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In Javascript, when is a new scope created? (with a new function and in a "with" statement) Are these the only

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In Javascript, when is a new scope created? The 2 situations I know of are:

  1. with a new function (update on 2012/09, I think it needs to be a function invocation, not just a function definition)
  2. in a "with" statement

as a note, any new block (in if-then-else, loops, or just beginning a block for no other reason) won't create a new scope.

Is there a third situation where a new scope is created besides the two situations above? Thanks.

In Javascript, when is a new scope created? The 2 situations I know of are:

  1. with a new function (update on 2012/09, I think it needs to be a function invocation, not just a function definition)
  2. in a "with" statement

as a note, any new block (in if-then-else, loops, or just beginning a block for no other reason) won't create a new scope.

Is there a third situation where a new scope is created besides the two situations above? Thanks.

Share edited Sep 11, 2012 at 19:28 nonopolarity asked Apr 28, 2010 at 15:26 nonopolaritynonopolarity 151k142 gold badges490 silver badges782 bronze badges 3
  • 1 EcmaScript 5 strict mode no longer supports the with statement. – fpmurphy Commented Apr 28, 2010 at 15:50
  • 3 Jian Lin, it's customary to accept answer's to your questions. Not so much for the answerer's rep, but so that the rest of the interested munity knows that a conclusion was reached. – Justin Johnson Commented Apr 30, 2010 at 22:56
  • re definition vs. invokation: I believe scope is a property of the function. When the function is called, a context is created to hold the values for the variables in the function's scope. – allyourcode Commented Sep 12, 2012 at 21:09
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3 Answers 3

Reset to default 9

Yes, there is a third case where the scope chain is augmented (besides the let mozilla-extension that Shog9 mentions), when a catch block is evaluated:

The production Catch : catch (Identifier ) Block is evaluated as follows:

  1. Let C be the parameter that has been passed to this production.

  2. Create a new object as if by the expression new Object().

  3. Create a property in the object Result(2). The property's name is Identifier, valueisC. value, and attributes are { DontDelete }.

  4. Add Result(2) to the front of the scope chain.

  5. Evaluate Block.

  6. Remove Result(2) from the front of the scope chain.

  7. Return Result(5).

So basically, a new object is created, with a property named like the Identifier passed to catch, this new object is added to the scope chain, so we are able to use that identifier within the catch block.

try {
  throw "error";
} catch (identifier) {
  // `identifier` accessible here..
}

But keep in mind that it only augments the current scope temporarily, to introduce the catch Identifier, any variable declared inside will be simply hoisted to the top of it enclosing function.

There's also the let statement. Keep in mind that let (...) {}, like with (...){}, does not create a new scope for variables introduced within the block. However, let definitions can create variables scoped to the block (any block) they're defined in.

Fair warning: as has been pointed out in ments, while let is part of JavaScript 1.7 (the Mozilla dialect of ECMA-262/ECMAScript), it is not part of ECMAScript, and will likely not work any time soon in browsers other than Firefox. Also note that while with can be used as a stand-in for let statements in current implementations of ECMAScript, the "strict" mode proposed for the pending ECMA-262 5th edition disallows this as well. If you're concerned about writing cross-plaform, future-proof code (and you should be...) then stick with functions for scope control!

this will also refer to the object you are in:

a = {
   f: function(){ ... },
   b: function(){this.f()}
}
a.b() //calls a.f

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