I am a little bit confused with the code below. It's obviously a two arrow function written in es6 code but I do not understand exactly some parts.
The 2nd parameter named done is an empty function that does nothing? Or it is executed with a simple return as the result from the second arrow anonymous function?
the XXXX.load is a promise function that returns some results. How the caller of the Index can get the results of the 2nd parameter done ie done(null, result) ?
What is the equivalent code in es5?
const Index = (name, done = () => {}) => (dispatch, getState) => {
return XXXX.load()
.then((result) => {
dispatch({type:OK});
done(null, result);
})
.catch((error) => {
dispatch({type:ERROR});
done(error);
});
};
Let's go one by one:
Index (name, done = () => {}) defines a default value for done in case none is provided when Index is called. This helps down to road to not do any checks in case done is null/undefined. You could also write it like thisconst Index = (name, done) => (dispatch, getState) => {
if (!done) {
done = () => {}
}
}
caller will just pass a function as the second argument when calling Index.A general note: Index actually returns a function that will expect a dispatch and/or a getState param.
The 2nd parameter named done is an empty function that does nothing?
It's a parameter. It takes whatever value you give it.
The default value, which is assigned if the caller doesn't pass a second argument, is a function that does nothing. This lets it be called without throwing an undefined is not a function error or having an explicit test to see if it is a function or not.
How the caller of the Index can get the results of the 2nd parameter done ie done(null, result) ?
By passing its own function as the second argument.
What is the equivalent code in es5?
var Index = function(name, done) {
if (!done) done = function() {};
return function(dispatch, getState) {
return XXXX.load()
.then(function(result) {
dispatch({
type: OK
});
done(null, result);
})
.catch(function(error) {
dispatch({
type: ERROR
});
done(error);
});
}
};
done. Default values prevents runtime crashes.2 and 3 can be understood by seeing below code: (simply run it and see the consoles.
const DEFAULT_FUNCTION_VALUE = ()=> {};
const XXXX = {
load: function() {
return new Promise(function(resolve, reject) {
setTimeout(function() {
resolve({data: 'from XXXX.load'});
},2000);
});
}
}
const Index = function(name='', done=DEFAULT_FUNCTION_VALUE) {
return function(dispatch, getState) {
return XXXX.load().then(function(result) {
console.log({result});
done(result);
}).catch(function(error) {
console.log(error);
});
}
}
function doneImplementation(data) {
console.log('Data from done- ', data);
}
Index('', doneImplementation)();