Enforcing toString()

JavaScript usually automatically converts values to the type that a method or operator needs, which can lead to a variety of bugs. As a counter-measure, Brian McKenna (@puffnfresh) suggests using the following code for your tests:

    Object.prototype.valueOf = function () {
        throw new Error('Use an explicit toString');
    };

What is the effect of this code? You now can’t use the plus operator to convert an object to string, any more:

    > var obj = {};

    > 'Hello '+obj
    Error: Use an explicit toString

    > String(obj)
    '[object Object]'
    > obj.toString()
    '[object Object]'

    > 'Hello '+String(obj)
    'Hello [object Object]'

How does this work?
To convert an object to a specific primitive type T, it is first converted to any primitive value which is then converted to T. The former conversion happens in two steps [1]:

• Call method valueOf(). If it returns a primitive, we are done.
  


• Otherwise, call method toString(). If it returns a primitive, we are done.
  


• Otherwise, throw an error.

The above order of first invoking valueOf() and then toString() is chosen if the final conversion is to a number. If the final conversion is to string then toString() is invoked first. The plus operator converts to either number or string, but it produces a first primitive via the “number” algorithm [2].

Without the code snippet at the beginning of this post, Object.prototype.valueOf() returns this (an object) and is inherited by all objects that don’t override this method:

    > var obj = {};
    > obj.valueOf() === obj
    true

The plus operator therefore eventually calls toString(). The code snippet prevents that and throws an error before the operator can get to that method.
Note that the error message is not always completely correct:

    > Number(obj)
    Error: Use an explicit toString

But this trick can still be useful. If an object really wants to be converted to a number then it will bring its own valueOf(), anyway.

References

1. Coercing objects to primitives


2. What is {} + {} in JavaScript?