🚀 JavaScript for Backend Developers

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JavaScript for Backend Developers — Part 1: The Complete Execution Lifecycle Explained (2026 Guide) | FreeLearning365

Part 1 of 7 · JavaScript for Backend Developers

Stop Fearing JavaScript.
Master the Execution Lifecycle.

You write Java, C#, Python, Go — and you're brilliant at it. Then someone hands you a .js file and the panic begins. This series destroys that panic — starting with the one mental model every backend engineer needs: how JavaScript actually runs.

📖 ~45 min deep read 🧪 Interactive quizzes ⚡ Real production code 🤖 AI-powered workflow

01 · Why Backend Developers Panic About JavaScript

Let's name the fear honestly. As a backend developer, you've built systems where the rules are explicit. Java tells you the type. Go tells you the concurrency model. Python gives you a REPL and a GIL. Everything has a spec, a compiler, a safety net.

Then JavaScript shows up and — from the outside — it looks like a language designed by committee in a coffee shop during a thunderstorm. It runs on the client. It's asynchronous in a way that feels chaotic. It has this (which is somehow different every time). It has NaN and undefined and null and void 0. And somehow, it also runs your entire backend on Node.js.

Here is the truth that nobody tells you: JavaScript isn't harder than the languages you already know — it's just differently shaped. The rules exist. They're precise. They're spec'd in ECMA-262. Once you internalize how the engine executes your code, the chaos dissolves into a beautifully deterministic machine.

🧠

The insight that changes everything:

In Java, you reason about threads. In Go, you reason about goroutines. In JavaScript, you reason about one thread, one stack, and a queue. That's it. Every async mystery — setTimeout(0) firing last, promises resolving "later", microtasks before macrotasks — collapses into one simple mental model.

What This 7-Part Series Will Do for You

🧩

Part 1 — Execution Lifecycle

Parsing, AST, JIT, call stack, hoisting, closures, the event loop.

⚡

Part 2 — Async Patterns

Callbacks → Promises → async/await, concurrency primitives, error flow.

🧱

Part 3 — Objects & Prototypes

Prototype chain, classes, this rules, immutability patterns.

🛡️

Part 4 — Modern Syntax

ES modules, optional chaining, destructuring, generators, iterators.

🌐

Part 5 — DOM & Browser

Events, rendering pipeline, performance, memory leaks, DevTools.

🧪

Part 6 — Testing & Quality

Unit tests, mocking, linting, type safety with JSDoc & TypeScript.

🤖

Part 7 — AI & Production

AI-assisted JS, production patterns, observability, deploy strategies.

🏆

Capstone — Real Project

Build a full-stack tool end-to-end using everything you learned.

🎯

Who this is for: Developers who write backend code daily (Java, C#, Python, Go, PHP, Ruby) and want to become genuinely comfortable — not just "copy from Stack Overflow" comfortable — with JavaScript and Node.js.

02 · The 30,000-Foot Mental Model

Before we dive into parsing and stacks, let's draw the map. When you load an HTML page containing a <script> tag — or you run node app.js — here's the full journey your code takes:

Stage	What Happens	Where
1. Fetch & Parse	Source text is scanned and broken into tokens, then assembled into an Abstract Syntax Tree (AST).	Parser
2. Compile / Interpret	The AST is walked to produce bytecode. Hot functions get JIT-compiled to machine code.	Ignition + TurboFan (V8)
3. Execute	Global execution context is created, pushed onto the call stack, and code starts running.	Call Stack
4. Suspend & Resume	Async callbacks wait in queues (microtask / macrotask) until the stack is empty.	Event Loop
5. Complete	When the stack drains and queues empty, the runtime goes idle (or the process exits).	Runtime

That's the whole life of a JavaScript program. Every framework, every library, every "magic" thing you've seen in React or Express or Next.js is built on this exact pipeline. No exceptions.

💡

Reframe: You don't need to memorize every spec detail. You need one coherent model that lets you predict what code will do. By the end of this article, you'll be able to look at any piece of JavaScript and mentally trace its execution — the same way you mentally trace a stack frame in Java.

