Question

In underscore.js, the following code seems to add _.isNumber()

  // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp.
  each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp'], function(name) {
    _['is' + name] = function(obj) {
      return toString.call(obj) == '[object ' + name + ']';
    };
  });

but there must be something else going on, because copying this code straight into firefox causes it to fail as toString.call(5) returns [xpconnect wrapped native prototype] — so its obviously doing something else somewhere – but I can’t figure out what.

Here’s an example of the results in firefox:

http://jsbin.com/uviyaz/2/edit

Answer 1

Your jsbin is showing window.toString, where as underscore is using Object.prototype.toString, they have aliased it to toString locally in their code.

See http://jsbin.com/uviyaz/3/edit

See also underscore’s source where they do it:

  // Create quick reference variables for speed access to core prototypes.
  var slice            = ArrayProto.slice,
      unshift          = ArrayProto.unshift,
      toString         = ObjProto.toString,
      hasOwnProperty   = ObjProto.hasOwnProperty;