> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# TypeScript Parameter Properties
Parameter properties are a TypeScript syntactic shorthand that allows you to declare and initialize class member fields directly within a constructor's signature. By prefixing a constructor parameter with an access modifier (`public`, `private`, `protected`) or the `readonly` keyword, the TypeScript compiler implicitly declares a class property of the same name and automatically assigns the parameter's value to that property upon instantiation.
## Syntax Comparison
Without parameter properties, declaring and initializing a class field requires three distinct steps: property declaration, constructor parameter definition, and explicit assignment.
```typescript theme={"dark"}
class ExplicitClass {
public readonly id: number;
private name: string;
constructor(id: number, name: string) {
this.id = id;
this.name = name;
}
}
```
Using parameter properties, the declaration and assignment are condensed into the constructor signature:
```typescript theme={"dark"}
class ParameterPropertyClass {
constructor(public readonly id: number, private name: string) {
// The compiler automatically generates:
// this.id = id;
// this.name = name;
}
}
```
## Technical Mechanics
1. **Required Modifiers:** A parameter is only treated as a parameter property if it is prefixed with at least one of the following modifiers: `public`, `private`, `protected`, or `readonly`. If no modifier is present, it remains a standard local constructor parameter.
2. **Execution Order:** The implicit assignment of parameter properties occurs before any code within the constructor body is executed. If a class inherits from a base class, the assignment occurs immediately after the `super()` call.
3. **Combination with Default Values:** Parameter properties fully support default parameter values. If a default value is provided and the argument is omitted during instantiation, the class property is initialized with the default value.
```typescript theme={"dark"}
class Configuration {
constructor(
protected environment: string = "development",
public readonly timeoutMs: number = 3000
) {}
}
```
## JavaScript Emission
During compilation, TypeScript strips the type annotations and access modifiers, emitting standard JavaScript assignment logic within the constructor.
The following TypeScript code:
```typescript theme={"dark"}
class User {
constructor(private username: string) {}
}
```
Compiles to the following JavaScript (ES2015+):
```javascript theme={"dark"}
class User {
constructor(username) {
this.username = username;
}
}
```
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