In ECMAScript 6, you can use the Spread syntax:

let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
arr1.push(...arr2);

console.log(arr1)

Spread syntax is available in all major browsers (that excludes IE11). For the current compatibility, see this (continuously updated) compatibility table.

However, see Jack Giffin's reply below for more comments on performance. It seems concat is still better and faster than the spread operator.

If you're only appending a single variable, then push() works just fine. If you need to append another array, use concat():

var ar1 = [1, 2, 3];
var ar2 = [4, 5, 6];

var ar3 = ar1.concat(ar2);

alert(ar1);
alert(ar2);
alert(ar3);

The concat does not affect ar1 and ar2 unless reassigned, for example:

var ar1 = [1, 2, 3];
var ar2 = [4, 5, 6];

ar1 = ar1.concat(ar2);
alert(ar1);

There is a lot of great information on JavaScript Reference.

#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