new Array(width - this.length + 1).join(char)

This ^ is effectively saying "make an empty array with n number of slots, then join the empty slots together using the char to separate each empty slot. So, if char = "0", and n = 3, we get 000.

width - this.length + 1 is used to determine the number of characters needed to be added to beginning of the string.

Then we add that to the beginning of the original string: this

+1

You need the + 1 because of how join works.

new Array(1).join('0') = ""      // wrong
new Array(1+1).join('0') = "0"   // correct
new Array(2).join('0') = "0"     // wrong
new Array(2+1).join('0') = "00"  // correct

If you want to map objects to something (in this case a property). I think Array.prototype.map is what you're looking for if you want to code functionally.

(fiddle)

If you want to support older browsers, that are not ES5 compliant you can shim it (there is a polyfill on the MDN page above). Another alternative would be to use underscorejs's pluck method:

var users = [
      {name: "Joe", age: 22},
      {name: "Kevin", age: 24},
      {name: "Peter", age: 21}
    ];
var result = _.pluck(users,'name').join(",")