#JavaScript (ES6), 19 bytes

Takes input as an array of characters.

<!-- language-all: lang-javascript -->

    s=>`!${s.join`!`}!`

[Try it online!][TIO-jzguh10n]

[JavaScript (Node.js)]: https://nodejs.org
[TIO-jzguh10n]: https://tio.run/##hYxNC4JAFEX371e8gUAlGpwpKwvb5SID21cwg184iiNOtIl@u2mrFoGXC@cuDlfJpzRJV7aPRaPTrM@D3gQHQWYvQ5UuG0HEm4g@0Y3RdUZrXdi5faWUCoYcl7hCD9fAuO/5DAe4jI9gfDPC23IAV9wdnKN1aywH4M@TNCnKXOkIZbXLoLp05xRwSB6f4uP@O6MwhiisslYVukVTtFKr0ZBq8n1KIAR@OthO/wE "JavaScript (Node.js) – Try It Online"


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#JavaScript (ES6), 23 bytes

Takes input as a string.

<!-- language-all: lang-javascript -->

    s=>['',...s,''].join`!`

[Try it online!][TIO-jzgud1r0]

[JavaScript (Node.js)]: https://nodejs.org
[TIO-jzgud1r0]: https://tio.run/##fYxNC4JAGITv7694Pa2SLe6WlYXd8pCB3SvYxS9cxRU3@vubdooODQPPwAyj5EuafGyG57LXRWmr2Jr4eCPEp5Qan5AHVbrphSNsrnuju5J2unYrVzDkuMI1hrgBxqMwYjghYHwG49sZ4Y4DBMLDBZJ7TzyAnxNpCpSV0inKdl9Cex0vBeCkKjtnp8MnpkkGadKWg6r1gKYepFbzQqp/x386x4EvC8@zbw "JavaScript (Node.js) – Try It Online"