I want my chrome browser to forcefully pop up dialog box for download(i.e Save As). Without user manually changing the settings for Downloads in chrome browser.
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There is no way to do what you're suggesting for security reasons. You don't have rights to choose where the file will go on their OS. JS won't go any deeper than the user's browser. What your asking for is essentially FS (file system) read-write permissions.
Make sure to do a little searching around before you ask new questions, as this one was already answered here: Directory Chooser in HTML page