Parameter Properties

TypeScript constructor parameter properties: an accessibility modifier (public/private/protected) or `readonly` on a constructor parameter declares a same-named class field and assigns it automatically — no separate field declaration or `this.x = x` needed.

class City {
    constructor(public name: string, public population: number, private countryCode: string, readonly founded: number) {}

    describe(): string {
        return this.name + " (" + this.countryCode + "), pop " + String(this.population) + ", founded " + String(this.founded);
    }
}

const c = new City("Thessaloniki", 1030000, "GR", -315);
console.log(c.describe());
console.log(c.name);
console.log(c.founded);

// Parameter properties may carry defaults, and a default may reference an
// earlier parameter (ADR-00599) — here countryCode defaults to "GR" and a
// second constructor param can build on an earlier one.
class Town {
    constructor(public name: string, public countryCode: string = "GR") {}
}
const t = new Town("Katerini");
console.log(t.name, t.countryCode);  // Katerini GR

View source on GitHubexamples/language/parameter_properties.ts

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