What does the exclamation mark AFTER a JS function name do? [duplicate]

I’ve seen posts about placing the exclamation before the function name e.g. !functionName(){} but I came across this today and I’m not sure what purpose the ! fulfills:

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This is from a React app

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example usage:

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I get what the changeTrendValue method does, but I don’t get how the ! in the changeValue callback affects the result