Prepending "http://" to a URL that doesn't already contain "http://"

Question

I have an `input` field that saves a URL, I'd like this saved input to recognize when "Http//" is absent from the start of the variable but have no idea where to begin... is it possible to check only a portion of a string? - then have a function that will append if necessary?

Accepted Answer

A simple solution for what you want is the following:

```
var prefix = 'http://';
if (s.substr(0, prefix.length) !== prefix)
{
    s = prefix + s;
}
```

However there are a few things you should be aware of...

The test here is case-sensitive. This means that if the string is initially `Http://example.com` this will change it to `http://Http://example.com` which is probably not what you want. You probably should also not modify any string starting with `foo://` otherwise you could end up with something like `http://https://example.com`.

On the other hand if you receive an input such as `example.com?redirect=http://othersite.com` then you probably do want to prepend `http://` so just searching for `://` might not be good enough for a general solution.

Alternative approaches

Using a regular expression:

```
if (!s.match(/^[a-zA-Z]+:\/\//))
{
    s = 'http://' + s;
}
```

Using a URI parsing library such as JS-URI.

```
if (new URI(s).scheme === null)
{
    s = 'http://' + s;
}
```

Prepending "http://" to a URL that doesn't already contain "http://"

Solution

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