What is Control Flow?

A fundamental skill in programming is controlling how a program executes. The control of program execution is categorized using three control structures. They are "sequential", "selection", and "repetition". So far, you've written straightforward code that uses sequential JavaScript statements that execute one after another. More complex decision-making programs use selection and repetition. Programs also implement different types of looping structures, such as looping until some condition is met (sentinel) or looping a specific number of times (counter control). As you read through this section, you should notice that control structures in JavaScript use "reserved keywords" (if, else, switch).

Control flow

When your program contains more than one statement, the statements are executed as if they are a story, from top to bottom. This example program has two statements. The first one asks the user for a number, and the second, which is executed after the first, shows the square of that number. 
let theNumber = Number(prompt("Pick a number"));

console.log("Your number is the square root of " +
            theNumber * theNumber);

The function Number converts a value to a number. We need that conversion because the result of prompt is a string value, and we want a number. There are similar functions called String and Boolean that convert values to those types.

Here is the rather trivial schematic representation of straight-line control flow:


Source: Marijn Haverbeke, https://eloquentjavascript.net/02_program_structure.html
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