Now in ECMAScript2015 (a.k.a. ES6), you can use the spread operator to append multiple items at once:

var arr = [1];
var newItems = [2, 3];
arr.push(...newItems);
console.log(arr);

See Kangax's ES6 compatibility table to see what browsers are compatible

you were pretty close with the spread operator, your code just needed a little change, with this change you can pass multiple items as separated values or even an array with multiple items, even multiple arrays with multiple values.

they end always being a plain array of strings, here is your working code.

const cart = {
  contents: [],
  addItem: (...items) => {
    cart.contents = [...cart.contents, ...items];
  },
  removeItem(deletedItem) {
    this.contents.splice(this.contents.indexOf(deletedItem), 1);
  }
};

// multiple values
cart.addItem("laptop", "guitar", "phone");
console.log(`The cart contains: ${cart.contents}`);

// array of values
cart.addItem(['new laptop', 'new guitar', 'new phone']);
console.log(`The cart contains: ${cart.contents}`);

// multiple arrays
cart.addItem(['inner laptop1', 'inner guitar1', 'inner phone1'], ['inner laptop2', 'inner guitar2', 'inner phone2'], ['inner laptop3', 'inner guitar3', 'inner phone3']);
console.log(`The cart contains: ${cart.contents}`);