JavaScriptConcepts/HigherOrderFunction/polyfills.html at main · adityaSrivastava29/JavaScriptConcepts · GitHub

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Array Polyfills</title>
    <style>
      body {
        font-family: "Monaco", "Menlo", monospace;
        max-width: 1200px;
        margin: 0 auto;
        padding: 20px;
        background: #1a1a1a;
        color: #e0e0e0;
        line-height: 1.6;
      }

      .container {
        display: grid;
        grid-template-columns: 1fr;
        gap: 30px;
        margin-bottom: 30px;
      }
      .method-section {
        background: #2a2a2a;
        padding: 20px;
        border-radius: 10px;
        border-left: 4px solid #4caf50;
      }

      .method-section h2 {
        color: #4caf50;
        margin-top: 0;
        display: flex;
        align-items: center;
        gap: 10px;
      }

      .code-block {
        background: #1e1e1e;
        padding: 15px;
        border-radius: 8px;
        margin: 15px 0;
        overflow-x: auto;
        border: 1px solid #333;
      }

      .code-block pre {
        margin: 0;
        color: #f8f8f2;
      }

      .keyword {
        color: #ff79c6;
      }

      .function {
        color: #50fa7b;
      }

      .string {
        color: #f1fa8c;
      }

      .comment {
        color: #6272a4;
      }

      .number {
        color: #bd93f9;
      }

      .test-section {
        background: #2a2a2a;
        padding: 20px;
        border-radius: 10px;
        margin-top: 20px;
        grid-column: 1 / -1;
      }

      .test-section h2 {
        color: #ff6b6b;
        margin-top: 0;
      }

      button {
        background: #4caf50;
        color: white;
        border: none;
        padding: 10px 20px;
        border-radius: 5px;
        cursor: pointer;
        margin: 5px;
        font-family: inherit;
      }

      button:hover {
        background: #45a049;
      }

      .output {
        background: #1e1e1e;
        padding: 15px;
        border-radius: 8px;
        margin-top: 15px;
        min-height: 100px;
        border: 1px solid #333;
      }

      .concept {
        background: #1e3a8a;
        padding: 15px;
        border-radius: 8px;
        margin: 15px 0;
        color: #bfdbfe;
      }

      .concept h3 {
        color: #60a5fa;
        margin-top: 0;
      }
    </style>
  </head>

  <body>
    <h1>🚀 Array Method Polyfills: Understanding the Magic</h1>

    <div class="container">
      <!-- KEY CONCEPTS SECTION -->
      <div class="test-section" style="border-left: 4px solid #ff6b6b">
        <h2>🎯 Key Concepts to Remember</h2>

        <div
          style="
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 20px;
            margin-bottom: 20px;
          "
        >
          <div class="concept">
            <h3>1. The Pattern</h3>
            <p>All three polyfills follow the same structure:</p>
            <ul>
              <li>Validate the callback function</li>
              <li>
                Create a result container (array for map/filter, any type for
                reduce)
              </li>
              <li>Loop through the original array</li>
              <li>Apply the callback function with proper parameters</li>
              <li>Return the result</li>
            </ul>
          </div>

          <div class="concept">
            <h3>2. Important Details</h3>
            <ul>
              <li>
                <strong>Sparse arrays:</strong> We check
                <code>i in this</code> to handle arrays with missing indices
              </li>
              <li>
                <strong>thisArg parameter:</strong> We use
                <code>callback.call(thisArg, ...)</code> to set the
                <code>this</code> context
              </li>
              <li>
                <strong>Callback parameters:</strong> Each method passes
                different parameters:
                <ul>
                  <li>map/filter: <code>(element, index, array)</code></li>
                  <li>
                    reduce: <code>(accumulator, element, index, array)</code>
                  </li>
                </ul>
              </li>
            </ul>
          </div>

          <div class="concept">
            <h3>3. The Mental Models</h3>
            <ul>
              <li>
                <strong>Map:</strong> Transform each item → Same length,
                different values
              </li>
              <li>
                <strong>Filter:</strong> Keep some items → Shorter or same
                length, same values
              </li>
              <li>
                <strong>Reduce:</strong> Combine all items → Single result of
                any type
              </li>
            </ul>
          </div>