03 · Parsing → AST → Bytecode

When the JavaScript engine receives your source code, it doesn't "run" it immediately. It first has to understand it. That process has three recognizable stages that will feel familiar if you've ever worked with a compiler:

Lexical analysis (tokenization) — The source is chopped into tokens: keywords, identifiers, operators, literals.
Syntactic analysis (parsing) — Tokens are arranged into a tree that respects the grammar of the language. This is the AST.
Bytecode generation — The AST is walked to produce an intermediate representation the interpreter can execute quickly.

Watch the Pipeline in Action

Take a trivial-looking line of JavaScript:

snippet.js JavaScript

const total = price * quantity + tax;

Roughly, here's the AST the parser builds (simplified):

AST (simplified) Tree

VariableDeclaration (const)
└── VariableDeclarator
    ├── Identifier: total
    └── BinaryExpression (+)
        ├── BinaryExpression (*)
        │   ├── Identifier: price
        │   └── Identifier: quantity
        └── Identifier: tax

Every JavaScript engine — V8 (Chrome/Node), SpiderMonkey (Firefox), JavaScriptCore (Safari) — goes through a version of this. The AST is what the engine actually reasons about. It's why you can inspect ASTs with tools like acorn, @babel/parser, or espree (the one ESLint uses).

Why This Matters for You as a Backend Dev

🧭

Understanding the AST unlocks three superpowers:

1. You understand why "syntax errors" prevent all code from running — parsing happens before any execution, so a typo on line 500 kills line 1.
2. You can write codemods — tools that transform source code (e.g., migrate your entire codebase from require() to import).
3. You can read transpiler output — Babel, SWC, and TypeScript all produce ASTs, transform them, and generate new source.

Hands-On: See Your Own Code as an AST

If you've installed Node.js, you can inspect any snippet's AST in seconds. The built-in node:vm module is only for execution, but you can use the FreeLearning365 online tools for quick transformations — or just paste into AST Explorer.

Let's build a tiny real-world example: a "spot the difference" analyzer that checks whether two snippets share the same structure.

try-it.js — run with: node try-it.js JavaScript

// Parsing is fast and cheap — you can do it at runtime.
const sourceA = `const total = price * quantity + tax;`;
const sourceB = `const sum   = a * b + c;`;

// Function constructor parses the code but doesn't run it.
function isValidSyntax(src) {
  try {
    new Function(src);   // parsing happens here
    return true;
  } catch (err) {
    return false;
  }
}

console.log(isValidSyntax(sourceA)); // true
console.log(isValidSyntax(sourceB)); // true
console.log(isValidSyntax(`const = broken`)); // false

⚠️

Security note: new Function() and eval() execute code in the current scope and are dangerous with untrusted input. Use them for parsing checks or trusted internal tools only. For a safer AST, use a parser library like acorn.

04 · JIT Compilation & V8 Internals — Why JS Gets Fast

"JavaScript is interpreted" was true in 1995. It hasn't been true since around 2008. Modern engines use Just-In-Time (JIT) compilation, and understanding this is the key to writing fast JS.

The Two-Headed Dragon: Ignition + TurboFan

V8 (Chrome and Node.js) runs your code through two cooperating components:

Component	Role	Speed Trade-off
Ignition	Interpreter. Converts AST to bytecode and executes it. Starts fast.	Quick startup, slower execution per-op.
TurboFan	Optimizing compiler. Detects "hot" functions and compiles them to machine code.	Slower to compile, much faster to run.
Sparkplug / Maglev	Mid-tier compilers (newer additions) that fill the gap between Ignition and TurboFan.	Balanced trade-off for medium-hot code.

🏎️

Metaphor: Imagine you're teaching someone to drive a delivery route. First time, they follow the GPS step by step (Ignition — slow but flexible). After ten trips, they know the shortcuts and stop checking the GPS (TurboFan — fast but only valid for that specific route). If you suddenly change the route (different data types), they have to go back to the GPS. That's deoptimization.

Hidden Classes & Why Object Shape Matters

In V8, every object gets a hidden class (also called a "shape" or "map"). It describes the layout of the object's properties. When two objects have the same hidden class, property access can be done in a single CPU instruction. When they don't, V8 falls back to a dictionary lookup — orders of magnitude slower.

Watch how this innocuous code creates two different hidden classes:

hidden-classes.js JavaScript

// ⚠️ Two objects with the SAME properties but DIFFERENT insertion order
const a = { x: 1, y: 2 };
const b = { y: 2, x: 1 };

// V8 assigns them DIFFERENT hidden classes → no shared optimization.

