Javascript - Array.map with String.trim

Question

Why does the following not work? (Chrome, so no problem with missing Arrays.map)

[" a ", "b", " c", "d "].map(String.prototype.trim)

TypeError: String.prototype.trim called on null or undefined

Answer 1

map passes each element of the array as a parameter to the function:

[element1, e2].map(myFunction); // --> myFunction(element1); myFunction(e2)

String.prototype.trimis not a function that you pass in the string to be trimmed. Instead, you call the function as a method of this line:

" some string ".trim(); // "some string"

To use trimin .map, you need to do something like:

[" a ", "b", " c", "d "].map(function(e){return e.trim();});

Answer 2

In fact, since the function Array.map()must have currentElementas an argument, and String.prototype.trimdoes not accept any arguments, therefore, we cannot call it that way. So you have to make it hard:

[" a ", "b", " c", "d "].map(function(elem){
     return elem.trim();
});

Answer 3

Ramda ( Javascript):

> var R = require('ramda')
undefined
> [" a ", "b", " c", "d "].map(R.trim)
[ 'a', 'b', 'c', 'd' ]

node repl.

Answer 4

One shorter version with arrow function:

[" a ", "b", " c", "d "].map(e => e.trim());