1 + 1 will output 11 and not 2...

[mwinter]

If you're not sure whether you're dealing with a number or a string, be safe and convert it. There are several methods of doing this:

new Number(n);

No! That evaluates to a Number object, not a number. Whilst it should work, it will involve further conversion (through use of the valueOf method). Instead, call the constructor function as a function:

Number(n)

-n;

Just to point out the obvious: this will evaluate to a number with reversed sign.

Isn't the easiest way, then to do
a + 1*b; ?

There isn't really any objective answer to that, though for compactness I would use unary plus (+) and for readability, the Number constructor function:

a + (+b)
a + Number(b)

The parentheses emphasis the meaning, as well as help prevent an accident like confusing "+ +b" for "++b".

Yes, +"3" performed an average of 0.0053ms faster than parseInt("3", 10) over 100,000 iterations in SpiderMonkey.

Unary plus should be the fastest of all techniques.