// ✅ Better: keep consistent property order across your codebase
const c = { x: 1, y: 2 };
const d = { x: 3, y: 4 };
// c and d share the same hidden class → fast access.

⚡

Backend impact: In Node.js services that create thousands of objects per second (parsing payloads, hydrating entities), consistent object shape can be the difference between p99 latency of 20 ms and 200 ms. This is one reason why typed schemas (TypeScript interfaces, Zod, Joi) also pay off in raw performance.

Deoptimization: The Silent Killer

V8 optimizes aggressively under assumptions. When the assumption breaks, it "deopts" — throwing away the optimized code and falling back to bytecode.

deopt.js JavaScript

// V8 optimizes this assuming `a` and `b` are small integers.
function add(a, b) {
  return a + b;
}

add(1, 2);       // number path — optimized
add(3, 4);       // still optimized
add("a", "b");   // 💥 deopt — string concatenation path
add({}, []);      // 💥💥 worse — object coercion

The fix isn't to avoid mixed types everywhere (that's impractical), but to be consistent within hot loops. If a function is called 100,000 times in a JSON transform, keep its inputs homogeneous. If you must branch on type, do it outside the hot function.

05 · Execution Contexts & the Call Stack

Now we get to the part that will make everything click. When JavaScript runs code, it does so inside an Execution Context. Think of it as a sealed envelope with everything a piece of code needs to run:

📦

Variable Environment

All var, let, const, and function declarations.

🔗

Scope Chain

Pointer to the parent scope — how identifier lookups resolve.

🎯

`this` Binding

Determined at call time by how the function is invoked.

There are two kinds of execution contexts you'll meet constantly:

Global Execution Context (GEC) — created once when your program starts. In the browser it wraps window; in Node it wraps the module wrapper.
Function Execution Context (FEC) — created every time a function is called.

The Call Stack — Your Best Debugging Friend

The call stack is a LIFO (last in, first out) structure. When a function is called, its execution context is pushed. When it returns, it's popped. That's it.

Interactive — Call Stack Visualizer

Notice the pattern: push, push, push, pop, pop, pop. The stack traces you see in error messages are just a snapshot of this structure at the moment of the error. Once you can see the stack, debugging becomes twenty times faster.

🚨

"Stack overflow" isn't a metaphor. If you recurse without a base case, or have deeply nested callbacks, you genuinely exhaust the stack's memory. In the browser the limit is around 10,000–15,000 frames; in Node.js, similar. This is why recursive tree traversal of huge structures needs an explicit stack, not recursion.

Try It: Blow the Stack (Safely)

stack-overflow.js — run with: node stack-overflow.js JavaScript

function recurse(depth = 0) {
  return recurse(depth + 1);   // no base case!
}

try {
  recurse();
} catch (e) {
  console.log(`Caught: ${e.message}`);
  // → "Caught: Maximum call stack size exceeded"
}

That error message is the engine politely telling you it ran out of stack frames. You'll see it in production when recursive functions hit unexpectedly deep data.

06 · Hoisting, Demystified Once and For All

Hoisting is where 90% of backend developers give up on JavaScript. So let's destroy the mystery.

Hoisting is not "moving code to the top". It's a two-pass behavior of the execution context:

Creation phase: Before any line executes, the engine scans the scope and registers all declarations. Functions are fully defined; var variables are set to undefined; let/const are registered but left in the "Temporal Dead Zone" (TDZ).
Execution phase: Now code runs line by line. Assignments happen. Initializers for let/const become valid.

Three Rules That Explain Everything

Declaration	Hoisted?	Initial Value	TDZ?
`function foo() {}`	✅ Yes	The function itself	No
`var x = 5;`	✅ Yes (declaration only)	`undefined`	No
`let y = 5;`	✅ Yes (declaration only)	Uninitialized	Yes
`const z = 5;`	✅ Yes (declaration only)	Uninitialized	Yes
`class Foo {}`	✅ Yes	Uninitialized	Yes

The Classic Gotcha, Predicted Correctly

hoisting.js JavaScript

console.log(a);       // undefined  (var hoisted, value not yet assigned)
console.log(b);       // 💥 ReferenceError: Cannot access 'b' before initialization
console.log(c);       // [Function: c]  — function hoisting is full

var a = 1;
let b = 2;
function c() { return 3; }

🎓

Backend rule of thumb: Never use var in new code. Use const by default, let only when you must reassign. This eliminates 90% of hoisting bugs before they happen. ESLint rule prefer-const + no-var enforces it automatically.

