Programmers are used to writing programs that are executed in a linear fashion. As you write, test, and execute your program, you expect your instructions to run in the order that you wrote them. In Listing 1-1, you have a program that will first assign a variable a to the number 2. It will then assign a variable b to the number 3, followed by assigning a variable sum, the sum of a + b, and it will finally print a result to the console. There is no way this program will work if you try to print(sum) before you even managed to calculate the value for sum.
let a = 2
let ...
