Search Terms

https://github.com/Microsoft/TypeScript/search?q=type+guards+constants+narrowing&type=Issues

Suggestion

There is no way to use a saved result of a type guard evaluation for narrowing. Allow narrowing by

• storing assertions (booleans) of narrowing into constants
• impose narrowing based on the values carried by those constants

Use Cases

• reuse for narrowing evaluations, in case there are two places that narrow using the same guard on the same value
• declutter "if" expressions
• give names to narrowing facts for better readability

Examples

// now
            if (
                isLoneAvailableCapacity(feed) &&
                (isAvailableCapacitySubbeamPath(link) || isAvailableCapacityConnectivityLegPath(link)) &&
                toAvailableCapacityKey(feed.availableCapacity) === link.availableCapacityKey
            ) {


// could be
            const isAvailableCapacityPairdWithPath = isLoneAvailableCapacity(feed) &&
                (isAvailableCapacitySubbeamPath(link) || isAvailableCapacityConnectivityLegPath(link)) &&
                toAvailableCapacityKey(feed.availableCapacity) === link.availableCapacityKey

            if (isAvailableCapacityPairdWithPath) {
            }

Checklist

My suggestion meets these guidelines:

• [+] This wouldn't be a breaking change in existing TypeScript / JavaScript code
• [+] 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. new expression-level syntax)