Typescript object 可选的初始值设定项错误，默认属性在构造函数简写中声明

Question

When declaring a constructor like this:

export class Test {

    constructor(
            public name: string,
            public value: number = -1) {
    }
}

Keeping in mind that in the official TS documentation it is written:

Default-initialized parameters that come after all required parameters are treated as optional, and just like optional parameters, can be omitted when calling their respective function.

When trying this:

let test: Test = {name: 'xxx'};

I get this error:

TS2741: Property 'value' is missing in type '{ name: string; }' but required in type 'Test'.

And if I try to explicitly declare the parameter optional:

public value?: number = -1

I get this one:

TS1015: Parameter cannot have question mark and initializer.

Answer 1

There is a way to accomplish what you want. You need to declare value as a public property on your class and make it optional. Then you can take in a parameter in your constructor called value which will be local to that constructor function. Then in your constructor you can assign the value in the 'value' parameter to the 'value' property on your 'class' instance object.

class Test {
    public value?: number;

    constructor(
            public name: string,
        value: number = -1) {
        this.value = value;
    }
}

const a: Test = { name: '' };
const b= new Test('');