['1', '2', '3'].map(parseInt)

map(parseInt).js

// ['1', '2', '3'].map(parseInt) returns [1, NaN, NaN]; why?

// simplified `map`, to be able to step into the code through the debugger
function map(array, callback) {
  var mappedArray = [];
  for (var i = 0; i < array.length; i++) {
    mappedArray[i] = callback(array[i], i, array);
  }
  return mappedArray;
}

debugger;
var result = map(["1", "2", "3"], parseInt); 
console.log(result); // [1, NaN, NaN]

// compare with
var result2 = map(["1", "2", "3"], function(n) {return parseInt(n)});
console.log(result2); // [1, 2, 3]

// side experiment: what happens when we pass an extra argument to a function
function logName(first, second, third) {console.log(first, second, third)}
logName("first", "second", "third", "fourth"); // first second third