if (!cachedReturnValues[key]) {
    cachedReturnValues[key] = value;
}

In very few situations this is dangerous; it only can become an issue in for-in loops.

This argument is a lot stronger than you agree with. First and foremost javascript allows the use of variables outside of a block scope. But essentially are these variables outside their scope? My answer is no. Note, javascript allows variables to be used; “outside of block scope”, which doesn’t mean(or better put is not equivalent to) “outside of scope”. Javascript expands the scope of a variable declared in a block, beyond the scope of the block(contrary to what exists in most other programming/scripting language idioms).

Of course this is a bad design, but it is accepted javascript coding practise. And I can say at the minimum 10% of javascript code in live, distribution(libraries) and production, have codes written this way. And for no other reason, than because javascript allows it. Once existing code is rewriten by replacing “var” with “let”, of course bugs can and will creep into code written this way.

Qouting Douglas Crockford; “In most languages, it is generally best to declare variables at the site of first use. That turns out to be a bad practice in JavaScript because it does not have block scope. It is better to declare all variables at the top of each function.” — Awful Parts: Appendix A – JavaScript: The Good Parts

That is why why , @James gave the title of this article as “JS adolesence”. Javascript experts or wizards anywhere else in the known universe won’t have a problem with this. But there is little argument that the concept is clear, practical and objective. And a few coders may have developed such tewwwwible habits (like @james emphasized).

Or/and would you be unwilling to also accept that the ECMAscript Sages chose to introduce the “let” keyword, partly for this same reason????

For example;

//very convenient for someone out there simply because it's a //javascript ballpark
function buggyBugs(you,me)
{
     if (you>me)
                 {
                 // imagine a find and replace "var" with "let"
                   var we = you;    
                      return we;
                 }
     we = me;
     return we;
}

@James, fine article.

Thanks

function doSomething() {
  var x = 1,
      y = 100,
      z = 'hello';
}

Readable, and efficient. 🙂

I agree 100%. Dogma, cargo-culting, and complacency are developer growth killers.