Check truthy/falsy-ness for non-primitives to identify unreachable code

Suggestion

In some cases TypeScript can identify unreachable code and throws an error:

const isTrue = true;
if (isTrue === false) {
  console.log('Can never be reached.')
}

In other cases it is currently not possible, but it would be very helpful. (Maybe even with a suggestion what could have been meant - see array example.)

const isArray = ['hello'];
if (!isArray) {
  console.log('Can never be reached. You probably meant `if (!isArray.length) {`.')
}

const isObject = {};
if (!isObject) {
  console.log('Can never be reached.')
}

🔍 Search Terms

• reachable / unreachable
• dead code
• condition
• falsy / truthy
• primitive / non-primitive
• always

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code -> It can break code, but only to show potential bugs.
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Throw errors on unreachable code examples.

const isArray = ['hello'];
if (!isArray) { // error: This condition will always return false. Did you meant to check `isArray.length`?
  console.log('Can never be reached. You probably meant `if (!isArray.length) {`.')
}

const isObject = {};
if (!isObject) { // error: This condition will always return false.
  console.log('Can never be reached.')
}

// OR:
if (isArray) {
} else {
  console.log('Can never be reached. You probably meant `if (!isArray.length) {`.')
}

📃 Motivating Example

This week I saw a colleague which could have found a bug way sooner by introducing such a check. ❤️

💻 Use Cases

Finding bugs.

Start by reviewing TypeScript's existing unreachable-code and condition diagnostics, then compare their behavior with the array and object examples in the issue. Define how truthy/falsy non-primitives should be reported and whether the suggested length guidance is appropriate; done means the examples produce the intended diagnostics without changing emitted JavaScript.