program/courses/backend/advanced-javascript/week2/session-materials/slides/index.html at main · HackYourFuture-CPH/program · GitHub

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<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8" />
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    <title>Week 2 - Callbacks & Delayed Execution</title>
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  </head>
  <body>
    <div class="reveal">
      <div class="slides">
        <!-- Title -->
        <section>
          <h1>Callbacks & Delayed Execution</h1>
          <p>Why backends can't run top-to-bottom</p>
          <p class="dim small">Week 2</p>
        </section>

        <!-- Section: Why Async -->
        <section>
          <section>
            <h2>Why Async?</h2>
          </section>

          <section>
            <h3>The Problem</h3>
            <p>Database queries take time</p>
            <p>File reads take time</p>
            <p>Network calls take time</p>
            <p class="highlight" style="margin-top: 1em">
              If we waited, we'd serve one request at a time
            </p>
          </section>

          <section>
            <h3>The Cafe Analogy</h3>
            <blockquote>
              A waiter doesn't stand at the kitchen waiting for one order.<br /><br />
              They take the order, give it to the kitchen, and serve other
              tables while waiting.
            </blockquote>
          </section>

          <section>
            <h3>The Solution</h3>
            <p>"Start this operation..."</p>
            <p>"...call me back when you're done"</p>
            <p class="highlight" style="margin-top: 1em">That's a callback</p>
          </section>
        </section>

        <!-- Section: Functions as Values -->
        <section>
          <section>
            <h2>Functions as Values</h2>
            <p>First, we need to understand this</p>
          </section>

          <section>
            <h3>Functions Are Just Values</h3>
            <pre><code class="language-javascript">// Assign a function to a variable
const greet = function(name) {
  return `Hello, ${name}!`;
};

// Call it through the variable
greet("Alice");  // "Hello, Alice!"</code></pre>
          </section>

          <section>
            <h3>Pass Functions to Functions</h3>
            <pre><code class="language-javascript">function runTwice(fn, value) {
  console.log(fn(value));
  console.log(fn(value));
}

runTwice(greet, "Bob");
// "Hello, Bob!"
// "Hello, Bob!"</code></pre>
            <p class="dim">This is what makes callbacks possible</p>
          </section>

          <section>
            <h3>You Already Do This!</h3>
            <pre><code class="language-javascript">teas.map(tea => tea.name);
//       ^^^^^^^^^^^^^^^^
//       This IS a function passed to map</code></pre>
            <p>map, filter, forEach all take functions as arguments</p>
          </section>

          <section>
            <h3>Function Factories</h3>
            <p>A function that returns a function</p>
            <pre><code class="language-javascript">function createGreeter(greeting) {
  return function(name) {
    console.log(`${greeting}, ${name}!`);
  };
}

const sayHello = createGreeter("Hello");
const sayHi = createGreeter("Hi");

sayHello("Alice");  // "Hello, Alice!"
sayHi("Bob");       // "Hi, Bob!"</code></pre>
          </section>
        </section>

        <!-- Section: Callbacks -->
        <section>
          <section>
            <h2>Callbacks</h2>
            <p>A function you pass, to be called later</p>
          </section>

          <section>
            <h3>Synchronous Callbacks</h3>
            <p>Called immediately, during execution</p>
            <pre><code class="language-javascript">teas.map(tea => tea.name);
// callback runs NOW, for each item</code></pre>
          </section>

          <section>
            <h3>Asynchronous Callbacks</h3>
            <p>Called later, after something completes</p>
            <pre><code class="language-javascript">setTimeout(() => {
  console.log("Done!");
}, 1000);
// callback runs LATER, after 1000ms</code></pre>
          </section>
        </section>

        <!-- Section: reduce -->
        <section>
          <section>
            <h2>reduce</h2>
            <p>The "build up" method</p>
          </section>

          <section>
            <h3>The Mental Model</h3>
            <p>
              <span class="highlight">N items</span> &rarr;
              <span class="highlight">1 value</span>
            </p>
            <p>Sum, count, group, aggregate</p>
          </section>

          <section>
            <h3>The Accumulator</h3>
            <pre><code class="language-javascript">const total = numbers.reduce((accumulator, current) => {
  return accumulator + current;
}, 0);
// ^-- initial value</code></pre>
            <p class="dim">The accumulator builds up through each iteration</p>
          </section>

