText.replace(filterDecode[a],filterEncode[a]);
encodeText = returnString;
}
}
return returnString;
}
}
// Saying it is undefined
console.log(Url.encode("="));
// Encodes it just find
console.log(Url.encode("%3F"));
// Encodes both of them but for some odd reason encodes the
// equal sign twice.
console.log(Url.encode("%3F ="));
I do hope everything seems clear as to what my problem is about. I usually would just try searching on here for an answer, but this problem is so confusion, I don't know what I exactly should search for.
Thanks!

Some of your strings in filterDecode have special meaning as regular expressions. When converting the string to a regular expression, you need to wrap each character in [] so it will be matched literally.
You don't need to concatenate / when creating regexDe.
There's no need for the for(b...) loops. Use the regular expression in the replace() call and it will perform all the replacements at once, since it has the g flag.
Put the encode strings that contain % at the beginning of the array. Otherwise, when you encode something like = as %3D, a later iteration of the outer loop will re-encode that as %253D. You only want to encode this if it was in the original string, not an intermediate step.
var Url = {
filterDecode: ["%252F", "%253A", "%253D", "%253F", "%252B", "%2B", "%3A", "%3D", "%3F", "%2F", "%26"],
filterEncode: ["%2F", "%3A", "%3D", "%3F", "%2B", "+", ":", "=", "?", "/", "&"],
strToRe: function(str) {
let reStr = str.split("").map(c => '[' + c + ']').join('');
return new RegExp(reStr, "g");
},
decode: function(decodeText) {
let a;
let filterEncode = Url.filterEncode;
let filterDecode = Url.filterDecode;
for (a = 0; a < filterDecode.length; a++) {
decodeText = decodeText.replace(Url.strToRe(filterDecode[a]), filterEncode[a]);
}
return decodeText;
},
encode: function(encodeText) {
let a, b;
let filterEncode = Url.filterEncode;
let filterDecode = Url.filterDecode;
for (a = 0; a < filterEncode.length; a++) {
encodeText = encodeText.replace(Url.strToRe(filterEncode[a]), filterDecode[a]);
}
return encodeText;
}
}
console.log(Url.encode("="));
console.log(Url.decode("%3D"));
console.log(Url.encode("%3F"));
console.log(Url.decode("%253F"));
console.log(Url.encode("%3F ="));
console.log(Url.decode("%253F %3D"));

what causes the infinite loop in splice method?

I have made a code that inserts '-' in front of upper case
const caseSwitch = (str) => {
let strArr = [...str];
for(let i = 0; i < strArr.length; i++) {
if(strArr[i] === strArr[i].toUpperCase()) {
strArr.splice(i, 1, '-' + strArr[i]);
}
} return strArr.join('')
}
caseSwitch('camelCaseSnakeCase'); // 'camel-Case-Snake-Case'
but one thing made me really curious. In the splice part, when I change the number from 1 to 0
strArr.splice(i, 0, '-' + strArr[i]);
the warning sign 'RangeError: Potential infinite loop.' pops up.
but when I used 0, in another splice method that inserts '-' between two even numbers:
function insertHyphen(str) {
var strArr = str.split('');
var numArr = strArr.map(Number);
for(var i = 0; i < numArr.length; i++) {
if(numArr[i-1]%2===0 && numArr[i]%2===0) {
numArr.splice(i, 0, '-');
}
}
return numArr.join('');
}
insertHyphen('112233445566'); //'112-2334-4556-6'
it works just fine. But don't know why two same method work different in both codes.

Much easier, match all uppercase letters and replace it with itself prefixed with a -
const caseSwitch = (str) => str.replace(/([A-Z])/g, "-$1");
console.log(caseSwitch('camelCaseSnakeCase'));

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;
}