问题
Using Babel, I can see that
callback = () => {};
compiles to
callback = function callback() {};
which is what I expect. However I get an error when I try to use it with ||
callback = callback || () => {}
Which I'd expect to be equivalent to
callback = callback || function(){};
Why is this an error? Also, is there a more correct ES6 version of this familiar syntax?
回答1:
It fails because that is just not valid syntax.
Use the following to make it work:
callback = callback || (() => {})
If you don't wrap it that way, it would be interpreted as if you typed the following. But that is invalid syntax.
callback = (callback || ()) => {}
To extend on the evaluation of the assignment, see the specification of the AssignmentExpression. It consist of a ConditionalExpression or an ArrowFunction (or some other expressions I will disregard). So the interpreter will try to use your code as a conditional. But the () itself is not valid in that context as an expression is expected inside that ParenthesizedExpression. As a result, it will fail. If you instead group the expression as callback || (() => {}) both sides of the LogicalOrExpressions are valid expressions.
回答2:
Due to operator precedence, you have to wrap the arrow function in parentheses to make it work:
callback = callback || (() => {})
回答3:
If you're planning to use the || to provide a default value for the callback parameter to a function, it's easier to just write
function myfunc(callback = () => { }) {
callback("Hi 1252748");
}
No extra parens needed.
来源：https://stackoverflow.com/questions/39795104/how-to-use-arrow-function-with-operator