strict boolean expressions compilerOption

Search Terms

require strict boolean

Suggestion

add yet another --strict check into compilerOptions, to only allow expressions that evaluate to Boolean for all conditional branching - if, for, while, ?: ternary operator, etc

basically this TSLint rule, built into TS compiler
https://palantir.github.io/tslint/rules/strict-boolean-expressions/

Use Cases

often people check the existence of function, before calling it:
if (somFunc) someFunc()

or they think that function is actualy a property, and hence dont call it

if (someVal)... . where it should have been if (someVal()) ...

if if required a strict boolean expression, both of the above would have been errors.

Examples

instead above examples would be written:
if (!!someFunc) someFunc()

and for someValue():boolean

if (someValue())..

Checklist

My suggestion meets these guidelines:

• 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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reviewing TypeScript's compilerOptions handling and the type-checking paths for if, for, while, and conditional expressions. Compare the requested behavior with the linked TSLint strict-boolean-expressions rule. Done means a compiler option enforces Boolean-valued conditional expressions without changing emitted JavaScript.