Accessing by value and reference

When you assign a value to a variable, the JavaScript engine will determine whether the value is a primitive or reference value.

Access by Value

• If the value is a primitive value, when you access the variable, you manipulate the actual value stored in that variable. In other words, the variable that stores a primitive value is accessed by value.

Copying Primitive Values

• When you assign a variable that stores a primitive value to another, the value stored in the variable is created and copied into the new variable

var a = 1;
var b = a;
b = 5

console.log(a); // 1;
console.log(b); // 5;

The above works because we're accessing by value. We're copying the value to b and b is a new address in the stack. a and b have no relationship, when you change the value stored in the b variable, the value of the a variable doesn't change.

Access by Reference

• Unlike a primitive value, when you manipulate an object, you work on the REFERENCE of that object, rather than the actual object.

• It means that a variable that stores an object is ACCCESSED BY REFERENCE

Copying by reference value

• When you assign a reference value from one variable to another, the value stored in the variable is also copied in the location of the new variable.

• THE DIFFERENCE is that the value stored in BOTH variables now are the ADDRESS of the actual object stored in the heap!

Example

var a = {a: "a"}
var b = {b: "b"}
a = b;
b.b = "c";

console.log(a); // what would this print out?

Answer 1

var a = {a: "a"}
var b = {b: "b"}
a = b;
b.b = "c";

console.log(a); // {b: "c"}

When we copy a to b we're merely pointing to the same address location in the heap. When we change modify b we're modifying the location in memory. Therefore when we log a , since it's pointing to the same location, it'll have the modified object.

Resource: https://www.javascripttutorial.net/javascript-pass-by-value/