A pure function is a function that does not have a side-effect. Nothing the function does will ever affect a variable outside of it. This means that the input argument, when used in a computation, should not cause a side-effect, such as talking to the filesystem or opening a network connection, for example. Let's look at an example:
function notAPureFunction(filePath) { const fileContent = fs.readFileSync(filePath); const rows = fileContent.split(','); let sum = 0; rows.forEach(row => { sum += row; }); return sum;}
As we can see, our function opens up a file, loops through its rows, and calculates a sum of all the row contents. Unfortunately, the function talks to the filesystem, which is considered a side-effect. It may look ...
