我试图理解如何使用js Promise api来折射具有大量嵌套IF的代码。
例如,从localstorage获取JSON对象时,普通代码看起来像
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;
}
此代码的可读性不是那么好。所以我想可能是我可以利用js Promise将其重写为
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;
}
现在我想知道如何在内部抛出一个异常,以便catch可以捕获它并执行默认值?
这是js承诺的有效用法,我在浪费性能
答案 0 :(得分:1)
您可以使用Promise.reject()
引发错误:
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;
}
虽然我发现以下更易读和惯用。
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
}
}
正如您所知,Promise用于异步计算。任何其他用途可能会混淆其他程序员。
答案 1 :(得分:1)
您可以使用thrown抛出错误,然后在catch方法中处理它们
var p1 = new Promise(function(resolve, reject) {
resolve('Success');
});
p1.then(function(value) {
console.log(value); // "Success!"
throw 'oh, no!';
}).catch(function(e) {
console.log(e); // "oh, no!"
}).then(function(){
console.log('after a catch the chain is restored');
}, function () {
console.log('Not fired due to the catch');
});
// The following behaves the same as above
p1.then(function(value) {
console.log(value); // "Success!"
return Promise.reject('oh, no!');
}).catch(function(e) {
console.log(e); // "oh, no!"
}).then(function(){
console.log('after a catch the chain is restored');
}, function () {
console.log('Not fired due to the catch');
});
但是如果在异步函数中抛出一些错误,则永远不会调用catch。
// Errors thrown inside asynchronous functions will act like uncaught errors
var p2 = new Promise(function(resolve, reject) {
setTimeout(function() {
throw 'Uncaught Exception!';
}, 1000);
});
p2.catch(function(e) {
console.log(e); // This is never called
});
来源:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch
答案 2 :(得分:0)
throw
来引发任何错误。
MDN的例子:
throw "Error2"; // generates an exception with a string value
throw 42; // generates an exception with the value 42
throw true; // generates an exception with the value true
throw new Error("Error");
答案 3 :(得分:0)
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 : throw new Error("invalid json") )
//on more validation step if needed
.then( json => typeof json === 'object' ? json : throw new Error("invalid json") )
.then( valid_json => { return = valid_json } )
.catch( err => { ret = default; console.warn(err.message); } );
return ret;
}
你基本上可以执行以下操作,因为如果解析失败,它将被自动捕获。
function $storage (key, default) {
let ret;
let promise = new Promise( (y,n) => y(localStorage) )
.then(ls => JSON.parse(ls.getItem(key)) )
.then(valid_json => { return = valid_json } )
.catch(err => { ret = default; console.warn(err.message); } );
return ret;
}
答案 4 :(得分:0)
我看到的问题只是JSON.parse
,将它包装在一个更有用的函数中,你得到类似的东西:
function safeParse(x) {
try {
return JSON.parse(x);
} catch(e) {
// Log the problem
return null;
}
}
function parmval(key, defval) {
var json = safeParse(localStorage.get(key));
return (typeof json === "object") ? json : defval;
}
Promise是关于异步操作,而不是IF。