Syntax and Complex Types

JavaScript is flexible, powerful, and sometimes quirky. Below is a comprehensive guide covering syntax basics, complex types, and advanced structures you need to understand to write maintainable code.

Basic Syntax

Variables

let name = "Alice";    // Mutable variable
const PI = 3.14;       // Immutable constant
var legacy = 42;       // Avoid using var

Functions

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

const add = (a, b) => a + b;

Conditionals & Loops

if (score > 90) {
  console.log("Excellent");
} else {
  console.log("Keep trying");
}

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

Complex Types

Arrays

const colors = ["red", "green", "blue"];
console.log(colors[1]);  // "green"

Can hold mixed types (not recommended):

const mixed = [1, "hello", true, { key: "value" }];

Objects

const person = {
  name: "Alice",
  age: 30,
  greet: function() {
    return `Hi, I'm ${this.name}`;
  }
};

console.log(person.name);       // "Alice"
console.log(person.greet());    // "Hi, I'm Alice"

Nested Objects (Objects of Objects)

const company = {
  name: "TechCorp",
  address: {
    street: "123 Main St",
    city: "Tech City",
    location: {
      lat: 40.7128,
      lng: -74.0060
    }
  },
  employees: {
    manager: { name: "Bob", age: 45 },
    developer: { name: "Carol", age: 28 }
  }
};

console.log(company.address.city);              // "Tech City"
console.log(company.address.location.lat);     // 40.7128
console.log(company.employees.manager.name);   // "Bob"

Maps (Key-Value Store)

const map = new Map();
map.set('id', 1);
map.set('name', 'Alice');

console.log(map.get('name'));  // "Alice"
console.log(map.has('id'));    // true

Sets (Unique Collection of Values)

const set = new Set();
set.add(1);
set.add(2);
set.add(1);   // Duplicate ignored

console.log(set.has(1));   // true
console.log(set.size);     // 2

Symbols (Unique Identifiers)

const sym1 = Symbol('id');
const sym2 = Symbol('id');

console.log(sym1 === sym2);  // false

const obj = {
  [sym1]: 'unique value'
};

console.log(obj[sym1]);      // "unique value"

Special Types

Null vs Undefined

let a;
console.log(a);   // undefined

let b = null;
console.log(b);   // null

Type Checking

typeof 123        // "number"
typeof "hello"    // "string"
typeof true       // "boolean"
typeof {}         // "object"
typeof []         // "object"  (Arrays are objects)
typeof null       // "object"  // quirk in JS

Complex Type Example

const user = {
  id: 1,
  name: "Alice",
  roles: ["admin", "editor"],
  profile: {
    email: "alice@example.com",
    address: {
      street: "1 Wonderland Drive",
      city: "Wonderland",
      zip: "12345"
    }
  },
  isActive: true
};

console.log(user.profile.address.city);    // "Wonderland"
console.log(user.roles[0]);                // "admin"

Practical Tips

Avoid var; prefer let and const.
Use Maps when keys are not strings or need guaranteed key order.
Use Sets to store unique items efficiently.
Symbols are great for adding private-ish properties.
Deep cloning nested objects requires libraries like lodash or structured cloning.

Syntax and Complex Types

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