Why Google Engineers Avoid JavaScript Default Parameters (And What to Use Instead)

JavaScript default parameters, introduced in ES6, let developers assign fallback values to function arguments. While they appear to simplify code and improve readability, Google’s engineering team often deliberately avoids this feature for several reasons.

Possible subtle errors

The behavior of default parameters can clash with developers' intuition. A default value is applied only when the argument is undefined; passing null, an empty string, or 0 leaves the default untouched, which can lead to hard‑to‑track bugs.

function greet(name = "Guest") {
  console.log(`Hello, ${name}!`);
}

greet(undefined); // "Hello, Guest!"
greet(null);      // "Hello, null!"
greet("");       // "Hello, !"
greet(0);        // "Hello, 0!"

Impact on code predictability

Default parameters are evaluated at each function call rather than at definition time, meaning the default expression is recomputed on every invocation.

Complex interactions with closures and scope

Default parameters can reference earlier parameters, creating a special scope that is separate from the function body, which may cause confusing interactions.

Reduced code readability

When default‑parameter logic becomes complex, the function signature can become long and difficult to read.

Alternatives

Google engineers typically prefer explicit argument handling, such as manual checks or using object destructuring.

Or use an object‑destructuring pattern:

function processData(data, options) {
  const { format = "json", version = "1.0" } = options || {};
  // function body
}

In large teams and complex projects, clear parameter handling reduces errors and improves code quality. This does not mean default parameters must always be avoided; developers should understand their behavior and apply them judiciously.