You can perform some operations on the current value of the variable, and then write the result back to the same variable. See an example:
let num = 1; // declare the variable num and write the value 1 to it
num = num + 2; // write to num itself plus 2
alert(num); // shows 3
Without running the code, determine what will be displayed on the screen:
let num = 1;
num = num + 1;
num = num + 1;
alert(num);
Without running the code, determine what will be displayed on the screen:
let num = 1;
num = num + 2;
num = num + 3;
alert(num);