instanceof fails on multiple context

[obastemur]

We started to have this issue on JXcore - under limited memory environments (mobile, mini boards..)

Line: https://github.com/mishoo/UglifyJS2/blob/master/lib/utils.js#L223
Code:

 if (!(words instanceof Array)) words = words.split(" ");

'Array' here targets totally different memory reference when each of the sources run inside the 'vm' individually. This happens due to JavaScript engine optimization under very low resources. JS engine moves the reference of Array to another point while the optimized function code still checks for the same reference. Indeed this is JS engine related issue ( very tricky one ) but I don't know if it's something fixable in a short term. I bet this would also fail on iframe / webworker on browsers.

Suggestion: (not sure if this is cross browser but it does the magic on node/jxcore)

if(!Array.isArray(words)) ....

[obastemur]

function isSomething(thing, obj) {
  return Object.prototype.toString.call(obj) === '[object '+thing+']';
}

isSomething('Array', words)

This also works and it's cross browser.

If UglifyJS could execute all it's source files under the same VM at once, this wouldn't be a problem. Actually this would be the best way. We may not be sure when JS engine is going to issue a garbage collection and how it will effect the actual memory references / optimizations.

[alexlamsl]

vm is no longer in use after #636 and 99233c4