我可以防止意外覆盖TypeScript / JavaScript中的局部变量吗？

Question

Today I wasted an hour debugging a trivial issue, where a local variable named server was being initialized and configured - then, on one of the last lines in the same file, accidentally it was being redeclared, eg by another var server = ... statement, effectively creating a new variable named server , thus causing the previous variable to fall out of scope; yet, because these were the same type of variable, with the same name, everything else continued to work, making this fairly hard to debug.

Is there a TypeScript or JavaScript language feature, that prevents this sort of thing?

My thinking is that, declaring two variables with the same name, in the same scope, ought not to be allowed at all.

Perhaps there's a linter or some quality assurance tool that has the ability to check for and prevent this sort of thing? (and perhaps other "bad" patterns?)

Answer 1

Use let everywhere possible.

A let variable cannot be used before its declaration:

var x = 3;
function f() {
  console.log(x); // ReferenceError, x is not defined
  let x = 5;
}

Answer 2

Two options:

1. Use ECMA Script 6 and let .
2. Use jslint with var .

There is a closed issue about this on the GitHub/Microsoft/Typescript page. The recommendation is to target ECMA Script 6 and use let .

ECMA Script 6 with let

In ECMA Script 6 this would create an error :

let x = "foo";
let x = "bar"; // TypeScript with ECMA 6 will complain here
console.log(x);

Duplicate declaration, x

JSLint with var

Also, though the following won't throw a TypeScript error , the jslint tool will complain about it, even if you aren't using strict.

(function () {

    var x, y;
    x = "foo";
    y = "foo";

    function sayMsg() {
        // jslint will complain here
        var y = "bar";
    }

    sayMsg();

    // jslint will also complain here
    var x = "bar" + y;

}());

This is what jslint will tell you:

Redefinition of 'y' from line 3.

Combine this with the previous 'var' statement.