JavaScript asynchronous programming 4 -- promise error handling

charlee44 2021-05-04 12:08:22
javascript asynchronous programming promise error


1. summary

In the last article 《JavaScript Asynchronous programming 3——Promise Chain use of 》 in , adopt Promise Chain use of , Avoid multiple nested callbacks in the program ( Back to hell ). According to the previous article, we can know that ,Promise It's state based , success / The failed state will handle the corresponding callback function . generally speaking , The state of failure we want to be able to capture , Make it look like an anomaly (Error) Handle it the same way .

2. Detailed discussion

Promise Of then() Method has two parameters , One is a successful callback function , One is the failed callback function . You can omit the parameter of the failed callback function , While using Promise Of catch() Method , Catch the failed exception . for example , Let's improve on the example in the last article :

$(function () {
function get(url) {
return new Promise(function (resolve, reject) {
var req = new XMLHttpRequest();
req.open('GET', url);
req.onload = function () {
// Even if it's 404 It will also enter the corresponding function , So we need to detect the State
if (req.status == 200) {
// Fulfill the promise , Returns the response text
resolve(req.response);
} else {
// Done not done , Returns an error
reject(Error(req.statusText));
}
};
// The corresponding function in case of an error
req.onerror = function () {
reject(Error("Network Error"));
};
// Send a request
req.send();
});
}
function getImg(uri){
return new Promise(function(resolve, reject){
var img = new Image();
img.onload = function () {
resolve(img);
};
img.onerror = function () {
reject(Error("Load Image Error!"));
}
img.src = uri;
});
}
var addressUri = "./1.json";
get(addressUri).then(function (response) {
var imgJson = JSON.parse(response);
return getImg(imgJson[0]);
}).catch(function (error) {
console.error("Failed!", error);
}).then(function(img){
$(img).appendTo($('#container'));
}).catch(function(error){
console.error("Failed!", error);
});
});

The process before and after improvement is very similar , But there are subtle differences . The former passes through Promise Of then() Handling exceptions , Only one of the function's callback functions and the failed callback functions will run ; The latter passes catch() Handling exceptions , It's more like JavaScript Of try/catch, stay try{} The error in will immediately go to catch{} block . In this case , It's easy to do something like try/catch Non blocking asynchronous version of exception operation :

$(function () {
function get(url) {
return new Promise(function (resolve, reject) {
var req = new XMLHttpRequest();
req.open('GET', url);
req.onload = function () {
// Even if it's 404 It will also enter the corresponding function , So we need to detect the State
if (req.status == 200) {
// Fulfill the promise , Returns the response text
resolve(req.response);
} else {
// Done not done , Returns an error
reject(Error(req.statusText));
}
};
// The corresponding function in case of an error
req.onerror = function () {
reject(Error("Network Error"));
};
// Send a request
req.send();
});
}
function getImg(uri){
return new Promise(function(resolve, reject){
var img = new Image();
img.onload = function () {
resolve(img);
};
img.onerror = function () {
reject(Error("Load Image Error!"));
}
img.src = uri;
});
}
var addressUri = "./1.json";
get(addressUri).then(function (response) {
var imgJson = JSON.parse(response);
return getImg(imgJson[0]);
}).then(function(img){
$(img).appendTo($('#container'));
}).catch(function(error){
console.error("Failed!", error);
}).then(function(){
alert(" Image loading complete !");
});
});

In the improved example above , first then() And the second then() If there is an error in , The exception will be transferred to catch() in , And the third then(), The program will continue to run anyway .

3. Reference resources

  1. JavaScript Promises: An introduction
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https://qdmana.com/2021/05/20210504120609708d.html

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