How to compress a string in JavaScript

JavaScript, algorithms, strings

Here is one way to compress a string in JavaScript.
Space complexity: O(n) for storing a compressed version of the string
Time complexity: O(n). We must iterate through the entire string at least once.

compressStringTry in REPL
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function compress(string) {
    if (typeof string !== 'string') return null;
    if (string === '') return '';
    let trackingChar = string[0];
    let charCount = 0;
    let compressed = '';
    
    for (let i = 0; i < string.length; i++) {
        const char = string[i];
        if (char === trackingChar) {
            charCount++;
    } else if (char !== trackingChar) {
            compressed += trackingChar + (charCount === 1 ? '' : '' + charCount);   
            trackingChar = char;
            charCount = 1;
    }
  }
  
  compressed += trackingChar + (charCount === 1 ? '' : '' + charCount);
 
    return compressed.length < string.length ? compressed : string;
}
// ==== Tests
function runTests(func) {
  const inputOutput = [[null, null], ['', ''], ['AABBCC', 'AABBCC'], ['AAABCCDDDD', 'A3BC2D4'], ['BAAACCDDDD', 'BA3C2D4'], ['AAABAACCDDDD', 'A3BA2C2D4']];
  
  for (let i = 0; i < inputOutput.length; i++) {
    const input = inputOutput[i].slice(0, inputOutput[i].length-1);
  
    const expected = inputOutput[i][inputOutput[i].length-1];
    const result = func(...input);
    
    if (result !== expected) {
      return `Test failed: Expected - ${expected}. Got ${result}`;
    }
  }
  return 'All tests passed!'
}
runTests(compress); // All tests passed
// Test cases
// compress(null); // null
// compress(''); // ''
// compress('AABBCC') // 'AABBCC'
// compress('AAABCCDDDD') // 'A3BC2D4'
// compress('BAAACCDDDD') // 'BA3C2D4'
// compress('AAABAACCDDDD') // 'A3BA2C2D4'