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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Javascript Promise Cheatsheet</title>
<meta name="description" content="Javascript Promise Cheatsheet">
<meta name="author" content="Maurycy Zarzycki, Retrocade.net">
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.0/jquery.min.js"
integrity="sha256-cCueBR6CsyA4/9szpPfrX3s49M9vUU5BgtiJj06wt/s=" crossorigin="anonymous"></script>
<script src="js/prism.js"></script>
<script src="js/scripts.js"></script>
<link href="https://fonts.googleapis.com/css?family=Inconsolata|Open+Sans:400,400i,700" rel="stylesheet">
<link rel="stylesheet" href="css/prism.css">
<link rel="stylesheet" href="css/style.css">
</head>
<body>
<article>
<header>
<h1>JS Promise cheatsheet</h1>
<section>
<p>
This document contains various examples on how promises behave in different situations. It is created
and maintained by <a href="https://github.com/RetrocadeNet/">Maurycy Zarzycki, Retrocade.net</a>, its
source is available on <a href="https://github.com/RetrocadeNet/Promise-cheatsheet">Github</a>. All
suggestions and issues can be communicated via Github's
<a href="https://github.com/RetrocadeNet/Promise-cheatsheet/issues">Issues page</a>.
</p>
<p>
Recommended reading: <a href="https://promisesaplus.com/">Promises/A+</a>.
</p>
</section>
</header>
<section class="toc">
<header>
<h2>Table of Contents</h2>
</header>
<article>
<section>
<ul>
<li><a href="#promise-vocabulary">Promise vocabulary</a></li>
</ul>
</section>
</article>
</section>
<section class="promise-vocabulary">
<header>
<h2 id="promise-vocabulary">Promise vocabulary</h2>
</header>
<article>
<section>
<p>
This section refers specifically to the vocabulary used in this document to make finding specific examples easier, though the code samples themselves should be self explanatory.
</p>
<ul>
<li>
<strong>Promise</strong> - An object which represents the eventual result of an asynchronous
operation. <code>new Promise()</code>, <code>Promise.resolve()</code>,
<code>Promise.reject()</code> all return a promise object;
</li>
<li>
<strong>Executor</strong> - A function passed to Promise constructor, ie. <code>new Promise(<executor>)</code>
and takes two callback arguments, one for resolving the promise and one for rejecting it: <code>function
executor(resolve, reject)</code>;
</li>
<li>
<strong>Then()</strong> - A method of a promise object which takes two optional callback
arguments, one for handling the case when the promise is resolved (fulfilled) and another for
when it is rejected, ie: <code>.then(onResolved, onRejected)</code>. It returns a new promise
which is pending until Then is handled.
</li>
<li>
<strong>onResolved</strong> - One of the two possible promise handlers, defined in Then(), is
called when the promise to which it is attached is resolved. This can happen by: explicitly
resolving promise, returning from <i>onResolved</i> or <i>onRejected</i> any value other than a
rejected promise, not returning anything from either of those handlers.
</li>
<li>
<strong>onRejected</strong> - One of the two possible promise handlers, defined in Then(), is
called when the promise to which it is attached is rejected. This can happen by: explicitly
rejecting promise, returning from <i>onResolved</i> or <i>onRejected</i> a rejected promise,
throwing an error inside a promise handler.