          <div class="concept">
            <h3>4. Why This Matters</h3>
            <p>Understanding polyfills helps you:</p>
            <ul>
              <li>Debug issues with these methods</li>
              <li>Understand performance implications</li>
              <li>Write more efficient code</li>
              <li>Handle edge cases better</li>
              <li>Appreciate what the browser does for you!</li>
            </ul>
          </div>
        </div>
      </div>
      <!-- MAP POLYFILL -->
      <div class="method-section">
        <h2>🗺️ Array.prototype.map</h2>

        <div class="concept">
          <h3>Concept:</h3>
          <p>
            Map creates a NEW array by transforming each element using a
            callback function. It's like having a factory that processes each
            item and outputs a modified version.
          </p>
        </div>

        <div class="code-block">
          <pre><code><span class="comment">// Our custom map polyfill</span>
<span class="keyword">Array.prototype.</span><span class="function">myMap</span> = <span class="keyword">function</span>(<span class="function">callback</span>, <span class="function">thisArg</span>) {
    <span class="comment">// Check if callback is a function</span>
    <span class="keyword">if</span> (<span class="keyword">typeof</span> callback !== <span class="string">'function'</span>) {
        <span class="keyword">throw new</span> TypeError(callback + <span class="string">' is not a function'</span>);
    }

    <span class="comment">// Create new array to store results</span>
    <span class="keyword">const</span> result = [];

    <span class="comment">// Loop through each element</span>
    <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i < <span class="keyword">this</span>.length; i++) {
        <span class="comment">// Check if index exists (sparse arrays)</span>
        <span class="keyword">if</span> (i <span class="keyword">in</span> <span class="keyword">this</span>) {
            <span class="comment">// Call callback with: element, index, array</span>
            result[i] = callback.call(thisArg, <span class="keyword">this</span>[i], i, <span class="keyword">this</span>);
        }
    }

    <span class="keyword">return</span> result;
};</code></pre>
        </div>
      </div>

      <!-- FILTER POLYFILL -->
      <div class="method-section">
        <h2>🔍 Array.prototype.filter</h2>

        <div class="concept">
          <h3>Concept:</h3>
          <p>
            Filter creates a NEW array containing only elements that pass a
            test. Think of it as a bouncer that only lets certain items through
            based on your criteria.
          </p>
        </div>

        <div class="code-block">
          <pre><code><span class="comment">// Our custom filter polyfill</span>
<span class="keyword">Array.prototype.</span><span class="function">myFilter</span> = <span class="keyword">function</span>(<span class="function">callback</span>, <span class="function">thisArg</span>) {
    <span class="comment">// Check if callback is a function</span>
    <span class="keyword">if</span> (<span class="keyword">typeof</span> callback !== <span class="string">'function'</span>) {
        <span class="keyword">throw new</span> TypeError(callback + <span class="string">' is not a function'</span>);
    }

    <span class="comment">// Create new array for filtered results</span>
    <span class="keyword">const</span> result = [];

    <span class="comment">// Loop through each element</span>
    <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i < <span class="keyword">this</span>.length; i++) {
        <span class="comment">// Check if index exists</span>
        <span class="keyword">if</span> (i <span class="keyword">in</span> <span class="keyword">this</span>) {
            <span class="comment">// Test element with callback</span>
            <span class="keyword">if</span> (callback.call(thisArg, <span class="keyword">this</span>[i], i, <span class="keyword">this</span>)) {
                <span class="comment">// Only add if test passes</span>
                result.push(<span class="keyword">this</span>[i]);
            }
        }
    }

    <span class="keyword">return</span> result;
};</code></pre>
        </div>
      </div>

      <!-- REDUCE POLYFILL -->
      <div class="method-section">
        <h2>⚡ Array.prototype.reduce</h2>

        <div class="concept">
          <h3>Concept:</h3>
          <p>
            Reduce takes an array and "reduces" it to a single value by applying
            a function that combines elements. Like a snowball rolling downhill,
            gathering more snow (data) as it goes.
          </p>
        </div>

        <div class="code-block">
          <pre><code><span class="comment">// Our custom reduce polyfill</span>
<span class="keyword">Array.prototype.</span><span class="function">myReduce</span> = <span class="keyword">function</span>(<span class="function">callback</span>, <span class="function">initialValue</span>) {
    <span class="comment">// Check if callback is a function</span>
    <span class="keyword">if</span> (<span class="keyword">typeof</span> callback !== <span class="string">'function'</span>) {
        <span class="keyword">throw new</span> TypeError(callback + <span class="string">' is not a function'</span>);
    }

