If you wish you verify why is this, consider the following. Forget about String.prototype.trim at the moment. Consider this function: const f = function()

If you wish you verify why is this, consider the following. Forget about String.prototype.trim at the moment. Consider this function: const f = function() { console.log('receiver is', this, '. arguments are', arguments) } This function prints the receiver (aka "this"), and arguments. If you use it in the following way: ['a', 'b', 'c'].map(Function.prototype.call, f) The output is receiver is [String: 'a'] . arguments are { '0': 0, '1': [ 'a', 'b', 'c' ] } receiver is [String: 'b'] . arguments are { '0': 1, '1': [ 'a', 'b', 'c' ] } receiver is [String: 'c'] . arguments are { '0': 2, '1': [ 'a', 'b', 'c' ] } Meaning, that the receiver (aka "this") of the function will be the array element, and the arguments will be the array index and the array itself.