get all the elements of a particular form

document.forms["form_name"].getElementsByTagName("input");

You can use FormData if you want the values:

var form = document.getElementById('form-name');
var data = new FormData(form);
for (var [key, value] of data) {
    console.log(key, value)
}

You're all concentrating on the word 'get' in the question. Try the 'elements' property of any form which is a collection that you can iterate through i.e. you write your own 'get' function.

Example:

function getFormElelemets(formName){
  var elements = document.forms[formName].elements;
  for (i=0; i<elements.length; i++){
    some code...
  }
}

Hope that helps.

document.getElementById("someFormId").elements;

This collection will also contain <select>, <textarea> and <button> elements (among others), but you probably want that.