How to use javascripts replace () on dynamic string

Working with an expression obtained from the api source code. Can't get it to work like they have. Or it doesn't work as I would expect.

//original expression
// returns an anonymous function signature for example 'function(a, b)'
var args = ['a', 'b', 'c'];
    return 'function ' + (args[1] || '').replace('/[\s\r\n]+/', ' ');

//1 - this works
var args = ['a', 'b', 'c'];

var string1 = 'function ' + (args[1] || '');
console.log(string1.replace(/function/, 'fn'));    
// fn b

//2- this.does not(using the api expression construct)
console.log('function' + (args[1] || '').replace(/function/, 'fn'));  
// function b

      

        
        
        
      

    

Question: Why doesn't the replacement work at # 2? to get "fn b" in a single expression. myplunk thanks