Does strict mode prohibit statement level function

问题:

"use strict";

if (true) {
  function foo() {
  }
}

In PhpStorm this code shows an error:

Function statement not at top level of a program or function is prohibited

However, Chrome happily executes it, even in the debugger and without any console output.

Now is it prohibited or not?

回答1:

Yes, in ES5 they are prohibited (and in strict mode, all implementations throw). See also Kangax' great article for function statements in sloppy mode.

However, in ES6 they are block-level function declarations with new semantics. See also What are the precise semantics of block-level functions in ES6?. This seems to be what Chrome implements here; foo is not available outside of the if block.