JavaScript indexOf greater than -1 false when it's clearly not?

Question

The value of the field with the id 'post_url' is http://www.example.com/. Shouldn't JavaScript return true for anything greater than -1?

alert('1a = '+id_('post_url').value.indexOf('http:'));//      0
alert('1b = '+id_('post_url').value.indexOf('http:') > -1);// false
alert('2a = '+id_('post_url').value.indexOf('www.'));//       7
alert('2b = '+id_('post_url').value.indexOf('www.') > -1);//  false

Answer 1

You're comparing a string and a number. You're concatenating the result of indexOf (which I assume is -1) to a string before you do your > comparison. This is equivalent to NaN > -1.

"1b-1" > -1 // => false

Answer 2

Check Operator Precedence The addition/string concatenation has greater precedence than relational comparison.

Your code needs some brackets/grouping operator to cure that:

alert( '1b = ' + ( id_('post_url').value.indexOf('http:')>-1 ) ); // 1b = true

Answer 3

You are attempting to evaluate a string concatenation as a number, which is returning false.

'2b'+id_('post_url').value.indexOf('www.')>-1

...using order of operations, will add the string, 2B to the index returned by, id_('post_url').value.indexOf('www.')

Thus, casting the entire thing as a string, and evaluating whether the string is greater than -1.

Here's how you need to write your alert.

alert(id_('post_url').value.indexOf('www.')>-1);

That should alert 'true'.

Answer 4

'2b'+id_('post_url').value.indexOf('www.')

You are adding 2b to index of, so it won't remain an integer but will have value as string of 2b+ whatever index you have. So that is the reason for what you are observing.