understanding javascript function call

Question

still learning to javascript. How does this below code work? when main(params) is called does it mean it under-the-hood calls dowork(callbackXYZ, options, params) ?

const { dowork } = require('some_lib');

async function callbackXYZ(src, tgt, params) => {
    // ...logic
}
const main = dowork(callbackXYZ, options);
await main(params);

Answer 1

Here's a simplified code example. It removes the async stuff, since it's not needed to understand.

 // The callback you provide to `dowork` // The internals of `dowork` will give invoke it with the given args function callbackXYZ(src, tgt, params) { console.log("src is:", src); console.log("tgt is:", tgt); console.log("params is:", params); } // The options you provide to `dowork` var options = {src: "foo", tgt: "bar"}; // Invoking `dowork` receives your callback and options, and returns // a new function that has access to both of these arguments. const main = dowork(callbackXYZ, options); // The params you provide to the returned function const params = {my: "params"}; // Invoking the returned function main(params); // Here's the `dowork`. It receives your callback and options, and it // returns a new function that expects you to pass it params. // So that returned function has reference to the callback, options // and params. // The body of the returned function, simply invokes your callback and // passes it data from the options and params you provided function dowork(callback, opts) { return function(params) { callback(opts.src, opts.tgt, params); } }

So dowork receives your callbackXYZ and the opts and returns a function that you can invoke, passing in the params .

When you invoke that returned function, and pass it the params , that returned function invokes your original callback, and passes it data both from the options and the params .

Answer 2

No!

• dowork is function which returns another function
• that another function is assigned to const main
• then this main (function which returns promise) is called

You can also run this code without declaring anything:

...
await dowork(callbackXYZ, options)(params);