What issues can arise from using the 'do' keyword in JavaScript?

In JavaScript, do is a reserved keyword used to start a do…while loop. Because it is reserved, it cannot be used as an identifier (variable, function, or property name) without quoting. Attempting to declare a variable named do results in a SyntaxError:

var do = 5; // ❌ SyntaxError

In strict mode, the restriction is stricter: using a reserved word as an identifier is always disallowed. Even in non‑strict mode, some engines may allow it in certain contexts, but it is considered bad practice and can lead to confusing bugs.

If you need to use a property named do, you must quote it:

const obj = { 'do': 1 };
console.log(obj['do']);

Therefore, the main issue is that do cannot be used as a plain identifier, and developers must avoid naming variables or functions do to maintain compatibility and readability.