Hoisting Inside Functions — The Real Head-Scratcher

Inside a function, hoisting still applies. Function declarations are hoisted to the top of the function's scope, while var declarations do too — but not their values.

nested-hoisting.js JavaScript

function outer() {
  console.log(inner);     // [Function: inner]  — hoisted
  console.log(value);     // undefined         — var hoisted

  function inner() {
    return 'inside';
  }

  var value = 42;
  return inner() + value;
}

console.log(outer());  // "inside42"

07 · Scope Chains & Closures — The Aha Moment

Closures are the single most powerful concept in JavaScript. Every interview asks about them. Every framework uses them. And they're shockingly simple once you see them clearly.

A closure is a function bundled together with its surrounding scope. Even after the outer function returns, the inner function still remembers the variables it captured.

🎒

The backpack metaphor: When a function is created, it gets a backpack. Into that backpack go copies of references to every variable from its enclosing scopes. Wherever the function travels — passed as a callback, returned from a factory, stored in an object — the backpack goes with it. The function can always open its backpack and find those variables.

Closure in 10 Lines

closure.js JavaScript

function createCounter() {
  let count = 0;         // captured by the returned function

  return {
    increment() { count += 1; return count; },
    decrement() { count -= 1; return count; },
    current()   { return count; }
  };
}

const c1 = createCounter();
const c2 = createCounter();

c1.increment(); // 1
c1.increment(); // 2
c2.increment(); // 1  — separate state!

console.log(c1.current()); // 2
console.log(c2.current()); // 1

Notice what's happening: count is not on any global scope. It's not accessible from outside. It lives only inside each closure. This is how JavaScript gives you private state — a pattern that predates #private fields and classes by two decades.

Real-World Closure: The Perfect Debounce

Here's a closure you'll see in every serious frontend and backend codebase — a debounce function:

debounce.js JavaScript

function debounce(fn, delayMs) {
  let timerId = null;      // ← captured forever

  return function(...args) {
    clearTimeout(timerId);
    timerId = setTimeout(() => fn.apply(this, args), delayMs);
  };
}

// Usage in an Express route:
const logSearch = debounce((term) => {
  console.log('Logging search term:', term);
}, 500);

Each call to debounce() creates a new timerId variable. That variable survives forever inside the returned function. Even though debounce returned, the timer ID persists — ready to cancel the previous pending call.

The Classic Loop Bug (and Its Modern Fix)

❌ Don't — var leaks

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 10);
}
// Output: 3, 3, 3  ← all share same `i`

✅ Do — let is per-iteration

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 10);
}
// Output: 0, 1, 2  ← each gets own `i`

Why does let fix it? Because each iteration of a for loop with let creates a new binding for that variable. Each closure captures its own copy. With var, there's one binding shared across all iterations — so by the time the callbacks run, i has already finished at 3.

08 · The Event Loop — The Heart of Asynchronous JavaScript

If closures are the most powerful feature, the event loop is the most misunderstood. Every backend developer eventually writes code like this and gets surprised:

surprise.js — predict the output before reading further JavaScript

console.log('1');

setTimeout(() => console.log('2'), 0);

Promise.resolve().then(() => console.log('3'));

console.log('4');

// Output order: 1, 4, 3, 2

If you guessed 1, 2, 3, 4 — you're not alone. If you guessed 1, 4, 3, 2 — congratulations, you already understand the event loop intuitively. Here's the full picture.

The Complete Architecture

Component	Purpose	Examples
Call Stack	Where synchronous code runs, one frame at a time.	Function calls, expressions
Web APIs / Node APIs	Offloaded work the engine itself can't do synchronously.	`setTimeout`, `fetch`, `fs.readFile`
Microtask Queue	High-priority callbacks. Drained fully between macrotasks.	Promise `.then`, `queueMicrotask`, `MutationObserver`
Macrotask Queue	Low-priority callbacks. One per loop tick.	`setTimeout`, `setInterval`, I/O callbacks
Event Loop	The orchestrator. Picks tasks from queues when the stack is empty.	—

The Golden Rules

Synchronous code always runs first, top to bottom.
When the call stack is empty, the event loop checks the microtask queue. It drains every microtask before moving on.
Then it picks one macrotask. Runs it. Then again drains the microtask queue.
Repeat forever.

