Can we disallow to use || on optional types by lint?

Question

I want to disallow following code:

function(num: undefined|number) {
  return num || 10;
}

I want use ?? instead of ||.

Answer 1

You can use the rule @typescript-eslint/prefer-nullish-coalescing:

This rule reports when you may consider replacing:

• An || operator with ??
• An ||= operator with ??=

Also note that the rule documentation states:

CAUTION

This rule will not work as expected if strictNullChecks is not enabled.

To enable it add "@typescript-eslint/prefer-nullish-coalescing": "error" to the rules in the linter configuration.

Read the documentation page for additional configuration options for the rule.