javascript: Why can some functions be bound and mapped and others not? -

out of curiosity:

the mdn taught me how shortcut function application this:

trim = function.prototype.call.bind(string.prototype.trim)  join = function.prototype.call.bind(array.prototype.join) 

now, can map trim, not join reason. join takes ',' default argument (separator), should fine, instead uses array index:

> trim = function.prototype.call.bind(string.prototype.trim) call() > [' a','b '].map(trim) ["a", "b"] > join = function.prototype.call.bind(array.prototype.join) call() > [['a','b'],['c','d']].map(join) ["a0b", "c1d"] 

why?

also, if wanted different separator? passing bind doesn't work, since prepended existing arguments (at time 1 of elements of list on map). takes role of strings join , strings join act separators if there separate:

> joins = function.prototype.call.bind(array.prototype.join,';') call() > [['a','b'],['c','d']].map(joins) [";", ";"] 

i have researched , found:

answers explaining thing this , solutions equivalent bind shortcut referenced

similar explanations, solution using thisarg again, mdn's page map

a duplicate question duplicate answers

the function pass map receives 3 arguments. map works this.

array.prototype.map = function(f) {   var result = [];   for(var = 0; < this.length; i++) {     result.push(f(this[i], i, this));   }   return result; } 

so when run code [['a','b'],['c','d']].map(join), what's happening inside map

join(['a', 'b'], 0, [['a','b'],['c','d']]) join(['c', 'd'], 1, [['a','b'],['c','d']]) 

to result want write function can generate join functions. e.g.

function joinsep(separator) {   return function(array) {     return array.join(separator)   } } var joins = joinsep(';'); [['a','b'],['c','d']].map(joins)