问题
What exactly is the difference between undefined and void 0 ?
Which is preferred and why?
回答1:
The difference is that some browsers allow you to overwrite the value of undefined. However, void(anything) always returns real undefined.
undefined = 1;
console.log(!!undefined); //true
console.log(!!void(0)); //false
回答2:
undefined has normal variable semantics that not even strict mode can fix and requires run-time look-up. It can be shadowed
like any other variable, and the default global variable undefined is not read-only in ES3.
void 0 is effectively a compile time bulletproof constant for undefined with no look-up requirements. It is
like writing null or true, instead of looking up a variable value. It works out of the box without any safety arguments and is shorter to write. It is better in every way.
回答3:
Use undefined. Its more commonly known than void(0).
回答4:
JS is very loose at syntax, parentheses here are optional, void 0 and void(0) are equivalent.
For second question, you need to use undefined directly while avoiding unneeded operand evaluation to retrieve the same undefined value.
More info in the reference: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/void
来源：https://stackoverflow.com/questions/5716976/javascript-undefined-vs-void-0