window, this, undefined vs not defined — Flow Diagram — JavaScript

Accessing an identifier — what you actually get

flowchart TD
    S["<b>Access identifier <code>x</code></b><br/>reading from a scope"]
      --> Q{"Was <code>x</code> declared?"}
    Q -->|"<b>i</b> · No"| NDF["<b>ReferenceError</b><br/>x is not defined"]
    Q -->|"Yes"| K{"Declared how?"}
    K -->|"<b>ii</b> · var (global)"| WIN["Stored on <b>window</b><br/>window.x works"]
    K -->|"<b>iii</b> · let / const (global)"| SCR["Stored in <b>Script scope</b><br/>window.x is undefined"]
    WIN --> V{"Value assigned yet?"}
    SCR --> V
    V -->|"<b>iv</b> · No"| UND["Returns <b>undefined</b><br/>(Phase 1 placeholder)"]
    V -->|"<b>v</b> · Yes"| VAL["Returns the <b>real value</b>"]
    S -.->|"<b>vi</b> · meanwhile"| TH["<b>this</b> depends on how<br/>the function was called"]
 
    classDef ok   fill:#e7efd9,stroke:#587640,stroke-width:2px,color:#1a1915;
    classDef warn fill:#fdecd3,stroke:#c2410c,stroke-width:2px,color:#1a1915;
    classDef err  fill:#f5d4cf,stroke:#991b1b,stroke-width:2px,color:#1a1915;
    classDef node fill:#f5efe1,stroke:#6a8a4f,stroke-width:2px,color:#1a1915;
    class S,Q,K,V node
    class WIN,SCR,UND,TH warn
    class VAL ok
    class NDF err

Never declared

No entry in memory at all. Reading the name throws `ReferenceError: x is not defined` — it's an error, not a value.
var at global

Attached to the `window` object. Readable as `x`, `this.x`, and `window.x` — all three are identical.
let / const at global

Stored in the Script scope (separate memory). `window.x` is `undefined` — it never attaches to the global object.
Declared, unassigned

Phase 1 put the placeholder `undefined` there. Accessing it returns `undefined` — declared, just no value yet.
Declared + assigned

Returns the real value written in code. This is the normal, happy path.
this — orthogonal

`this` is not resolved by the scope chain. Its value depends entirely on how the function was called (global / method / arrow / new / bound).

Accessing an identifier — what you actually get

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