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| **Concept**       | **Details**                                                                 |
|-------------------|------------------------------------------------------------------------------|
| **Promise State** | `pending`, `fulfilled`, `rejected`                                           |
| **Microtask**     | `.then`, `await`, `queueMicrotask()`                                         |
| **Task**          | `setTimeout`, `setInterval`, `fetch`                                         |
| **Event Loop**    | Runs call stack → microtasks → tasks                                         |
| **async/await**   | Syntactic sugar for Promises                                                 |
| **Error Handling**| Use `.catch()` or `try/catch`                                                |

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## JavaScript `this` Binding — At a Glance

| How You Call It                     | What `this` Refers To            |
|-------------------------------------|----------------------------------|
| `regularFunction()`                 | `global` / `undefined`           |
| `obj.method()`                      | `obj`                            |
| `arrowFunction()`                   | Inherited from lexical scope     |
| `method.call(obj)` / `apply(obj)`   | `obj`                            |
| `method.bind(obj)`                  | `obj` (returned function bound)  |
| `setTimeout(function)`              | `global` / `undefined`           |

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| Syntax                      | What `this` refers to           |
|-----------------------------|---------------------------------|
| Regular function            | `undefined` (strict) / `window` |
| Object method               | The object                      |
| Arrow function              | Lexical surrounding scope       |
| `call(obj)` / `apply(obj)`  | `obj`                           |
| `bind(obj)`                 | `obj` (returns new function)    |
| `setTimeout` (regular fn)   | `undefined` / `window`          |
| `setTimeout` (arrow fn)     | Inherits from outer context     |

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| Concept                  | Key Idea                                                                 |
|--------------------------|--------------------------------------------------------------------------|
| Scope                    | Controls visibility of variables                                         |
| Lexical Scope            | Inner functions access variables from parent scopes                      |
| Hoisting                 | Declarations are moved to top before execution                           |
| TDZ (Temporal Dead Zone) | `let`/`const` exist but aren’t usable until initialized                  |
| Shadowing                | Inner scope overrides variables with same name                           |
| Module Scope             | JS Modules have isolated scope; vars aren't global unless exported       |
| Call Stack               | JS engine's way to manage function calls using LIFO                      |

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| Scope Type         | Description                                                                 |
|--------------------|-----------------------------------------------------------------------------|
| **Global Scope**   | Accessible everywhere (outside any function or block)                       |
| **Function Scope** | `var` declarations inside a function are limited to it                      |
| **Block Scope**    | `let` and `const` inside `{}` blocks are accessible only inside the block   |
| **Lexical Scope**  | Inner functions access outer variables from where they were defined         |
| **Module Scope**   | In ES Modules, variables are scoped to that file unless exported            |

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| Feature                        | Function Declaration  | Function Expression  | Arrow Function            |
|--------------------------------|-----------------------|----------------------|---------------------------|
| **Named**                      | Yes                   | Optional             | Optional                  |
| **Hoisted**                    | Yes                   | No                   | No                        |
| **Lexical `this`**             | No                    | No                   | Yes                       |
| **Can be used as constructor** | Yes                   | Yes                  | No                        |
| **Can be anonymous**           | No                    | Yes                  | Yes                       |
| **Common Use**                 | General purpose       | Callbacks            | One-liners, Array methods |

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| Concept                         | Key Insight                                                                  |
|---------------------------------|------------------------------------------------------------------------------|
| **Closure**                     | Inner functions keep reference to outer variables                            |
| **Callback**                    | A function passed to another function                                        |
| **HOF (Higher-Order Function)** | A function that takes or returns another function                            |
| **Execution Context**           | Each function call creates a new isolated scope                              |
| **Microtask vs Task Queue**     | Promises (microtasks) are prioritized and run before setTimeout (task queue) |

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    ┌────────────────────────┐
    │   JavaScript Code      │
    └────────────────────────┘
               │
               ▼
        ┌────────────┐
        │ Call Stack │ ◄────────────────────────────┐
        └────────────┘                              │
               │                                    │
               ▼                                    │

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## JavaScript Memory: Stack vs Heap

| **Stack**                        | **Heap**                                          |
|----------------------------------|---------------------------------------------------|
| Fast, simple memory storage      | Large, dynamic memory                             |
| Holds **primitive** values       | Holds **reference** values (objects, arrays)      |
| Copy by **value**                | Variables store **memory addresses (references)** |

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## JavaScript Key Concepts – Day 1 Summary

| Term            | Meaning                                                 |
|-----------------|---------------------------------------------------------|
| `var`           | Function scoped, hoisted with `undefined`               |
| `let` / `const` | Block scoped, exist in Temporal Dead Zone (TDZ)         | 
| Primitives      | Copied by value (stored in the stack)                   |
| References      | Copied by reference (stored in the heap)                |
| Scope Chain     | Functions access variables from outer scopes            |
| Closure         | Functions remember outer variables even after execution |