为什么在lodash中使用`+ start`而不是`start`

Question

https://github.com/lodash/lodash/blob/3.7.0/lodash.src.js#L2781

What does +start mean in this snippet start = start == null ? 0 : (+start || 0); start = start == null ? 0 : (+start || 0); ?

In my opinion, +start equals 0+start . and we'v already known start is a number.

so why not use start = start == null ? 0 : (start || 0); start = start == null ? 0 : (start || 0); ?

Is there anything I misunderstand.? I am really confused.

Answer 1

You don't know, at the start of that method, that start is a number. The documentation says it ought to be , but when has the documentation always been exactly right?

There isn't any real difference between +start and 0+start , as both will coerce start into a number (so will -start , but it obviously inverts the sign in the process).

Assuming the docs are right and start is already a number, +start is a no-op and doesn't cause any problems or break anything. If start is not a number (the next most likely type is probably string), then this will coerce it into a number before using it.