The replace filter breaks when the variable is undefined.

It throws the following error:

An exception has been thrown during the rendering of a template ("s.getTemplateLine is not a function")

The difference in behaviour can be seen at:

https://cdpn.io/codebymikey/debug/jOWwpwR/XBkGRqXWjWKA vs https://twigfiddle.com/ljg25g

twing was also configured with strict_variables: false, so there's a difference in behaviour somewhere with regards to how undefined/null variables are handled in the PHP version compared to the Javascript version.

It might also be worth adding a check around e.getTemplateLine to check if the function exists before trying to call it - it'd make it easier to know the root cause of the exception.

edit: The issue is with locutus' strtr implementation.

TypeError: Cannot read property 'length' of null
    at strtr (/project/path/node_modules/locutus/php/strings/strtr.js:68:16)
    at _do (/project/path/node_modules/twing/dist/cjs/lib/extension/core/filters/replace.js:23:16)
    at replace (/project/path/node_modules/twing/dist/cjs/lib/extension/core/filters/replace.js:26:32)
    at /project/path/node_modules/twing/dist/cjs/lib/callable-wrapper.js:46:29
    at Object.doDisplay (eval at getTemplatesModule (/project/path/node_modules/twing/dist/cjs/lib/environment.js:489:24), <anonymous>:29:593)
    at async Promise.all (index 1)
    at async testTwigTemplates (/project/path/scripts/twig-tester.js:204:5)

We should be able to work around the issue by checking it's not empty before passing it to strtr.