var res={
optionMsg:""
}
""+ res.data || res.optionMsg
//"undefined"
res.data || res.optionMsg
//
res.data || res.optionMsg+""
//
Why do I return undefined before the text?
text is returned after the correct
what is the case?
operator + is higher than | | has higher priority , so "Operation failure" + res.data | | res.optionMsg equals ("Operation failure" + res.data) | | res.optionMsg , res.data is undefined, so it is also equivalent to ("Operation failure" + undefined) | | res.optionMsg is equivalent to "Operation failure undefined" | | res.optionMsg
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