Prevent variables from being used if not guaranteed initialization.

What rule do you want to change? https://eslint.org/docs/rules/init-declarations It may be it’s own rule because I couldn’t really find a good fit for this.

Does this change cause the rule to produce more or fewer warnings? More

How will the change be implemented? (New option, new default behavior, etc.)? New option

Please provide some example code that this change will affect:

Bad: abc may be left uninitialized before used.

function foo() {
  let abc;
  
  if (returnsTrue()) {
    abc = 123; 
  }
  useIt(abc); // Oops, abc is used but may be uninitialized. Possible error?
}

foo();

Good: abc is guaranteed to be initialized when used.

function foo() {
  let abc;
  
  if (returnsTrue()) {
    abc = 123; 
  } else {
    return; // this return will make sure abc is initialized before use
  }

  useIt(abc);
}

foo();

What does the rule currently do for this code? Nothing.

What will the rule do after it’s changed? It will prevent variables from being used if it’s not guaranteed that they have been initialized.

[mdjermanovic]

There is a recently accepted proposal for no-unassigned-vars rule in #12497, with the ongoing discussion about its scope, which may include code path analysis. It looks like the suggested enhancements in the discussion there are same as this proposal.

[ilyavolodin]

I think this rule is getting into a realm of typing (flow/typescript). I think those tools would be better to cover something like this, because ESLint will never be able to check this fully without going through all function calls (which we don’t do).