Transitioning to strict mode

Claus Reinke claus.reinke at
Mon Feb 18 14:29:38 PST 2013

>> Out of curiosity, what does your favorite test coverage tool report 
>> for the source below? And what does it report when you comment
>> out the directive?
> :-p Ok, there are exceptions if your code depends on semantic changes 
> described in the third section of the article (dynamic this/eval/arguments).
> That's you case with how you define isStrict (dynamic this)
> So: if your code does *not* depend on semantic changes, all instances of 
> setting to an undeclared variable will be caught.

Just wanted to shake your faith in testing :-) The example code might
look unlikely, but real code is more complex and might evolve nasty
behavior without such artificial tuning.

You still need more than statement or branch coverage. Otherwise,
we might get 100% "coverage" while missing edge cases

    function raise() {
      "use strict";
      if( Math.random()>0.5 || (Math.random()<0.5) && (variable = 0)) 

    raise(); // adjust probabilities and call numbers until we get
                // "reliable" 100% branch coverage with no errors; then
                // wait for the odd assignment to happen anyway, in
                // production, not reproducably

Throwing or not throwing Reference Errors is also a semantics change, 
and since errors can be caught, we can react to their presence/absence,
giving another avenue for accidental semantics changes.

Undeclared variables are likely to be unintended, and likely to lead to
bugs, but they might also still let the code run successfully to completion 
where strict mode errors do or don't, depending on circumstances.

Testing increases confidence (sometimes too much so) but cannot
prove correctness, only the absence of selected errors.

What I'd like to understand is why likely static scoping problems
should lead to a runtime error, forcing the dependence on testing. 

If they'd lead to compile time errors (for strict code), there'd be no 
chance of missing them on the developer engine, independent of 
incomplete test suite or ancient customer engines. Wouldn't that 
remove one of the concerns against using strict mode? What am I 


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