Why does this javascript snippet '([][[]]+[])[!+[...' returns 'deadlines'?

My boss wanted to make a joke, and said that the most important thing in our job was:

([][[]]+[])[!+[]+!+[]]+([][[]]+[])[!+[]+!+[]+!+[]]+(![]+[])[+!+[]]+([][[]]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]+([![]]+[][[]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+([][[]]+[])[!+[]+!+[]+!+[]]+(![]+[])[!+[]+!+[]+!+[]]

When I executed it (in javascript) I got string deadlines, how can this be? I tried to guess how it is working, but I don't get it (I work with javascript for a couple of months).

via Perostek Balveda