> ## Documentation Index
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# TypeScript Member Access
The `.` (dot) operator, formally known as the member access operator or property accessor, is a synchronous, left-to-right associative operator used to resolve properties, methods, or nested namespaces. In TypeScript, it functions simultaneously as a runtime property accessor and a compile-time type resolution mechanism.
```ebnf theme={"dark"}
MemberExpression ::= LeftHandSideExpression "." IdentifierName
```
## Technical Mechanics
**1. Left-Hand Side (LHS) Evaluation**
The compiler evaluates the `LeftHandSideExpression`'s AST node to determine its semantic *Type* (an internal checker object). The LHS must resolve to a type capable of holding members, such as an `interface`, `type` alias (object shape), `class`, `namespace`, or a primitive with a corresponding wrapper object (e.g., `String`, `Number`).
**2. Right-Hand Side (RHS) Resolution**
The `IdentifierName` on the right is not evaluated as an expression; it is treated as a literal, static identifier. It must conform to valid identifier naming rules. Dynamic property access requires bracket notation (`[]`) instead of the dot operator.
**3. Compile-Time Type Checking**
When the dot operator is applied, the TypeScript compiler performs a property lookup on the semantic *Type* resolved from the LHS.
* If the `IdentifierName` exists on the LHS *Type*, the expression evaluates to the type of that specific member.
* If the `IdentifierName` does not exist, the compiler emits a `ts(2339)` error: `Property '[IdentifierName]' does not exist on type '[LHS Type]'`.
## TypeScript-Specific Compiler Behaviors
* **Visibility Enforcement:** The compiler intercepts dot operator access to check class access modifiers (`private`, `protected`). If the LHS is an instance of a class and the RHS is a restricted member accessed outside its permitted lexical scope, TypeScript throws a compile-time error, regardless of runtime validity.
* **Mutability Constraints:** If the resolved RHS member is decorated with the `readonly` modifier, the compiler allows the dot operator to be used in an r-value context (reading) but rejects it in an l-value context (assignment).
* **Control Flow Analysis:** The dot operator facilitates type narrowing when accessing a discriminant property on a union type. Mere access of a property via the dot operator does not narrow the type; control flow narrowing occurs only when that accessed property is evaluated in a type guard or conditional check (e.g., `if (obj.type === 'A')`).
* **Strict Null Checking:** Under `--strictNullChecks`, if the LHS type includes `null` or `undefined`, the compiler rejects the dot operator access with a `ts(2531)` or `ts(2532)` error, requiring type guards or nullability operators.
## Nullability and Member Access
TypeScript handles member access on potentially nullish types through specific syntax interactions:
* **Optional Chaining (`?.`):** This is a distinct, single token that alters the evaluation semantics. If the LHS evaluates to `null` or `undefined`, the operation short-circuits, returning `undefined` without throwing a runtime `TypeError`.
```ebnf theme={"dark"}
OptionalChain ::= LeftHandSideExpression "?." IdentifierName
```
```typescript theme={"dark"}
declare const user: { name: string } | null;
const userName = user?.name;
```
* **Non-Null Assertion (`!`):** Unlike optional chaining, `!.` is not a single token or a variant of the dot operator. In TypeScript, `!` is the non-null assertion postfix operator, and `.` is the standard member access operator. They are entirely separate tokens evaluated sequentially. The expression `obj!.prop` is parsed as `(obj!) . prop`. The `!` forces the compiler to strip `null` and `undefined` from the LHS type before the `.` operator performs the member lookup.
```typescript theme={"dark"}
declare const config: { timeout: number } | undefined;
const timeout = config!.timeout; // Parsed as (config!).timeout
```
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