[thght]

I would use my dynamic type checker/enforcer to make it pure:

  var types= require( 'types.js' );

  function sum(arr) {

    arr = types.forceArray( arr );
    var z = 0;
    for (var i = 0; i < arr.length; i++) {
      z += types.forceNumber( arr[i], 0 );
    }
    return z;
  }