Why are function declarations handled differently in different browsers?

This answer is outdated since the release of ES6 in 2015. See What are the precise semantics of block-level functions in ES6? for how it works since then.

Function declarations are not valid in blocks. You have undefined behaviour which is undefined.

Function declarations at a top level (either global or top level within a function) are hoisted.

Function declarations inside blocks are a syntax error in strict mode

(function () { 
  "use strict"; 
  if (true) { 
    function g() { } 

SyntaxError: In strict mode code, functions can only be declared at top level or immediately within another function.

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