Why Ts Complains With Function Declarations Inside Function Body

I have this error from TS: It's pretty clear why the error occurs: function outer(){ if (true) { function inner(){ // nested function declaration } }

Solution 1:

Would a function expression be a better choice

Yes. The following is the way to go:

functionouter() {
  if (true) {
    const inner = function() { // OK
    }
  }
}

Why?

• ES modules are in strict mode by default.
• strict mode does not allow function declarations in blocks

Reason why it was disallowed is covered in the original JavaScript specification. Short version: The behaviour was inconsistent between implementations.

NOTE Several widely used implementations of ECMAScript are known to support the use of FunctionDeclaration as a Statement. However there are significant and irreconcilable variations among the implementations in the semantics applied to such FunctionDeclarations. Because of these irreconcilable differences, the use of a FunctionDeclaration as a Statement results in code that is not reliably portable among implementations. It is recommended that ECMAScript implementations either disallow this usage of FunctionDeclaration or issue a warning when such a usage is encountered. Future editions of ECMAScript may define alternative portable means for declaring functions in a Statement context.

So when strict mode came into being (ES5) it made it disallowed.