Javascript "for" syntax -

so in this question asked how javascript compressed. question answered yet following snippet confuses me had ask question. here is:

for (y = 0; $ = 'zxqj`_^zwvuqonmkjihgca@8$  ' [y++];)       with(_.split($)) _ = join(pop()); eval(_)  

where syntax come from? kind of understand third operation ([y++]), why in brackets , why isn't there semicolon before it? also, @ line 2: looks me _ variable being declared, kind of variable type with(_.split($))?

any for command can broken down. start "classic" loop:

for( i=0; i<end; i++) 

this translates to:

• set i=0
• repeat following long i<end:
  • run code in block
  • increment i

so, applying same code:

• set y=0
• as long character in position y of string exists (ie. y not past length of string), assign character $ , increment y, then:
  • run code in block
  • do nothing (there nothing after second semicolon)

it's ugly hell, still makes sense if break down.