Assume we have two variables a and b:

let a = 5;
let b = 7;

Let us swap the values. First we get the sum of the two variables and assign it to a

a = a + b; // a = 12

Now, let us reasign the value of b with the value of a by getting the difference

b = a - b;  // b = 12 - 7 = 5

Now, let us reasign the value of a with the value of b by getting the difference

a = a - b;  // b = 12 - 5 = 7

Let us see what our variables have by printing them out.

console.log(a); // 7
console.log(b); // 5

This is how the whole code should look like:

let a = 5;
let b = 7;
console.log('The value of a and b before swapping.');
console.log('The value of a is, ' a); // The value of a is 5
console.log('The value of b is, ' b); // The value of b is 7
// Get the sum
a = a + b; // a = 12
// Reasign b with the value of a
b = a - b;  // b = 12 - 7 = 5
// Reasign a with the value of b
a = a - b;  // b = 12 - 5 = 7
console.log('The value of a and b after swapping.');
console.log('The value of a is, ' a); // The value of a is 7
console.log('The value of b is, ' b); // The value of b is 5

That is it! We have swapped our values using arithmetic operations.