🔑

The single most important rule: Microtasks always run before the next macrotask. This is why promise callbacks always beat setTimeout(0). Once this rule is baked into your brain, you'll predict async behavior correctly every time.

Interactive: Trace the Event Loop

Interactive — Event Loop Simulator

Microtask Starvation — A Real Production Risk

Because microtasks drain completely before the next macrotask, a self-scheduling microtask can freeze your entire application. This is a genuine production bug:

starvation.js — never do this JavaScript

// 💀 This will hang the browser or Node event loop forever.
function starve() {
  Promise.resolve().then(starve);   // schedules itself
}
starve();

// The macrotask queue will NEVER get a chance to run.
// setTimeout callbacks, I/O, UI updates — all frozen.

🛑

Backend impact: A misbehaving library that recursively resolves promises can lock your Node.js server completely. CPU hits 100%, health checks fail, Kubernetes kills the pod, restart loop begins. Always prefer setImmediate or setTimeout when you need to break up heavy work across ticks.

09 · Anatomy of Real Production Code

Theory is great, but you're a backend developer — you want to see how this applies to code that actually ships. Let's dissect a real-world pattern: a self-initializing, AI-friendly, production-grade module that has the shape of code you'll write on the job.

We'll walk through a small "service health monitor" — the kind of thing that would live in a Node.js microservice. Every line below exercises everything we've covered.

health-monitor.js — production pattern JavaScript

// ============================================================
// 1. IIFE — Immediately Invoked Function Expression
//    Creates a private scope. Nothing leaks to global.
// ============================================================
(function () {
  'use strict';   // opt into strict mode — catches silent errors

  // 2. Closure-scoped state — invisible from outside
  const CHECK_INTERVAL_MS = 30_000;
  const checks = new Map();
  let timerHandle = null;

  // 3. Function declaration — hoisted, so order doesn't matter
  async function runCheck(name, url) {
    const startedAt = performance.now();
    try {
      const res = await fetch(url, { method: 'HEAD' });
      const latency = performance.now() - startedAt;
      checks.set(name, { ok: res.ok, latency, at: new Date() });
    } catch (err) {
      checks.set(name, { ok: false, error: err.message, at: new Date() });
    }
  }

  // 4. Debounced logger — closure captures timer state
  const logSummary = (function () {
    let pending = null;
    return function () {
      clearTimeout(pending);
      pending = setTimeout(() => {
        for (const [name, info] of checks) {
          console.log(`[health] ${name}: ${info.ok ? 'UP' : 'DOWN'}`);
        }
      }, 300);
    };
  })();

  // 5. Public API — only what we want exposed
  const monitor = {
    register(name, url) { checks.set(name, { ok: null, url }); },
    async start() {
      for (const [name, info] of checks) {
        await runCheck(name, info.url);
      }
      logSummary();
      timerHandle = setInterval(() => {
        for (const [name, info] of checks) runCheck(name, info.url);
        logSummary();
      }, CHECK_INTERVAL_MS);
    },
    stop() { clearInterval(timerHandle); timerHandle = null; },
    snapshot() { return Object.fromEntries(checks); }
  };

  // 6. Expose to the world (Node: module.exports, browser: window)
  if (typeof module !== 'undefined' && module.exports) module.exports = monitor;
  else if (typeof window !== 'undefined') window.healthMonitor = monitor;

})();

What Just Happened — Line by Line

#	Concept Applied	Why It Matters
1	IIFE	Private scope. No accidental globals. Runs immediately.
2	Closure state	`checks` and `timerHandle` are unreachable from outside.
3	Function hoisting	`runCheck` is callable before its textual definition.
4	IIFE returning a function	Classic closure pattern — private `pending` variable.
5	Object literal API	Clean interface; internals stay private.
6	Environment detection	Works in Node and browser with one file.

🤖

AI Corner: Paste this module into Claude, ChatGPT, or Copilot and ask: "Refactor this to use modern ES modules, add TypeScript types via JSDoc, and identify any potential race conditions in the interval logic." You'll get a first-pass upgrade in seconds. The AI is only as good as your understanding of what to ask — and now you understand every line, you can prompt precisely.

