#JavaScript (ES6), 19 bytes

Takes input as an array of characters.

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

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#JavaScript (ES6),  23  20 bytes

Saved 3 bytes thanks to @ShieruAsakoto

Takes input as a string.

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

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#JavaScript (ES6), 22 bytes

Suggested by @tjjfvi

Takes input as a string.

s=>s.replace(/|/g,"!")

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JavaScript (ES6), 19 bytes

Takes input as an array of characters.

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

Try it online!

JavaScript (ES6),  23  20 bytes

Saved 3 bytes thanks to @ShieruAsakoto

Takes input as a string.

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

Try it online!

JavaScript (ES6), 22 bytes

Suggested by @tjjfvi

Takes input as a string.

s=>s.replace(/|/g,"!")

Try it online!

#JavaScript (ES6), 19 bytes

Takes input as an array of characters.

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

Try it online!

#JavaScript (ES6), 22 bytes

Suggested by @tjjfvi

Takes input as a string.

s=>s.replace(/|/g,"!")

Try it online!

#JavaScript (ES6), 23 bytes

Takes input as a string.

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

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#JavaScript (ES6), 19 bytes

Takes input as an array of characters.

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

Try it online!

#JavaScript (ES6),  23  20 bytes

Saved 3 bytes thanks to @ShieruAsakoto

Takes input as a string.

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

Try it online!

#JavaScript (ES6), 22 bytes

Suggested by @tjjfvi

Takes input as a string.

s=>s.replace(/|/g,"!")

Try it online!

#JavaScript (ES6), 19 bytes

Takes input as an array of characters.

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

Try it online!

#JavaScript (ES6), 23 bytes

Takes input as a string.

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

Try it online!

#JavaScript (ES6), 19 bytes

Takes input as an array of characters.

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

Try it online!

#JavaScript (ES6), 22 bytes

Suggested by @tjjfvi

Takes input as a string.

s=>s.replace(/|/g,"!")

Try it online!

#JavaScript (ES6), 23 bytes

Takes input as a string.

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

Try it online!