no-inner-declarations.md

no-inner-declarations

disallow function or variable declarations in nested blocks

Do not declare block scoped identifiers (function and var) inside of a block scope (such as an if)

Example:

Good

function foo(bar) {
  var quux = null;
  if (bar) {
    quux = 1;
  } else {
    quux = 2;
  }
  if (thing) {
    quux = 3;
  }
  return quux;
}

Bad, redefining variables and functions in different scopes

function foo(bar) {
  if (bar) {
    var quux = 1;
  } else {
    var quux = 2;
  }
  return quux;
}

if (thing) {
  function foo() {
    return 3;
  }
}

Justification

var is inherently block scoped, which if you're writing modern ES2015+ code, is an oddity as function, const and let will be block scoped. This might trip you up - especially in refactoring code - when seeing a declaration in a block, assuming it to be scoped to that block. By enabling this rule, we prevent this being a problem - as var is specifically disallowed inside block scopes - its nearest upper scope must be a function!

As for functions, well, prior to ES2015 they are function scoped like vars, however in ES6 compliant code they are block scoped. This, again, is a potential gotcha, and as such it is easier to disallow block scoped functions, and just live happily ever after by scoping your functions to the nearest function scope.

How to disable

There's little reason to disable this within individual files, but if you feel like disabling it for the whole project, here is how:

package.json (project-wide)

"eslintConfig": {
  "extends": ["strict/es5"],
  "rules": {
    "no-inner-declarations": 0
  }
}