Bug: linkStyle can't specify IDs that don't exist at the point in the diagram

[Yash-Singh1]

Describe the bug
A clear and concise description of what the bug is.

If I try to specify a linkstyle with a id that exists after the current linkStyle, Mermaid.js throws an error.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://mermaid.live/edit#pako:eNo1js0KwjAQhF8l7Lk9eI1QUPQJ6jGXpbu1wfyUdHMIpe_uijqnge9jmB2mTAwWngXXxTyuLhlN8Ok1SgtsTob8tgZsNuXE5y--mL4fjLrQQeQS0ZNO7B_oQBaO7MBqJZ6xBnHg0qFqXQmF7-QlF7Azho07wCp5bGkCK6XyX7p51EfxZx1vfqY3UQ
2. Notice error

Expected behavior
A clear and concise description of what you expected to happen.

linkStyles are applied after.

Screenshots
If applicable, add screenshots to help explain your problem.

Code Sample
If applicable, add the code sample or a link to the live editor.

https://mermaid.live/edit#pako:eNo1js0KwjAQhF8l7Lk9eI1QUPQJ6jGXpbu1wfyUdHMIpe_uijqnge9jmB2mTAwWngXXxTyuLhlN8Ok1SgtsTob8tgZsNuXE5y--mL4fjLrQQeQS0ZNO7B_oQBaO7MBqJZ6xBnHg0qFqXQmF7-QlF7Azho07wCp5bGkCK6XyX7p51EfxZx1vfqY3UQ

Versions: mermaid.live v9.1.3

Additional context
Add any other context about the problem here.

N/a

[RounakJoshi09]

Hi @Yash-Singh1 @nirname ,

I hope all is well. While investigating a particular bug, I noticed an issue with how linkStyle functions when applied to non-existent links. after reading the documentation , I found out that linkStyles applies on links in 0-indexed fashion. So, in this code
linkStyle 1 display:none; A --> B
we are applying linkStyle to the second link (according to 0-indexed) which doesnot even exists. This results in an unclear error message: TypeError: Cannot set properties of undefined (setting 'style').

The error message doesn't effectively communicate the underlying issue to the user.
Could you please assign this issue to me? Additionally, would it be possible to generate a more descriptive error message for such cases to enhance user experience?

Thank you for your time and consideration. I'm looking forward to your response.

Best regards,
Rounak

[Yash-Singh1]

Could you please assign this issue to me?

Done

Additionally, would it be possible to generate a more descriptive error message for such cases to enhance user experience?

Yes, that would be possible by throwing a parse error. Here is an example

mermaid/packages/mermaid/src/diagrams/git/gitGraphAst.js

Lines 158 to 166 in 12a4707

	let error = new Error('Incorrect usage of "merge". Cannot merge a branch to itself');
	error.hash = {
	text: 'merge ' + otherBranch,
	token: 'merge ' + otherBranch,
	line: '1',
	loc: { first_line: 1, last_line: 1, first_column: 1, last_column: 1 },
	expected: ['branch abc'],
	};
	throw error;