Filling a dropdown list based on another dropdown list in the same html form

You can achieve this using an object to hold the values and their associated dropdown's descriptions. In order to do this, you firstly need to add an event listener to your dropdown so that it will detect a change when you pick a new fruit. Using the change event listener, you can retrieve the value of the option which was selected using this.value.

Using the value from the option selected, you can proceed to get its associated dropdown's values from the object called prices (this will return an array). Once you've gotten this array, you can loop through it and "build" a string of HTML using .reduce() to place as the options for the price select tag. Once you've built this string, you can append it inside the select tag using .innerHTML which "converts" your HTML string to DOM objects (real elements rather than just text):

const prices = {"apple":[{value:3,desc:"Apple 1kg 3€"},{value:5,desc:"Apple 2kg 5€"},{value:7,desc:"Apple 3kg 7€"}],
             "banana":[{value:3,desc:"Banana 2kg 3.5€"},{value:5,desc:"Banana 4kg 7€"},{value:7,desc:"Banana 5kg 11€"}],
             "peach":[{value:3,desc:"Peach 1.5kg 3€"},{value:5,desc:"Peach 3kg 6€"},{value:7,desc:"Peach 4kg 7€"}]}

const price = document.querySelector('[name=price]');
document.querySelector('[name=fruit]').addEventListener('change', function(e) {
  price.innerHTML = prices[this.value].reduce((acc, elem) => `${acc}<option value="${elem.value}">${elem.desc}</option>`, "");
});

<select name="fruit">
  <option value="apple">Apple</option>
  <option value="banana">Banana</option>
  <option value="peach">Peach</option>
</select>
<br />
<select name="price">
  <option value="3">Apple 1kg 3€</option>
  <option value="5">Apple 2kg 5€</option>
  <option value="7">Apple 3kg 7€</option>
</select>

If you don't feel comfortable using .reduce() you can use a regular for loop instead:

...  
let options = "";
for(obj of prices[this.value]) {
  options += '<option value="' +obj.value +'">' +obj.desc +'</option>';
}
price.innerHTML = options;
...