Check string contains substring in javascript

Question

I'm building a node application and a module of this application checks whether the nameservers of a given domain name point to AWS.

Using the dns module I have the below code:

dns.resolveNs(domain, function (err, hostnames) { console.log(hostnames); console.log(hostnames.indexOf('awsdns') > -1); });

hostnames outputs an array of hostnames and the specific domain I've used has the following hostname structure (x4):

ns-xx.awsdns-xx.xxx

But console.log(hostnames.indexOf('awsdns') > -1); returns false?

hostnames is an array. e.g. ['ns-xx.awsdns-xx.xxx', 'ns-xx.awsdns-xx.xxx', 'ns-xx.awsdns-xx.xxx', 'ns-xx.awsdns-xx.xxx'] — leaksterrr
– leaksterrr, Commented Jul 26, 2015 at 15:41
A bit curious why you need the check at all, if all of the entries are known to have that substring...? — T.J. Crowder
– T.J. Crowder, Commented Jul 26, 2015 at 15:46

T.J. Crowder · Accepted Answer · 2015-07-26 15:45:30Z

If hostnames is an array, then hostnames.indexOf('awsdns') is looking for an entry that exactly matches (entire string) 'awsdns'.

To look for substrings in the array, use some:

console.log(hostnames.some(function(name) { return name.indexOf('awsdns') > -1; });

Or with ES6 syntax:

console.log(hostnames.some((name) => name.indexOf('awsdns') > -1));

Live Example:

var a = ['12.foo.12', '21.bar.21', '42.foo.42']; // Fails there's no string that entirely matches 'bar': snippet.log(a.indexOf('bar') > -1); // Works, looks for substrings: snippet.log(a.some(function(name) { return name.indexOf('bar') > -1; }));

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 --> <script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

T.J. Crowder · Accepted Answer · 2015-07-26 15:57:33Z

1

try

hostnames[0].indexOf('awsdns') > -1;

Since hostnames is an array, you need to check index of an actual hostname, not the array.

Note that this only works because you've said that if any of the entries has the substring, they all will. (Which is extremely unusual.) Otherwise, it would fail if the first entry didn't but a subsequent one did.

edited Jul 26, 2015 at 15:57

T.J. Crowder

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answered Jul 26, 2015 at 15:43

GPicazo

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8 Comments

GPicazo Over a year ago

He didn't specify. The point of the my answer is to point out what he was doing wrong, not to write write the code for him since he doesn't specify what exactly he needs to do with this.

leaksterrr Over a year ago

Due to the nature of aws nameservers, 'awsdns' will always appear in the first record and there will always only ever be one set of nameservers attached to a domain.

T.J. Crowder Over a year ago

@leaksterrr: You mean if any of them have it, they'll all have it, but it's possible that you'll have an array where none of them does? (E.g., not related to AWS...)

leaksterrr Over a year ago

@T.J.Crowder yes, in which case separate handlers are in place for those occurences.

T.J. Crowder Over a year ago

@leaksterrr: That was really important information to include in the question.

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