Why is this crazy javascript not causing a syntax error?

Question

Today I played with some Javascript snippets and noticed that this code actually runs:

{{for(var i = 0; i < 3; i++) {{{{ alert(i); }}}}}} 

You can try it yourself for jsFiddle .

Why is this done without any syntax errors? What do duplicate brackets mean? Javascript just ignoring duplicate curly braces?

Answer 1

{ x++; } { x++; } is a Block Statement .

{{{ x++; }}} {{{ x++; }}} is a block inside a block inside a block.

Code is executed inside each block. Therefore, adding extra {} around someting does nothing.

Answer 2

It creates a new block, which is actually useless ¹ since JavaScript does not have a block ^{2 area} .

_{¹ This is a beautiful oxymoron.
² nonetheless}

Answer 3

Brackets in brackets are just delimited blocks of code. Your sample can expand to:

 { { for(var i = 0; i < 3; i++) { { { { alert(i); } } } } } } 

which is stupid but subtle