Javascript - Apply trim function to each string in an array

Question

Want to trim each string in an array, e.g., given

x = [' aa ', ' bb '];

output

['aa', 'bb']

My first trial is

x.map(String.prototype.trim.apply)

It got "TypeError: Function.prototype.apply was called on undefined, which is a undefined and not a function" in chromium.

Then I tried

x.map(function(s) { return String.prototype.trim.apply(s); });

It works. What's the difference?

Answer 1

String.prototype.trim.apply is the Function.prototype.apply method without being bound to trim. map will invoke it with the string, the index and the array as arguments and nothing (undefined) for the thisArg - however, apply expects to be called on functions:

var apply = String.prototype.trim.apply;
apply.call(undefined, x[0], 0, x) // TypeError

What you can do is passing the trim function as the context for call:

[' aa ', ' bb '].map(Function.prototype.call, String.prototype.trim)
// ['aa', 'bb']

What happens here is

var call = Function.prototype.call,
    trim = String.prototype.trim;
call.call(trim, x[0], 0, x) ≡
      trim.call(x[0], 0, x) ≡
            x[0].trim(0, x); // the arguments don't matter to trim