    <span class="comment">// Handle empty arrays</span>
    <span class="keyword">if</span> (<span class="keyword">this</span>.length === <span class="number">0</span> && arguments.length < <span class="number">2</span>) {
        <span class="keyword">throw new</span> TypeError(<span class="string">'Reduce of empty array with no initial value'</span>);
    }

    <span class="keyword">let</span> accumulator;
    <span class="keyword">let</span> startIndex = <span class="number">0</span>;

    <span class="comment">// Set up initial values</span>
    <span class="keyword">if</span> (arguments.length >= <span class="number">2</span>) {
        accumulator = initialValue;
    } <span class="keyword">else</span> {
        <span class="comment">// Find first valid element as initial value</span>
        <span class="keyword">while</span> (startIndex < <span class="keyword">this</span>.length && !(startIndex <span class="keyword">in</span> <span class="keyword">this</span>)) {
            startIndex++;
        }
        accumulator = <span class="keyword">this</span>[startIndex];
        startIndex++;
    }

    <span class="comment">// Process each element</span>
    <span class="keyword">for</span> (<span class="keyword">let</span> i = startIndex; i < <span class="keyword">this</span>.length; i++) {
        <span class="keyword">if</span> (i <span class="keyword">in</span> <span class="keyword">this</span>) {
            <span class="comment">// Call callback with: accumulator, element, index, array</span>
            accumulator = callback(accumulator, <span class="keyword">this</span>[i], i, <span class="keyword">this</span>);
        }
    }

    <span class="keyword">return</span> accumulator;
};</code></pre>
        </div>
      </div>
    </div>

    <!-- EDITABLE POLYFILLS SECTION -->
    <div class="test-section" style="border-left: 4px solid #ffd700">
      <h2>✏️ Edit & Test Your Own Polyfills</h2>
      <p>Modify the polyfills below and test your changes instantly!</p>

      <div
        style="
          display: grid;
          grid-template-columns: 1fr 1fr 1fr;
          gap: 15px;
          margin-bottom: 20px;
        "
      >
        <div>
          <h3 style="color: #4caf50; margin-bottom: 10px">Map Polyfill</h3>
          <textarea
            id="mapPolyfill"
            style="
              width: 100%;
              height: 200px;
              background: #1e1e1e;
              color: #f8f8f2;
              border: 1px solid #333;
              border-radius: 5px;
              padding: 10px;
              font-family: Monaco, monospace;
              font-size: 12px;
              resize: vertical;
            "
          >
Array.prototype.myMap = function(callback, thisArg) {
    if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
    }

    const result = [];

    for (let i = 0; i < this.length; i++) {
        if (i in this) {
            result[i] = callback.call(thisArg, this[i], i, this);
        }
    }

    return result;
};</textarea
          >
        </div>

        <div>
          <h3 style="color: #4caf50; margin-bottom: 10px">Filter Polyfill</h3>
          <textarea
            id="filterPolyfill"
            style="
              width: 100%;
              height: 200px;
              background: #1e1e1e;
              color: #f8f8f2;
              border: 1px solid #333;
              border-radius: 5px;
              padding: 10px;
              font-family: Monaco, monospace;
              font-size: 12px;
              resize: vertical;
            "
          >
Array.prototype.myFilter = function(callback, thisArg) {
    if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
    }

    const result = [];

    for (let i = 0; i < this.length; i++) {
        if (i in this) {
            if (callback.call(thisArg, this[i], i, this)) {
                result.push(this[i]);
            }
        }
    }

    return result;
};</textarea
          >
        </div>

        <div>
          <h3 style="color: #4caf50; margin-bottom: 10px">Reduce Polyfill</h3>
          <textarea
            id="reducePolyfill"
            style="
              width: 100%;
              height: 200px;
              background: #1e1e1e;
              color: #f8f8f2;
              border: 1px solid #333;
              border-radius: 5px;
              padding: 10px;
              font-family: Monaco, monospace;
              font-size: 12px;
              resize: vertical;
            "
          >
Array.prototype.myReduce = function(callback, initialValue) {
    if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
    }

    if (this.length === 0 && arguments.length < 2) {
        throw new TypeError('Reduce of empty array with no initial value');
    }

    let accumulator;
    let startIndex = 0;

    if (arguments.length >= 2) {
        accumulator = initialValue;
    } else {
        while (startIndex < this.length && !(startIndex in this)) {
            startIndex++;
        }
        accumulator = this[startIndex];
        startIndex++;
    }

    for (let i = startIndex; i < this.length; i++) {
        if (i in this) {
            accumulator = callback(accumulator, this[i], i, this);
        }
    }

    return accumulator;
};</textarea
          >
        </div>
      </div>

      <div style="text-align: center; margin-bottom: 20px">
        <button
          onclick="updatePolyfills()"
          style="background: #ffd700; color: #1a1a1a; font-weight: bold"
        >
          🔄 Update Polyfills
        </button>
        <button onclick="resetPolyfills()" style="background: #ff6b6b">
          ↩️ Reset to Original
        </button>
      </div>
    </div>

