What is the meaning of 'catch(…)' with no '{…}' block following it?

Question

I see a piece of code similar to the one below in some npm package:

this.func(callback).then(function() {
  ...
  return x;
}).then(function() {
  ...
  return y;
}).then(function() {
  ...
  return z;
}).then(function() {
  mocha.run(function(failures) {
    ...
    callback(failures);
  });
}).catch(callback);

Questions:

1. What is the meaning of this catch(callback) with no {...} block following it?

2. I would like to add a finally clause to execute the callback , but every syntax that I'm trying seems to fail:

• .catch(callback).finally(callback);
• .catch(callback).finally(callback());
• .catch(callback).finally{callback()};
• .catch(callback).finally(){callback()};

Answer 1

In your case, then and catch refer to Promise's prototype and not to the native catch implementation. Check this example to understand it better:

let doSomething = () {
   return new Promise((resolve, reject) => {
        try { 
         reject(); 
       } catch(e) {
         reject(); 
        } finally {
          console.log('done');
        }
   });
}

doSomething().then(() => {}).catch(() => {});

Note that anything you'd do, catch will be called.

Answer 2

In your case catch function refers to your callback function which you are passing in catch block

//first case
function callback(){}

catch(callback);
//second case 
catch(function(){})

Both case will work

and for finally It is still lacking browser support, you can check here at bottom of this page

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally

and check this for how to do finally in alternative way.

what is the equivalent of bluebird Promise.finally in native ES6 promises?

Answer 3

you can try like this. For more detail on promise: see here

doSomething(()=>{//do something})
.then(()=>{})
.catch((error)=> { console.log(error); })
.finally(()=> { //finally block here });

Answer 4

Question 1: The Promise API calls the function passed in the catch whenever a promise gets " rejected ". So consider the following code:

// method 1: define the callback function
var callback = function(error){
    console.error(error); // console the error
};
this.func.then(...).catch(callback);

// method 2: pass the function assigned to "callback" variable itself 

this.func.then(...).catch(function(error){
    console.error(error); // console the error
});

You are just telling the promise returned by the above (in your code) function call(s) that: "Hey, whenever you fail to do the task, call this function callback that I (or someone else) have defined."

Question 2: The first method in your list of four methods should work.