when to use toString() and valueOf()?

[wjm1314]

Example(Unexpected result):

var foo = {
toString: function() { return 'foo'},
valueOf: function() { return 5}
}
console.log(foo + 'bar'); //5bar
console.log([foo,'bar'].join('')); //foobar

[pero5ar]

The join is actually perfectly expected. You're joining array elements into a string so it triggers toString for each element.

The + is also expected, although a bit less, the thing is that + is primarily an arithmetic operator so it triggers valueOf, however the valueOf a string primitive is that string primitive and since one of the values in the expression remained a string the + falls back to being a concatenation operator. This can be seen as a bit WTF, however situations similar to that are covered in the Funny math section.

[wjm1314]

oh~~~, I got it! Thank you!