Uncaught SyntaxError: \8 and \9 are not allowed in strict mode

Question

I work on a project using Angular v7 and just today encountered this error Uncaught SyntaxError: \8 and \9 are not allowed in strict mode. at runtime that will not go away

I've been able to trace it back to the angular-essential-select module (2.0.12) that I use and cannot replace at the moment. I have not modified my tsconfig.json or my angular.json and I definitely not have strict mode enabled.

The problem is that my app compiles properly, but does not work because of this error.

I've tried everything I could think of (from a simple npm ci to a full clone of my repo on which I have not pushed anything right before it broke) and I'm out of idea. If anyone wants to chime in, it'll be appreciated.

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Update:
I tried a repo clone on a new machine: the issue does not translate on my other machine (which is a good point).

My best guess is that a global npm package was updated for whatever reason and is causing this mess or some npm cache shenanigans.

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Update #2:

I had checked only on Google Chrome, but Richard Hogg's comment made me check on Firefox: it works fine.
Guess that since both Google Chrome and MS Edge run on the same Chromium core, it could be an issue with a Chromium update... Back to figuring this thing out!

Answer 1

So, as guessed in my question, the culprit for my issue was indeed the angular-essential-select module. In its css definition, it contains the problematic \9.

Removing the module fixes the issue.

As for the why, the Uncaught SyntaxError: \8 and \9 are not allowed in strict mode. comes from a CSS "hack" to tell the browser to use a specific CSS rule if in IE 8/9/10/11 (respectively: \8, \9, \10 and \11).
I think it's because the latest Chromium build does not support this CSS hack anymore, the modules using it break.

I'd advise to check the line the error points to (ie. vendor.js:10000 in Angular) to find out which module causes the issue and replace said module with a newer one.

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If you need to keep the problematic module and don't care about strict mode, check out Richard Hogg's answer

Answer 2

I'm having the same issue at my job, the app was running fine and then it showed up after I updated VScode but I feel like its coming from the browser cause it worked with MS Edge and now it doesn't work at all, very strange.. if you found the issue please post it.

Edit: So we or my colleague found a fix for now.. it was to add the noImplicitUseStrict, he explained in my poor second hand info that the browsers have updated strict mode to actually being strict! before the browser just allowed it now you have to add this line of code into the tsconfig.json, if this helps great otherwise sorry and good luck!

"strict": false, "noImplicitUseStrict": true,