</li>
</ul>
</section>
</article>
</section>
<section class="promise-group">
<header>
<h2>Basic promise behaviors</h2>
</header>
<article class="promise-article">
<header>
<h3>Promise executor is called immediately upon promise creation</h3>
</header>
<section>
<pre class="code"><code>
<script>
var promise = new Promise(function () {
log("Promise was run.");
});
log("End of script.");
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Then() resolution is asynchronous</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.resolve().then(function () {
log("On resolved");
}, function () {
log("On rejected - not run because promise is resolved")
});
Promise.reject().then(function () {
log("On resolved - not run because promise is rejected");
}, function () {
log("On rejected")
});
log("End of script.");
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Then() by default resolve the returned promise, with the single exception of returning a rejected
promise</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.resolve().then(function () {
// Nothing is returned
}).then(function () {
log("Resolved from nothing returned");
return true;
}).then(function () {
log("Resolved from return true");
return false;
}).then(function () {
log("Resolved from return false");
return null;
}).then(function () {
log("Resolved from return null");
return Promise.reject();
}).then(function () {
log("Not called because a rejected promise was returned");
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Rejected promise skips all onResolved handlers</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.reject().then(function () {
log("Never called #1");
}).then(function () {
log("Never called #2");
}).then(function () {
log("Never called #3");
}).then(function () {
log("Never called #4");
}).catch(function () {
log("First available onRejected handler")
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Throwing an error rejects a promise</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.resolve().then(function () {
throw new Error();
}).then(function () {
log("Never called because of the thrown error");
}).catch(function () {
log("Throwing an error rejects the promise");
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Returnin nothing inside catch stops the rejection</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.resolve().then(function () {
throw new Error();
}).then(function () {
log("Never called because of the thrown error");
}).catch(function () {
log("Throwing an error rejects the promise");
}).then(function(){
log("Called because nothing was returned from catch");
}).catch(function(){
log("Never called");
})
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Nested promises are handled first</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.resolve().then(function () {
return Promise.resolve().then(function () {
log("This promise tree has to finish first.");
}).then(function () {
log("Yet another step inside.");
}).then(function () {
log("Resolved the nested promise tree.");
});
}).then(function () {
log("This is only handled once the inner promise is finished.");
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Passing non-function to then ignores that 'then'</h3>
</header>
<section>
<pre class="code"><code>
<script>
Promise.resolve("This string is logged")
.then("This will not be logged")
.then(log);
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
</section>
<section class="promise-group">
<header>
<h2>Complex promise behaviors</h2>
</header>
<article class="promise-article">
<header>
<h3>Calling then() twice on the same promise is valid and creates two separate promise branches</h3>
</header>
<section>
<pre class="code"><code>
<script>
var promise = Promise.resolve();
promise.then(function(){
log('Branch #1 - do a reject');
return Promise.reject();
}).then(function(){
log("Resolve handler not called because previous promise was rejected")
});
promise.then(function(){
log('Branch #2 - do a resolve');
return Promise.resolve();
}).catch(function(){
log('Caught reject - not called because rejection happened in the other branch')
});
log("End of script.");
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Errors thrown inside the executor automatically reject the promise</h3>
</header>
<section>
<pre class="code"><code>
<script>
var promise = new Promise(function(){
log("Throwing an error");
throw new Error("Error");
});
promise.catch(function(){
log("Error was caught")
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Returning a resolved promise in executor does not resolve the promise</h3>
</header>
<section>
<pre class="code"><code>
<script>
var promise = new Promise(function(){
log("Returning a resolved promise");
return Promise.resolve();
});
promise.then(function(){
log("This is never called")
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Resolving the promise multiple times ignores the subsequent calls</h3>
</header>
<section>
<pre class="code"><code>
<script>
var promise = new Promise(function(resolve){
log("First resolve call");
resolve();
log("Second resolve call");
resolve();
log("Third resolve call");
resolve();
});
promise.then(function(){
log("Resolved the promise")
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
<article class="promise-article">
<header>
<h3>Doing Promise.resolve() on another promise passes it on</h3>
</header>
<section>
<pre class="code"><code>
<script>
// Test 1
var promiseTest1 = new Promise(function (resolve, reject) {
resolve("Resolved 1");
});
Promise.resolve(promiseTest1)
.then(function(data){
console.log(data);
log('Test1::Then - ' + data);
})
.catch(function(data){
log('Test1::Catch - ' + data);
});
// Test 2
Promise.resolve(Promise.resolve('Resolved 2'))
.then(function(data){
log('Test2::Then - ' + data);
})
.catch(function(data){
log('Test2::Catch - ' + data);
});
// Test 3
var promiseTest3 = new Promise(function (resolve, reject) {
reject("Rejected 3");
});
Promise.resolve(promiseTest3)
.then(function(data){
console.log(data);
log('Test3::Then - ' + data);
})
.catch(function(data){
log('Test3::Catch - ' + data);
});
// Test 4
Promise.resolve(Promise.reject('Rejected 4'))
.then(function(data){
log('Test4::Then - ' + data);
})
.catch(function(data){
log('Test4::Catch - ' + data);
});
</script>
</code></pre>
<ol class="results"></ol>
</section>
</article>
</section>
</article>
</body>
</html>