consistent-return

A function that returns a value on one path and nothing on another sometimes gives its caller undefined. That can be on purpose, but often someone forgot a value, or the function runs off the end by accident.

The function's first return sets the expectation. Every return that differs from it is reported, and so is a function that returns a value but can also run off the end.

Configure

Off by default. To turn it on:

[lint.rules]
"consistent-return" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"consistent-return" = ["error", {
  treatUndefinedAsUnspecified = false,
}]

Examples

Reported:

function findUser(id) {
  if (!id) {
    return;
  }
  return users.get(id);
}

function discount(order) {
  if (order.total > 100) {
    return 10;
  }
}

Not reported:

function findUser(id) {
  if (!id) {
    return undefined;
  }
  return users.get(id);
}

function discount(order) {
  if (order.total > 100) {
    return 10;
  }
  return 0;
}

function log(message) {
  if (!enabled) {
    return;
  }
  console.log(message);
}

Options

function findUser(id) {
  if (!id) {
    return undefined;
  }
  return users.get(id);
}

When not to use it

In TypeScript, the compiler option noImplicitReturns finds the same mistakes and knows the function's return type.