What does the error message "keys is not a function" mean in Firefox?

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This error message usually means that a JavaScript code is trying to use the "keys" method on a variable that is not an object or array. In other words, the code is attempting to access the "keys" property of a non-object value, which is not supported and results in an error. To fix this issue, you should check if the variable being used is actually an object or array before accessing its "keys" property.