object 内的等号解构花括号

Question

I saw this statement in Graphql directive definition:

const { resolve = defaultFieldResolver } = field;

I know the part const { resolve } = field; means getting resolve property of the field object out and storing it in a local variable resolve . But what I'm coming across for the first time is the = defaultFieldResolver part. What does the equal sign do here? I've done a thorough google search but maybe I'm not aware of the right keyword to search or this is probably something new.

Here's the link of the article where I saw this.

Thanks a bunch.

Answer 1

That means if field contains a resolve property, extract it:

 const defaultFieldResolver = 'defaultFieldResolver'; const field = { resolve: 'resolve' }; const { resolve = defaultFieldResolver } = field; console.log(resolve);

If field doesn't contain a resolve property, assign defaultFieldResolver to the resolve variable instead.

 const defaultFieldResolver = 'defaultFieldResolver'; const field = {}; const { resolve = defaultFieldResolver } = field; console.log(resolve);