          <section>
            <h3>Sum Example</h3>
            <pre><code class="language-javascript">const total = teas.reduce((sum, tea) => {
  return sum + tea.stockCount;
}, 0);
// Total stock across all teas</code></pre>
          </section>

          <section>
            <h3>Grouping Example</h3>
            <pre><code class="language-javascript">const byOrigin = teas.reduce((groups, tea) => {
  if (!groups[tea.origin]) {
    groups[tea.origin] = [];
  }
  groups[tea.origin].push(tea.name);
  return groups;
}, {});
// { Japan: ["Sencha", ...], China: [...] }</code></pre>
          </section>
        </section>

        <!-- Section: setTimeout -->
        <section>
          <section>
            <h2>setTimeout</h2>
            <p>The simplest async function</p>
          </section>

          <section>
            <h3>Surprising Output</h3>
            <pre><code class="language-javascript">console.log("1. First");

setTimeout(() => {
  console.log("2. Second");
}, 1000);

console.log("3. Third");</code></pre>
            <p>What order do these print?</p>
          </section>

          <section>
            <h3>The Answer</h3>
            <pre><code class="language-javascript">// Output:
// 1. First
// 3. Third
// 2. Second  (after 1 second)</code></pre>
            <p class="highlight">JavaScript doesn't wait. It keeps going.</p>
          </section>

          <section>
            <h3>Simulating Database Delay</h3>
            <pre><code class="language-javascript">function findTeaById(id, callback) {
  setTimeout(() => {
    const tea = teas.find(t => t.id === id);
    callback(tea);
  }, 500);  // 500ms "database" delay
}

findTeaById(3, tea => {
  console.log("Found:", tea.name);
});</code></pre>
          </section>
        </section>

        <!-- Section: File System -->
        <section>
          <section>
            <h2>File System</h2>
            <p>Real backend async</p>
          </section>

          <section>
            <h3>Reading Files</h3>
            <pre><code class="language-javascript">import fs from "fs";

fs.readFile("./data.json", "utf8", (error, data) => {
  if (error) {
    console.error("Failed:", error.message);
    return;
  }
  console.log("Got data:", data);
});</code></pre>
          </section>

          <section>
            <h3>Error-First Pattern</h3>
            <p>Node.js convention: <code>(error, result)</code></p>
            <pre><code class="language-javascript">fs.readFile(path, (error, data) => {
  if (error) {
    // Handle error FIRST
    return;
  }
  // Then use data
});</code></pre>
            <p class="highlight">Always check error before using result</p>
          </section>
        </section>

        <!-- Section: Building Understanding -->
        <section>
          <section>
            <h2>Build Your Own</h2>
            <p>Understanding how it works</p>
          </section>

          <section>
            <h3>Build myForEach</h3>
            <pre><code class="language-javascript">function myForEach(array, callback) {
  for (let i = 0; i < array.length; i++) {
    callback(array[i], i, array);
  }
}

myForEach(teas, tea => console.log(tea.name));</code></pre>
            <p class="dim">That's all forEach is!</p>
          </section>

          <section>
            <h3>Build myMap</h3>
            <pre><code class="language-javascript">function myMap(array, callback) {
  const result = [];
  for (let i = 0; i < array.length; i++) {
    result.push(callback(array[i], i, array));
  }
  return result;
}

myMap(teas, tea => tea.name);  // ["Sencha", ...]</code></pre>
          </section>
        </section>

        <!-- Summary -->
        <section>
          <section>
            <h2>Summary</h2>
          </section>

          <section>
            <h3>Key Concepts</h3>
            <p>
              <span class="highlight">Functions as values</span> - pass them
              around
            </p>
            <p>
              <span class="highlight">Callbacks</span> - functions called later
            </p>
            <p>
              <span class="highlight">reduce</span> - build up a single value
            </p>
            <p>
              <span class="highlight">Async</span> - don't wait, use callbacks
            </p>
          </section>

          <section>
            <h3>Next Week</h3>
            <p>Callback nesting gets messy...</p>
            <pre><code class="language-javascript">doFirst(result1 => {
  doSecond(result1, result2 => {
    doThird(result2, result3 => {
      // "Callback hell"
    });
  });
});</code></pre>
            <p class="highlight">Promises solve this!</p>
          </section>

          <section>
            <h2>Time to Practice</h2>
            <p class="emoji">🍵</p>
          </section>
        </section>
      </div>
    </div>

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