1252748 1252748 - 2 months ago 9
Javascript Question

How to use arrow function with || operator

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?

str str
Answer

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.