    <div class="test-section">
      <h2>🧪 Interactive Testing Ground</h2>
      <p>
        Click the buttons to see our polyfills in action and compare with native
        methods!
      </p>

      <div>
        <button onclick="testMap()">Test Map Polyfill</button>
        <button onclick="testFilter()">Test Filter Polyfill</button>
        <button onclick="testReduce()">Test Reduce Polyfill</button>
        <button onclick="runAllTests()">Run All Tests</button>
        <button onclick="clearOutput()">Clear Output</button>
      </div>

      <div class="output" id="output">
        Click any button above to see the polyfills in action!
      </div>
    </div>

    <script>
      // Our polyfill implementations (same as shown above)
      Array.prototype.myMap = function (callback, thisArg) {
        if (typeof callback !== "function") {
          throw new TypeError(callback + " is not a function");
        }

        const result = [];

        for (let i = 0; i < this.length; i++) {
          if (i in this) {
            result[i] = callback.call(thisArg, this[i], i, this);
          }
        }

        return result;
      };

      Array.prototype.myFilter = function (callback, thisArg) {
        if (typeof callback !== "function") {
          throw new TypeError(callback + " is not a function");
        }

        const result = [];

        for (let i = 0; i < this.length; i++) {
          if (i in this) {
            if (callback.call(thisArg, this[i], i, this)) {
              result.push(this[i]);
            }
          }
        }

        return result;
      };

      Array.prototype.myReduce = function (callback, initialValue) {
        if (typeof callback !== "function") {
          throw new TypeError(callback + " is not a function");
        }

        if (this.length === 0 && arguments.length < 2) {
          throw new TypeError("Reduce of empty array with no initial value");
        }

        let accumulator;
        let startIndex = 0;

        if (arguments.length >= 2) {
          accumulator = initialValue;
        } else {
          while (startIndex < this.length && !(startIndex in this)) {
            startIndex++;
          }
          accumulator = this[startIndex];
          startIndex++;
        }

        for (let i = startIndex; i < this.length; i++) {
          if (i in this) {
            accumulator = callback(accumulator, this[i], i, this);
          }
        }

        return accumulator;
      };

      const output = document.getElementById("output");

      function log(message) {
        output.innerHTML += message + "<br>";
        output.scrollTop = output.scrollHeight;
      }

      function updatePolyfills() {
        try {
          // Get the code from textareas
          const mapCode = document.getElementById("mapPolyfill").value;
          const filterCode = document.getElementById("filterPolyfill").value;
          const reduceCode = document.getElementById("reducePolyfill").value;

          // Remove existing polyfills
          delete Array.prototype.myMap;
          delete Array.prototype.myFilter;
          delete Array.prototype.myReduce;

          // Execute the new code
          eval(mapCode);
          eval(filterCode);
          eval(reduceCode);

          log(
            '<span style="color: #ffd700; font-weight: bold;">✅ Polyfills updated successfully! Try testing them now.</span><br>'
          );
        } catch (error) {
          log(
            `<span style="color: #ff6b6b; font-weight: bold;">❌ Error updating polyfills: ${error.message}</span><br>`
          );
        }
      }

      function resetPolyfills() {
        // Reset textareas to original code
        document.getElementById(
          "mapPolyfill"
        ).value = `Array.prototype.myMap = function(callback, thisArg) {
    if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
    }

    const result = [];

    for (let i = 0; i < this.length; i++) {
        if (i in this) {
            result[i] = callback.call(thisArg, this[i], i, this);
        }
    }

    return result;
};`;

        document.getElementById(
          "filterPolyfill"
        ).value = `Array.prototype.myFilter = function(callback, thisArg) {
    if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
    }

    const result = [];

    for (let i = 0; i < this.length; i++) {
        if (i in this) {
            if (callback.call(thisArg, this[i], i, this)) {
                result.push(this[i]);
            }
        }
    }

    return result;
};`;

