How to form an object based on key from another array of object?

If you're not very familiar with javascript, arrays and objects, it's probably better to keep it simple (avoid arrow functions and fancy es6 notation).

A simple Array#reduce should do the trick:

const availableList = [
    {id: 1, key: 'emp_id'},
    {id: 2, key: 'firstname'},
    {id: 3, key: 'lastname'},
    {id: 4, key: 'mobile'},
    {id: 5, key: 'email'},
];

var newObject = availableList.reduce(function (result, item) {
  result[item.key] = '';
  return result;
}, {});

console.log(newObject);

Some comments in case you're familiar with Array#reduce:

• The parameter I've named result is the value you use to "reduce" the array into a single variable.
• The parameter I've named item is the value of item of array during each iteration.
• The original value of result is the value specified as the second parameter of reduce (which in this case is {})
• Inside the function passed to the reduce, you have to return the value that will be passed in as the result parameter in the next iteration
• So in each iteration, you get the object with they keys you've added up to that point, you add another key with a value of empty string and then return it so that it can be used for the next iteration.
• Once it's done iterating, it will return the last value, that was returned by the inside function. That value gets assigned to the variable newObject

Here's my take on it using Object.fromEntries():

const availableList = [
  { id: 1, key: "emp_id" },
  { id: 2, key: "firstname" },
  { id: 3, key: "lastname" },
  { id: 4, key: "mobile" },
  { id: 5, key: "email" },
];

const result = Object.fromEntries(availableList.map((item) => [item.key, ""]))

console.log(result);

You have an array of properties. So as you suggested in your question, you need to use a forEach to iterate all the items. The items are objects in this case. So you can access all their properties just writing obj.property.

Let's see how can I add a property to an object. I could do:

obj.property5 = 'something'

Or I could do:

obj['property5'] = 'something'

Since you have something unpredictable as the name of the property you should use the second one. Here there's the code:

var secondObject = {};

availableList.forEach(function(item, index) {
  secondObject[item.key] = '';
});