10 · AI Corner — 10× Your JavaScript Productivity

This is 2026. Nobody writes JavaScript in isolation anymore. The developers who thrive are the ones who pair with AI deliberately — not to skip learning, but to compress the boring parts so they can focus on architecture and edge cases.

Ten Prompts That Will Change How You Write JS

🔍

Explain My Stack Trace

"Here's a stack trace from Node.js. Explain the async boundary that caused the error and suggest 3 defensive fixes."

🧪

Generate Tests

"Write Jest tests for this function covering: empty input, null, very large arrays, and unexpected types."

🔁

Convert Callbacks to async/await

"Refactor this callback-heavy function to async/await. Preserve error semantics and cancellation."

🧠

Predict the Output

"What is logged, in what order, and why? Explain using the event loop model."

🚀

Performance Audit

"Analyze this loop for deoptimization risks. What object shapes, type changes, or hidden class issues exist?"

📝

Add JSDoc + Types

"Add complete JSDoc with generics for this utility module. Use @template where useful."

🛡️

Security Review

"Review this file for prototype pollution, ReDoS, and unsafe eval patterns."

🧩

Design Alternative

"Give me three different architectural approaches to this problem, with trade-offs."

🎯

The meta-skill: The best AI users aren't those who prompt the most — they're those who can evaluate the output. If you don't know why microtasks drain before macrotasks, an AI can fool you with confident-sounding nonsense. Understanding is the prerequisite for leverage.

11 · Interactive Knowledge Check

Eight questions. No partial credit for guessing. Take them seriously — each one maps to a production scenario you'll hit within your first month of writing real JavaScript.

🧠

Part 1 — Execution Lifecycle Quiz

Answer all eight to unlock the cheat sheet.

Score: 0 / 8

12 · Cheat Sheet & What Comes Next

The Execution Lifecycle — One-Page Summary

Concept	One-Line Rule
Parsing	Syntax errors prevent all code from running, even code above the error.
AST	Your code becomes a tree before it becomes anything else. Tools consume the tree.
JIT	Consistent types + consistent object shape = fast. Mixed types = deopt.
Execution Context	Each function call gets its own context: variables, scope chain, `this`.
Call Stack	LIFO. Push on call, pop on return. Deep recursion overflows it.
Hoisting	Functions hoist fully. `var` hoists as `undefined`. `let`/`const` hoist but stay in TDZ.
Closure	A function + its captured scope. Enables private state. Watch loop variables.
Event Loop	Stack empty → drain microtasks → one macrotask → repeat.
Microtask	Promises, `queueMicrotask`. Runs before any macrotask. Can starve the loop.
Macrotask	`setTimeout`, I/O. Runs one at a time.

Do / Don't — A Backend Developer's Rules

✅ DO

Use const by default, let when reassignment is required.
Keep object shapes consistent in hot loops.
Use closures for genuinely private state.
Prefer Promise / async/await over callbacks.
Break long synchronous work with setImmediate.
Run node --trace-deopt when investigating performance.
Add ESLint with no-var, prefer-const, no-floating-promises.

❌ DON'T

Don't use var in new code (ever).
Don't assume setTimeout(0) runs "now".
Don't schedule microtasks recursively.
Don't mix types in hot functions or object shapes.
Don't capture loop variables with var.
Don't use eval or new Function with untrusted input.
Don't block the event loop with while(true) or heavy CPU in a callback.

What's Coming in Part 2

Part 2 takes the event loop knowledge you now have and turns it into mastery of asynchronous patterns:

Callback hell → Promises → async/await — the full evolution.
Error handling in async code: try/catch, .catch(), and unhandled rejections.
Promise.all, Promise.allSettled, Promise.race, Promise.any — and when to use each.
Cancellation with AbortController.
Building a resilient retry-with-backoff utility from scratch.
AI-assisted async debugging workflows.

📚

Practice before Part 2: Open Node.js and write a script that reproduces the 1, 4, 3, 2 output using your own log statements. Then modify it to show what happens when you await in the middle. Predict, then verify. That habit — predict, then verify — is what separates senior engineers from everyone else.

Part 1 of 7 · JavaScript for Backend Developers · FreeLearning365.com