Sourcemap link is not the last thing in file.

[MicahZoltu]

Rollup is generating a file that has a trailing newline. This MAY go against an implementation of the source map specification.

https://sourcemaps.info/spec.html#h.9ppdoan5f016

The generated code may include a line at the end of the source, with the following form:

//# sourceMappingURL=

One could interpret this to mean that the last characters of the file are the sourcemap comment, without a newline. If one did (as es-module-shims does currently), then the sourcemap comment in a rollup generated file would be invalid.

• Rollup Version: 1.21.4
• Operating System (or Browser): Windows Firefox
• Node Version: v12.3.0

How Do We Reproduce?

https://github.com/just-dont/sourcemaps-with-shims

Expected Behavior

There is no newline at the end of the sourceMappingURL comment.

Actual Behavior

There is a newline at the end of the sourceMappingURL comment.

Suggested fix:
Don't append a newline in order to maintain maximum compatibility.

[MicahZoltu]

Sorry to anyone subscribed to this repository, I accidentally hit ctrl-enter which submitted the issue before filling it in. I have updated the issue description now though.

[kzc]

Rollup is generating a file that has a trailing newline. This MAY go against an implementation of the source map specification.

I disagree. The source map spec is informal and editors commonly add either \n or \r\n to the end of files if a source map URL is added manually. Any good source map URL reading implementation would handle the presence or absence of newlines as all browsers currently do.

[MicahZoltu]

@kzc While I tend to agree that sourcemap users should be permissive in what they accept (and I believe there is an open PR for es-module-shims to make it more permissive), I also believe that Rollup should follow the strictest interpretation of the specification to obtain maximum compatibility. This follows the Robustness Principle design guideline:

Be conservative in what you send, be liberal in what you accept

[kzc]

The source map spec provided states:

There is an existing convention that should be supported for the use of source maps with eval’d code, it has the following form:

//@ sourceURL=foo.js

It is described here:

http://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/ 

It appears to be a dead link, but here's an archived copy of that web page:

https://web.archive.org/web/20190203041919/http://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/

An actual code fragment with a regular expression is provided:

    const reURIinComment = ///@ssourceURL=s*(S*?)s*$/m;

I assume that the web archive dropped the necessary backslashes to make this valid JS. If one fixes the regexp you can see that optional trailing whitespace - including newlines - is permissible:

    const reURIinComment = /\/\/@\ssourceURL=\s*(\S*?)\s*$/m;

Just to back up that assertion, I see that that web page makes mention of:

https://web.archive.org/web/20190203041940/https://pmuellr.blogspot.com/2009/06/debugger-friendly.html

which in turn mentions:

https://web.archive.org/web/20161221094120/https://bugs.webkit.org/show_bug.cgi?id=25475

which provides this valid JS:

    // if no URL, look for "//@ sourceURL=" decorator     
    if (!sourceURL) {
        var pattern = /.*\s*\/\/\s*@\s*sourceURL\s*=\s*(\S+)\s*/;
        var match = pattern.exec(source);
        
        if (match) {
            this.sourceURL = "eval:" + match[1];
        }
    }

So any source map URI at the end of the source code with or without trailing whitespace - including newlines - are conforming according to the sourcemap spec. es-module-shims does not conform to this spec.