Pet peeves:

• Using .map() (or any other array iteration method like .filter, etc), in JavaScript for simply going through the array instead of .forEach() or a loop. In general, use the array methods for their purpose:
• .map - I have an array, I another array where each item is based on the first one
• .flatMap - like .map but I use it when each new item also an array
• .filter - I have an array, I need less of it
• .find - I have an array, I need one item from it
• .findIndex - I have an array, I need the index of an item
• .some - I have an array, I need to check if at least one item passes a check
• .every - I have an array, I need to see if all items pass a check
• .forEach - I have an array, I need to do something with each item. Usually I don't want to return anything.
• .reduce/.reduceRight - I have an array, I need to make it into "one value". I'll have to check if I really need to use reduce for this.
• for, for..in, for..of, while, do..while loops if the problem can't (or doesn't need to) fit into one of the above.
• Using regular expressions when you need "something with text" instead of parsing, simple string manipulation, or basic arithmetic, and so on.
• Generating DOM content as a string of HTML.
• Explanations that say "This works fine" and then proceed to show a problem thus proving it doesn't work "fine".
• Unformatted code.