JavaScript中的“ typeof str”和“ typeof（str）”有什么区别？

Question

What is the difference between these two statements?

if (typeof errorMessage !== undefined)
{}

and

if (typeof (errorMessage) !== undefined)
{}

Answer 1

One of them has a pair of entirely superfluous parentheses.

The difference between typeof foo and typeof (foo) is the same as the difference between 1 + 1 and (1) + (1) .

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An an aside, the typeof operator will give you a string, so you should be comparing to "undefined" not undefined .