How do I sort characters of a string by appearance order?

I have the following code:

function main() {
  let input = prompt("Enter a string:").toLowerCase().replace(/\s/g, "");
  let process = "";

  for (let i = 0; i < input.length; i++) {
    let word = input[i];
    let repeatWord = "";
    for (let j = 0; j < word.length; j++) {
      repeatWord += word[j].repeat(j + 1);
    }
    process += repeatWord;
  }
  console.log(process);
}

How do I sort the characters of the string by order of first appearance?

For example, if I enter the string: "Learning Center", then the expected result should be "leeearrnnigct".

Use a Map to store a counter of all the characters in your string. Then, since Map preserves insertion order, you can simply iterate through the counter and use each key’s value to regenerate the string via the String repeat() method and the Array join() method.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function appearanceSort(str) {
  str = str.toLowerCase().replace(/\s/g, "");

  const counter = new Map();
  for (const char of str) {
    if (counter.has(char)) {
      counter.set(char, counter.get(char) + 1);
    } else {
      counter.set(char, 1);
    }
  }

  const strArr = [];
  for (const [key, value] of counter) {
    strArr.push(key.repeat(value));
  }

  return strArr.join("");
}

console.log(appearanceSort("Learning Center")); // leeearrnnigct

<!-- end snippet -->

However, the implementation above most likely won't work as expected with arbitrary unicode characters. To support grapheme clusters, you can use a library such as grapheme-splitter, or the browser built-in Intl.Segmenter, which looks like so:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function appearanceSort(str) {
  str = str.toLowerCase().replace(/\s/g, "");

  const segments = Array.from(new Intl.Segmenter().segment(str)).map(({
    segment
  }) => segment);

  const counter = new Map();
  for (const char of segments) {
    if (counter.has(char)) {
      counter.set(char, counter.get(char) + 1);
    } else {
      counter.set(char, 1);
    }
  }

  const strArr = [];
  for (const [key, value] of counter) {
    strArr.push(key.repeat(value));
  }

  return strArr.join("");
}

console.log(appearanceSort("👨‍👨‍👧‍👦 Learning 👨‍👨‍👧‍👦 Center")); // 👨‍👨‍👧‍👦👨‍👨‍👧‍👦leeearrnnnigct

<!-- end snippet -->

Note: Intl.Segmenter is currently not supported in Firefox. Use "Segmenter" in Intl to check for browser support and conditionally apply some fallback behavior.

My approach would be to create an array with all unique characters. Then iterate over it and count how many siblings each character has and then repeat the character as often as it appears.

function main(input: string) {
    const arr = input.toLocaleLowerCase().replace(/[ ]/g, '').split("");
    let uniqueLetters = [...new Set(arr)]
    let word = "";
    for (let i = 0; i < uniqueLetters.length; i++) {
        const letter = uniqueLetters[i];   
        const letterCount = arr.filter((l) => l == letter).length;
        word += letter.repeat(letterCount)
    }
    return word;
}

console.log(main("Learning Center")) // prints: leeearrnnnigct

Here is yet another version. This one will keep blanks and distinguish between upper/lower case letters:

<!-- begin snippet:js console:true -->
<!-- language:lang-js -->
const str="Learning Center".split(""),
freq={};
str.forEach(c=>freq[c]=(freq[c]??0)+1);
const res=str.map(c=>{
let s=c.repeat(freq[c]);
freq[c]=0;
return s
}).join("");

console.log(res);

<!-- end snippet -->

If you prefer a case insensitive solution without blanks then simply modify it to:

<!-- begin snippet:js console:true -->
<!-- language:lang-js -->
const str="Learning Center".toLowerCase().replaceAll(" ","").split(""),
freq={};
str.forEach(c=>freq[c]=(freq[c]??0)+1);
const res=str.map(c=>{
let s=c.repeat(freq[c]);
freq[c]=0;
return s
}).join("");

console.log(res);

<!-- end snippet -->

When you run through each characters in the string, create a new index and then use a third variable to swap the characters.

You can try the code below.

function suffle(target) {
  var chars = target.split("");
  var length = chars.length;

  for (var i = length - 1; i > 0; i--) {
    var j = Math.floor(Math.random() * (i + 1));
    var tmp = chars[i];
    chars[i] = chars[j];
    chars[j] = tmp;
  }

  return chars.join("");
}

console.log(suffle("hello"));