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# fetch-retry
Adds retry functionality to the `Fetch` API.
It wraps any `Fetch` API package (eg: [isomorphic-fetch](https://github.com/matthew-andrews/isomorphic-fetch), [cross-fetch](https://github.com/lquixada/cross-fetch), [isomorphic-unfetch](https://github.com/developit/unfetch) and etc.) and retries requests that fail due to network issues. It can also be configured to retry requests on specific HTTP status codes.
[](https://travis-ci.org/jonbern/fetch-retry)
## npm package
```javascript
npm install fetch-retry --save
```
## Example
`fetch-retry` is used the same way as `fetch`, but also accepts `retries`, `retryDelay`, and `retryOn` on the `options` object.
These properties are optional, and unless different defaults have been specified when requiring `fetch-retry`, these will default to 3 retries, with a 1000ms retry delay, and to only retry on network errors.
```javascript
require('es6-promise').polyfill();
var originalFetch = require('isomorphic-fetch');
var fetch = require('fetch-retry')(originalFetch);
```
```javascript
fetch(url, {
retries: 3,
retryDelay: 1000
})
.then(function(response) {
return response.json();
})
.then(function(json) {
// do something with the result
console.log(json);
});
```
or passing your own defaults:
```javascript
var originalFetch = require('isomorphic-fetch');
var fetch = require('fetch-retry')(originalFetch, {
retries: 5,
retryDelay: 800
});
```
> `fetch-retry` uses promises and requires you to polyfill the Promise API in order to support Internet Explorer.
## Example: Exponential backoff
The default behavior of `fetch-retry` is to wait a fixed amount of time between attempts, but it is also possible to customize this by passing a function as the `retryDelay` option. The function is supplied three arguments: `attempt` (starting at 0), `error` (in case of a network error), and `response`. It must return a number indicating the delay.
```javascript
fetch(url, {
retryDelay: function(attempt, error, response) {
return Math.pow(2, attempt) * 1000; // 1000, 2000, 4000
}
}).then(function(response) {
return response.json();
}).then(function(json) {
// do something with the result
console.log(json);
});
```
## Example: Retry on 503 (Service Unavailable)
The default behavior of `fetch-retry` is to only retry requests on network related issues, but it is also possible to configure it to retry on specific HTTP status codes. This is done by using the `retryOn` property, which expects an array of HTTP status codes.
```javascript
fetch(url, {
retryOn: [503]
})
.then(function(response) {
return response.json();
})
.then(function(json) {
// do something with the result
console.log(json);
});
```
## Example: Retry custom behavior
The `retryOn` option may also be specified as a function, in which case it will be supplied three arguments: `attempt` (starting at 0), `error` (in case of a network error), and `response`. Return a truthy value from this function in order to trigger a retry, any falsy value will result in the call to fetch either resolving (in case the last attempt resulted in a response), or rejecting (in case the last attempt resulted in an error).
```javascript
fetch(url, {
retryOn: function(attempt, error, response) {
// retry on any network error, or 4xx or 5xx status codes
if (error !== null || response.status >= 400) {
console.log(`retrying, attempt number ${attempt + 1}`);
return true;
}
})
.then(function(response) {
return response.json();
}).then(function(json) {
// do something with the result
console.log(json);
});
```
## Example: Retry custom behavior with async
The `retryOn` option may also be used with async and await for calling asyncronous functions:
```javascript
fetch(url, {
retryOn: async function(attempt, error, response) {
if (attempt > 3) return false;
if (error !== null) {
var json = await response.json();
if (json.property !== undefined) {
return true;
}
}
})
.then(function(response) {
return response.json();
}).then(function(json) {
// do something with the result
console.log(json);
});
```
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