has been solved. I don't know why both typeof and instanceOf thought it was string, but then it was right to use text==null.
when you add double quotation marks (text= "null"), you already define text as a string. So it must be of the string type.
you can judge whether it is null or not.
if (text = null) {}
because text='null' is a string type; typeof (null) output type is object, and of course false is returned if it is not the same type!
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