Question

This statement simply checks an “@” symbol in a textbox.

if (string1.indexOf("@")==-1){
 alert("Please input a valid email address!")
 document.example.email.focus()

This. if (string1.indexOf("@")==-1){ why do we use -1 instead of 0.

it baffles me, since 0 is been used almost everywhere as a ‘null’ or ’empty’ value.

Answer 1

In most programming languages, indexes are zero-based, meaning that the first position in an index (again in most programming languages, a string is an index of characters) will be 0 – hence 0 can’t be used to indicate nothing was found anywhere.

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To help clarify:

A string is an index of chars (characters, or single-symbol types). So, "[e-mail]" is an index containing 9 positions:

[0] = 'h'
[1] = 'i'
[2] = '@'
[3] = 'h'
[4] = 'o'
[5] = '.'
[6] = 'c'
[7] = 'o'
[8] = 'm'

Because indexes in JavaScript are zero-based they always start with their first position being 0. indexOf uses -1 to tell you that it couldn’t find the @ anywhere because it can’t use 0 since 0 is actually the first position of the index.

Even if strings weren’t indexes most languages would still use -1 to indicate it couldn’t find the character and 0 to indicate the first position for reasons of tradition and de facto standards.

In the above example, indexOf("@") would return 2, not 3. Again, because indexes are zero-based.