How to compute a property on a class in typescript?

Issue

In javascript I would have done it like this:

function a(b,c) {this.foo = b; this.bar = c; this.yep = b+c}
// undefined
b = new a(1,2)
// a {foo: 1, bar: 2, yep: 3}

But I haven't been able to find any way to do it in typescript. None of this works:

class A {
    foo: number;
    bar: number;
    yep: foo + bar;
}

class A {
    foo: number;
    bar: number;
    yep: this.foo + this.bar;
}

class A {
    foo: number;
    bar: number;
    let yep:number = this.foo + this.bar;
}

class A {
    foo: number;
    bar: number;
    yep: number;

    constructor() {
        this.yep = this.foo + this.bar;
    }
}

class A {
    foo: number;
    bar: number;

    get yep(): number {
        return this.foo + this.bar;
    }
}

class A {
    foo: number;
    bar: number;
    yep: function () {return this.get("foo") + this.get("bar")};
}

I initialize it like this:

somevar: A = {
    foo: 1,
    bar: 2
}

Also I tried this:

somevar: A = {
    foo: 1,
    bar: 2,
    this.yep: this.foo + this.bar
}

Thank you for your help. This math will have be more difficult and I'll need it more than once, so I don't want to put it in the template.

Solution

A is a class and not an interface, so you need to construct an instance. You cannot simply assign an object literal. It's not enough for the 'shape' to be compatible; it must be an instance of the class.

Variables declared with private, protected or public in the constructor will be added to the class.

For example:

class A {
  public yep: number;

  constructor(
      public foo: number, // will be transpiled in the constructor to: this.foo = foo;
      public bar: number  // will be transpiled in the constructor to: this.bar = bar;
  ) {
        this.yep = foo + bar; 
    }
}

const a: A = new A(1, 2);

Answered By - cartant