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Rooted by: Non Repeating Character Optimization
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  • Code 
    const firstNonRepeatingCharacter = (str) => {
  let count = str.split('').reduce((obj, char) => {
    obj[char] = ++obj[char] || 1;
    return obj;
  }, {});

  for (const key in count) {
    if (count[key] === 1) {
      return key;
    }
  }

  return null;
};
     Test Cases 
    // Since Node 10, we're using Mocha.
// You can use `chai` for assertions.
const chai = require("chai");
const assert = chai.assert;
// Uncomment the following line to disable truncating failure messages for deep equals, do:
// chai.config.truncateThreshold = 0;
// Since Node 12, we no longer include assertions from our deprecated custom test framework by default.
// Uncomment the following to use the old assertions:
// const Test = require("@codewars/test-compat");

describe("Solution", function() {
  it('should return the first non-repeating character', function() {
        assert.strictEqual(firstNonRepeatingCharacter('abcd'), 'a');
        assert.strictEqual(firstNonRepeatingCharacter('aabbcdc'), 'd');
    });

    it('should return the first non-repeating character when special characters are included', function() {
        assert.strictEqual(firstNonRepeatingCharacter('a@b@bc'), 'a');
    });

    it('should return null when all characters are repeating', function() {
        assert.strictEqual(firstNonRepeatingCharacter('aabbcc'), null);
    });

    it('should return the first character if it is the only one', function() {
        assert.strictEqual(firstNonRepeatingCharacter('z'), 'z');
    });

    it('should handle an empty string correctly', function() {
        assert.strictEqual(firstNonRepeatingCharacter(''), null);
    });

    it('should handle strings with numbers', function() {
        assert.strictEqual(firstNonRepeatingCharacter('1122a'), 'a');
    });
  
    it('should handle very long strings with the non-repeating character at the end', function() {
        const longString = 'a'.repeat(10000) + 'b'.repeat(10000) + 'c';
        assert.strictEqual(firstNonRepeatingCharacter(longString), 'c');
    });

    it('should handle very long strings with all characters repeating', function() {
        const longString = 'a'.repeat(20000) + 'b'.repeat(20000);
        assert.strictEqual(firstNonRepeatingCharacter(longString), null);
    });
});
    View Run Output
  • Code
    • const firstNonRepeatingCharacter = (str) => {
    • let chars = [], counts = [];
    • for(const char of str) {
    • let index = chars.indexOf(char);
    • if(index < 0) {
    • index = counts.length;
    • chars.push(char);
    • counts.push(0);
    • }
    • counts[index]++;
    • }
    • for(let index = 0; index < chars.length; index++) {
    • if(counts[index] === 1) {
    • return chars[index];
    • let count = str.split('').reduce((obj, char) => {
    • obj[char] = ++obj[char] || 1;
    • return obj;
    • }, {});
    • for (const key in count) {
    • if (count[key] === 1) {
    • return key;
    • }
    • }
    • return null;
    • };