Question

From Secrets of the Javascript Ninja (great walkthrough btw):

// We need to make sure that the new operator is always used
function User(first, last){ 
  if ( !(this instanceof User) ) 
    return new User(first, last); 

  this.name = first + " " + last; 
} 

var name = "Resig"; 
var user = User("John", name); 

assert( user, "This was defined correctly, even if it was by mistake." ); 
assert( name == "Resig", "The right name was maintained." );

Do any real-world code style guides make all constructors do this kind of safeguarding (the first two lines)? I believe linters will catch calls like User("John", "Resig") and warn about the missing new, won’t they?

Answer 1

I recommend you use strict mode and test in a browser that supports it. For example,

function User(first, last) {
    "use strict";
    this.name = first + " " + last;
}

this will throw in strict mode if you neglect the new. For example,

User("a", "b")

will throw a TypeError in Firefox. This allows you to catch errors without having to pay the penalty of an instanceof check. It throws a type error even if you call it outside of strict mode. This is because directly invoked strict mode funcitons have this bound to undefined instead of global. Setting a property on undefined results in a TypeError exception.