append object javscript

• JavaScript is case-sensitive. Calling new array() and new object() will throw a ReferenceError since they don't exist.
• It's better to avoid new Array() due to its error-prone behavior.
  Instead, assign the new array with = [val1, val2, val_n]. For objects, use = {}.
• There are many ways when it comes to extending an array (as shown in John's answer) but the safest way would be just to use concat instead of push. concat returns a new array, leaving the original array untouched. push mutates the calling array which should be avoided, especially if the array is globally defined.
• It's also a good practice to freeze the object as well as the new array in order to avoid unintended mutations. A frozen object is neither mutable nor extensible (shallowly).

Applying those points and to answer your two questions, you could define a function like this:

function appendObjTo(thatArray, newObj) {
  const frozenObj = Object.freeze(newObj);
  return Object.freeze(thatArray.concat(frozenObj));
}

Usage:

// Given
const myArray = ["A", "B"];
// "save it to a variable"
const newArray = appendObjTo(myArray, {hello: "world!"});
// returns: ["A", "B", {hello: "world!"}]. myArray did not change.