let myArray = [];
let commentData = {} as Dish;
commentData.id = 3;
commentData.name = 'something';
myArray.push(commentData);

It will work...

#Answer

Answer on how to push Comment (object) into Dish.comments (array) in TypeScript.

export interface Dish {
   id: number;
   name: string;
   image: string;
   category: string;
   label: string;
   price: string;
   featured: boolean;
   description: string;
   // comments: Comment[]; // remove this
   comments: Array<Comment>; // <--- change to this. everytime you want to add array use Array<YourInterface>
}

export interface Comment {
   rating: number;
   comment: string;
   author: string;
   date: string;
}

dish.comments.push(commentData);

View live code on TypeScript Playground and click RUN.

As you see in the above code. You need to change Comment[] into Array<Comment>.

#Explanation

Generic Type Variables

Array<T> or Array<Type>

You may already be familiar with this style of type from other languages such as Java and C#.

We have Generic Type Variables in typescript too.

Another way to use Generic Type Variables:

Here's an example of an array with multiple types:

let x: Array<string | number>
x = ["hello", "world", 2]

This second version is common if your array consists of different types of objects. For example:

interface Boat {
  name: string
}

interface SpaceShip {
  code: number
}

interface Wagon {
  active: boolean
}

let inventory: Array<Boat | SpaceShip | Wagon> = [];

let boatData: Boat = {
  name: "Boat 1"
}

let spaceShipData: SpaceShip = {
  code: 1234
}

let wagonData: Wagon = {
  active: true
}

inventory.push(boatData);
inventory.push(spaceShipData);
inventory.push(wagonData);

console.log(inventory);

View live code on TypeScript Playground and click RUN.

You can learn more about Generic Type Variables here and here

If you simply need to add the new commentData after each form submission (if I understood you correctly), all you need is this every time you want the new comments to be pushed to the existing dish.comments,

this.dish.comments = this.dish.comments.push(this.commentData); // assuming these are class properties, hence the 'this' usage

Does that work for you?

EDIT

Modify the first line

this.commentData = this.comment.value;

in dismiss() method to this,

this.commentData.author = this.comment.get('author').value;
this.commentData.rating = this.comment.get('rating').value;
this.commentData.comment = this.comment.get('comment').value;

If you need to add multiple objects to an array inside a loop:

let thingsArray = [];

someArray.forEach(doc => {
    let thingsObj = {} as Thingy;

    thingsObj.weekDue = doc.name;
    thingsObj.title = doc.age;
    thingsArray.push(thingsObj);

}).then(() => {
    console.log(thingsArray);
}

let arraylist= [];
let obj1 = {} as Class;
obj1.id = 1;
obj1.name = 'text';
let obj2 = {} as Class;
obj2.id = 1;
obj2.name = 'text';
arraylist =[obj1,obj2 ];

This work for me but I tried it with Classes