JavaScript Essentials for AI Agents: Syntax & Async Guide

Why JavaScript?

JavaScript runs everywhere:

• Web browsers


• Node.js servers


• Many automation tools


• Universal scripting

Basic Syntax

Variables

const name = "Alice";     // Constant
let age = 30;             // Mutable
var old = "avoid";        // Legacy, avoid

// const for most things, let when you need to reassign

Data Types

// Primitives
const str = "hello";
const num = 42;
const bool = true;
const nothing = null;
const notDefined = undefined;

// Objects
const obj = { name: "Alice", age: 30 };
const arr = [1, 2, 3];

Strings

const s = "Hello, World";
s.toLowerCase();           // "hello, world"
s.toUpperCase();           // "HELLO, WORLD"
s.split(", ");             // ["Hello", "World"]
s.replace("o", "0");       // "Hell0, World"
s.startsWith("He");        // true
s.includes("World");       // true
s.length;                  // 12

// Template literals
const name = "Alice";
`Hello, ${name}!`;         // "Hello, Alice!"

Arrays

const items = [1, 2, 3, 4, 5];
items[0];                  // 1
items[items.length - 1];   // 5
items.slice(1, 3);         // [2, 3]
items.push(6);             // Add to end
items.pop();               // Remove from end
items.includes(3);         // true
items.length;              // 5

// Iteration
items.forEach(x => console.log(x));
items.map(x => x * 2);     // [2, 4, 6, 8, 10]
items.filter(x => x > 2);  // [3, 4, 5]
items.find(x => x > 2);    // 3
items.reduce((a, b) => a + b, 0); // 15

Objects

const user = {
  name: "Alice",
  age: 30
};

user.name;                 // "Alice"
user["name"];              // "Alice"
user.email;                // undefined
user.email = "a@b.com";    // Add property
Object.keys(user);         // ["name", "age", "email"]
Object.values(user);       // ["Alice", 30, "a@b.com"]

// Destructuring
const { name, age } = user;

// Spread
const copy = { ...user };
const merged = { ...user, role: "admin" };

Control Flow

If/Else

if (age < 18) {
  console.log("Minor");
} else if (age < 65) {
  console.log("Adult");
} else {
  console.log("Senior");
}

Ternary

const status = age >= 18 ? "adult" : "minor";

Loops

// For
for (let i = 0; i < 5; i++) {
  console.log(i);
}

// For...of (arrays)
for (const item of items) {
  console.log(item);
}

// For...in (object keys)
for (const key in user) {
  console.log(key, user[key]);
}

// While
let count = 0;
while (count < 5) {
  console.log(count);
  count++;
}

Functions

Function Declaration

function greet(name) {
  return `Hello, ${name}!`;
}

Arrow Functions

const greet = (name) => `Hello, ${name}!`;

// Multiple statements
const process = (data) => {
  const result = transform(data);
  return result;
};

Default Parameters

function greet(name, greeting = "Hello") {
  return `${greeting}, ${name}!`;
}

Rest Parameters

function sum(...numbers) {
  return numbers.reduce((a, b) => a + b, 0);
}
sum(1, 2, 3, 4); // 10

Async/Await

// Promise-based
fetch(url)
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

// Async/await
async function getData() {
  try {
    const res = await fetch(url);
    const data = await res.json();
    return data;
  } catch (err) {
    console.error(err);
  }
}

Classes

class User {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }

  greet() {
    return `Hello, I'm ${this.name}`;
  }
}

const user = new User("Alice", "alice@example.com");
user.greet(); // "Hello, I'm Alice"

Modules

Export

// Named exports
export const name = "Alice";
export function greet() { }

// Default export
export default class User { }

Import

import User from './user.js';
import { name, greet } from './utils.js';
import * as utils from './utils.js';

Common Patterns

Nullish Coalescing

const value = input ?? "default";
// Uses "default" only if input is null/undefined

Optional Chaining

const street = user?.address?.street;
// Returns undefined instead of throwing if any is null

Spread Operator

// Arrays
const combined = [...arr1, ...arr2];

// Objects
const merged = { ...obj1, ...obj2 };

Destructuring

// Arrays
const [first, second] = [1, 2];

// Objects
const { name, age } = user;

// With rename
const { name: userName } = user;

Error Handling

try {
  riskyOperation();
} catch (error) {
  console.error(error.message);
} finally {
  cleanup();
}

// Throwing
throw new Error("Something went wrong");

Useful Methods

// JSON
JSON.parse('{"name":"Alice"}');
JSON.stringify({ name: "Alice" });

// Array methods
[1, 2, 3].join(", ");      // "1, 2, 3"
Array.isArray([1, 2, 3]);  // true

// Object methods
Object.assign({}, obj1, obj2);
Object.entries(obj);       // [[key, value], ...]

Conclusion

JavaScript essentials:

• Modern syntax (const/let, arrow functions)


• Powerful array methods


• Async/await for promises


• Object destructuring and spread


• Runs everywhere

Master these for effective JS work.

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Next: TypeScript Basics - Type-safe JavaScript