错误 TS2339：“温度”类型上不存在属性“摄氏度”

Question

I'm trying to make a Temperature class with typescript. The javascript version works, but typescript throws this error. Why does this example work with javascript, but fail with typescript? Here's the code to reproduce:

Temperature.js

class Temperature {
    constructor(celsius) {
        this.celsius = celsius;
    }
    get fahrenheit() {
        return this.celsius * 1.8 + 32;
    }
    set fahrenheit(value) {
        this.celsius = (value - 32) / 1.8;
    }
    static fromFahrenheit(value) {
        return new Temperature((value - 32) / 1.8);
    }
}

let test = new Temperature(100);
console.log(test.celsius);
console.log(test.fahrenheit);

Temperature.ts

class Temperature {
  constructor(celsius) {
    this.celsius = celsius;
  }
  get fahrenheit() {
    return this.celsius * 1.8 + 32;
  }
  set fahrenheit(value) {
    this.celsius = (value - 32) / 1.8;
  }

  static fromFahrenheit(value) {
    return new Temperature((value - 32) / 1.8);
  }
}

let test = new Temperature(100)
console.log(test.celsius);
console.log(test.fahrenheit);

Successful javascript output:

➜  ch6 node Temperature.js 
100
212
➜  ch6 

Compilation error:

➜  ch6 tsc --target es6 Temperature.ts 
Temperature.ts:3:10 - error TS2339: Property 'celsius' does not exist on type 'Temperature'.

3     this.celsius = celsius;
           ~~~~~~~

Temperature.ts:6:17 - error TS2339: Property 'celsius' does not exist on type 'Temperature'.

6     return this.celsius * 1.8 + 32;
                  ~~~~~~~

Temperature.ts:9:10 - error TS2339: Property 'celsius' does not exist on type 'Temperature'.

9     this.celsius = (value - 32) / 1.8;
           ~~~~~~~

Temperature.ts:18:18 - error TS2339: Property 'celsius' does not exist on type 'Temperature'.

18 console.log(test.celsius);
                    ~~~~~~~


Found 4 errors.

Answer 1

In a typescript class, we need to declare properties for them to compile correctly. See here for an example from the docs. When I add the property declaration, the file compiles successfully.

Temperature.ts

class Temperature {
  celsius: number;
  constructor(celsius) {
    this.celsius = celsius;
  }
  get fahrenheit() {
    return this.celsius * 1.8 + 32;
  }
  set fahrenheit(value) {
    this.celsius = (value - 32) / 1.8;
  }

  static fromFahrenheit(value) {
    return new Temperature((value - 32) / 1.8);
  }
}

let test = new Temperature(100)
console.log(test.celsius);
console.log(test.fahrenheit);

Compilation success:

➜  ch6 tsc --target es6 Temperature.ts
➜  ch6