New Rule: no-use-before-initialization

[adidahiya]

Suggested in microsoft/TypeScript#2234

If the typechecker knows that a value is undefined at a certain point in the program, this rule would disallow:

1. accessing properties or indexing into an object or array

   var arr: number[];
   var obj: any;
   arr.push(1);
   obj.foo = "bar";

2. returning a primitive value or using it in a RHS expression

   function dd(): number {
     var x: number;
     return x;
   }
   
   function ddd() {
     var x: number;
     x++;
   }

[pawciobiel]

Would it make sense to fix those too?

var a: number = 2;
var b: number;
var c: number = null;  // error
console.log(a + b);  // error
console.log(b + c);  // error
console.log(a + c);  // error

[adidahiya]

@pawciobiel yep, I think that would fall under the category of RHS (right-hand-side) expressions

[adidahiya]

blocked on #680 I believe

[andy-hanson]

no-use-before-declare seems to do this.

[adidahiya]

In v5.0 with the change to be a typed rule, no-use-before-declare does seem to do this.