how to use js Promise to eliminate pyramid of doom

Question

i'm trying to understand how to use js Promise api to refractor a code that has lots of nested IF.

example when getting JSON object from localstorage a normal code would look like

function $storage(key,default) {

  let json = localStorage.getItem(key);



  if(json === null) return default;



  try{  // <-- need try catch in case value was not valid json object

    json = JSON.parse(json); 

  } catch (e) {

    json = default;

  }

  return typeof json === 'object' ? json : default;

}

the readibility of this code is not that good. so i thought may be i can utilize js Promise to rewrite it into

function $storage (key, default) {

 let ret;



 let promise = new Promise( (y,n) => y(localStorage) )

 .then( ls => JSON.parse(ls.getItem(key)) )

 .then( json => typeof json === 'object' ? json : HOW_TO_THROW_ERROR() )

 //on more validation step if needed

 .then( json => typeof json === 'object' ? json : HOW_TO_THROW_ERROR() )

 .then( valid_json => { return = valid_json } )

 .catch( error => { ret = default; console.warn('json invalid',e); } );



 return ret;



}

now i want to know how can i throw an exception inside then so that the catch can caught it and execute default ?

is this valid usage of js promise of am i wasting performance

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Answer 1

You could use Promise.reject() to throw an error:

function $storage (key, default) {
 let ret;

 let promise = new Promise( (y,n) => y(localStorage) )
 .then( ls => JSON.parse(ls.getItem(key)) )
 .then( json => typeof json === 'object' ? json : Promise.reject("invalid json") )
 .then( valid_json => { return = valid_json } )
 .catch( err => { ret = default; console.warn(err.message); } );

 return ret;

}

Although I find the following more legible and idiomatic.

function $storage(key,default) {
  let json = localStorage.getItem(key);
  if(json === null ||  typeof json !== 'object') json = default;

  try{  
    json = JSON.parse(json); 
  } catch (e) {
    json = default;
  } finally {
  return json
  }
}

Promises are used, as you surely know, for asynchronous computation. Any other use might confuse other programmers.

how to use js Promise to eliminate pyramid of doom

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