        document.getElementById(
          "reducePolyfill"
        ).value = `Array.prototype.myReduce = function(callback, initialValue) {
    if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
    }

    if (this.length === 0 && arguments.length < 2) {
        throw new TypeError('Reduce of empty array with no initial value');
    }

    let accumulator;
    let startIndex = 0;

    if (arguments.length >= 2) {
        accumulator = initialValue;
    } else {
        while (startIndex < this.length && !(startIndex in this)) {
            startIndex++;
        }
        accumulator = this[startIndex];
        startIndex++;
    }

    for (let i = startIndex; i < this.length; i++) {
        if (i in this) {
            accumulator = callback(accumulator, this[i], i, this);
        }
    }

    return accumulator;
};`;

        // Update the polyfills
        updatePolyfills();
        log(
          '<span style="color: #4CAF50; font-weight: bold;">🔄 Polyfills reset to original code!</span><br>'
        );
      }

      function testMap() {
        log(
          '<span style="color: #4CAF50; font-weight: bold;">🗺️ TESTING MAP POLYFILL:</span>'
        );

        const numbers = [1, 2, 3, 4, 5];
        log(`Original array: [${numbers.join(", ")}]`);

        // Test our polyfill
        const ourResult = numbers.myMap((x) => x * 2);
        log(`Our myMap(x => x * 2): [${ourResult.join(", ")}]`);

        // Compare with native
        const nativeResult = numbers.map((x) => x * 2);
        log(`Native map(x => x * 2): [${nativeResult.join(", ")}]`);

        // Test with index
        const withIndex = numbers.myMap((x, i) => `${x}-${i}`);
        log(
          `With index myMap((x, i) => x + '-' + i): [${withIndex.join(", ")}]`
        );

        log("✅ Map test complete!<br><br>");
      }

      function testFilter() {
        log(
          '<span style="color: #4CAF50; font-weight: bold;">🔍 TESTING FILTER POLYFILL:</span>'
        );

        const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
        log(`Original array: [${numbers.join(", ")}]`);

        // Test our polyfill
        const ourResult = numbers.myFilter((x) => x % 2 === 0);
        log(`Our myFilter(x => x % 2 === 0): [${ourResult.join(", ")}]`);

        // Compare with native
        const nativeResult = numbers.filter((x) => x % 2 === 0);
        log(`Native filter(x => x % 2 === 0): [${nativeResult.join(", ")}]`);

        // Test with strings
        const words = ["apple", "banana", "cherry", "date"];
        const longWords = words.myFilter((word) => word.length > 5);
        log(`Filter long words: [${longWords.join(", ")}]`);

        log("✅ Filter test complete!<br><br>");
      }

      function testReduce() {
        log(
          '<span style="color: #4CAF50; font-weight: bold;">⚡ TESTING REDUCE POLYFILL:</span>'
        );

        const numbers = [1, 2, 3, 4, 5];
        log(`Original array: [${numbers.join(", ")}]`);

        // Test sum
        const sum = numbers.myReduce((acc, curr) => acc + curr, 0);
        log(`Sum myReduce((acc, curr) => acc + curr, 0): ${sum}`);

        // Test without initial value
        const sumNoInit = numbers.myReduce((acc, curr) => acc + curr);
        log(`Sum without initial: ${sumNoInit}`);

        // Test product
        const product = numbers.myReduce((acc, curr) => acc * curr, 1);
        log(`Product: ${product}`);

        // Test building an object
        const words = ["hello", "world", "test"];
        const wordLengths = words.myReduce((acc, word) => {
          acc[word] = word.length;
          return acc;
        }, {});
        log(`Word lengths object: ${JSON.stringify(wordLengths)}`);

        log("✅ Reduce test complete!<br><br>");
      }

      function runAllTests() {
        clearOutput();
        log(
          '<span style="color: #ff6b6b; font-weight: bold;">🚀 RUNNING ALL TESTS:</span><br>'
        );
        testMap();
        testFilter();
        testReduce();
        log(
          '<span style="color: #4CAF50; font-weight: bold;">🎉 ALL TESTS COMPLETED!</span>'
        );
      }

      function clearOutput() {
        output.innerHTML = "";
      }

      // Run a quick demo on load
      setTimeout(() => {
        log("Welcome! Click the buttons above to test our polyfills.");
        log(
          "Each polyfill recreates the exact behavior of the native methods."
        );
      }, 500);
    </script>
  </body>
</html>