Generate Symmetric Ascii
Omit the extra == true in conditions:

if(randomBool())


Use the ternary if operator to compact your code:

left_side.push(randomChoice(randomBool() ? chars : double_chars))


If this seems to dense for you, you can also extract the inner expression like this:

var options = randomBool() ? chars : double_chars;
left_side.push(randomChoice(options))