Objects are passed by reference. This means when we pass a variable assigned to an object into a function as an argument, the computer interprets the parameter name as pointing to the space in memory holding that object. As a result, functions which change object properties actually mutate the object permanently (even when the object is assigned to a const variable).

const spaceship = {
  homePlanet : 'Earth',
  color : 'silver'
};

let paintIt = obj => {
  obj.color = 'glorious gold'
};

paintIt(spaceship);

spaceship.color // Returns 'glorious gold'

Our function paintIt() permanently changed the color of our spaceship object. However, reassignment of the spaceship variable wouldn’t work in the same way:

let spaceship = {
  homePlanet : 'Earth',
  color : 'red'
};
let tryReassignment = obj => {
  obj = {
    identified : false, 
    'transport type' : 'flying'
  }
  console.log(obj) // Prints {'identified': false, 'transport type': 'flying'}

};
tryReassignment(spaceship) // The attempt at reassignment does not work.
spaceship // Still returns {homePlanet : 'Earth', color : 'red'};

spaceship = {
  identified : false, 
  'transport type': 'flying'
}; // Regular reassignment still works.

Let’s look at what happened in the code example:

• We declared this spaceship object with let. This allowed us to reassign it to a new object with identified and 'transport type' properties with no problems.
• When we tried the same thing using a function designed to reassign the object passed into it, the reassignment didn’t stick (even though calling console.log() on the object produced the expected result).
• When we passed spaceship into that function, obj became a reference to the memory location of the spaceship object, but not to the spaceship variable. This is because the obj parameter of the tryReassignment() function is a variable in its own right. The body of tryReassignment() has no knowledge of the spaceship variable at all!
• When we did the reassignment in the body of tryReassignment(), the obj variable came to refer to the memory location of the object {'identified' : false, 'transport type' : 'flying'}, while the spaceship variable was completely unchanged from its earlier value.