CODEWITHSUNDEEP
node.jskoa
mfc
mfc

Reputation: 3026

yields in koa routing

I'm trying to figure out the use of yield in koa routing.

as an example,

router.get('/data', function *(next) {
    this.body = yield someData;
});

If I wanted to send a static file, I could use koa-send like so :-

router.get('/data', function *(next) {
    yield send(this, 'file.html');
});

However if i assign the 2nd yield to this.body, it doesnt work.

so what does 

this.body = yield ...

actually mean and why don't I need to assign the 2nd yield to the body?

Upvotes: 1

Views: 965

Answers (1)

James Moore
James Moore

Reputation: 1901

If you peek inside the koa-send library you'll notice this:

ctx.body = fs.createReadStream(path);

Basically the library is assigning a stream to this.body, then when you were trying to assign what is returned by calling yield send(this, 'file.html'), which appears to be the file path and name, you're breaking / overwriting what the library was trying to do.

Now if you'd like, you could choose to not use koa-send and instead just do this:

this.body = fs.createReadStream('file.html');

Getting to your specific question

this.body = yield ...

You call yield (inside generators) on Promise/thunk/generator returning functions that do something asynchronous, which pauses execution in the function until the asynchronous task(s) are completed, and then restarts the function when the result is available.

I made a screencast a while back on understanding JavaScript Generators that you might find helpful:

http://knowthen.com/episode-2-understanding-javascript-generators/

Upvotes: 3

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