Normalize inputs to dry up and flatten code structure

normalized.js

// Now the data you'll be working with will ALWAYS be an array, so you can plan for that.
// You still needed the conditional (a ternary in this case), but you've pushed it to the top of your code
// allowing all subsequent code to work with it in a more uniform way.
const normalizedData = Array.isArray(data) ? data : [data];

const transformedData = normalizedData.map(myTransformerFunction);

not-normalized.js

let transformedData;

if (Array.isArray(data)) {
  transformedData = data.map(myTransformerFunction);
}
else {
  transformedData = [data].map(myTransformerFunction);
}

// Note the nearly identical work being done in each conditional branch?