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Examples of common patterns for promises in JS
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// ============================================================================= | |
// Example for unit testing an async func that doesn't already support promises | |
var Q = require('q'); // You must do `npm install q` first | |
// Async function that uses callbacks instead of promises | |
function authenticate(dataToSubmit, callback) { | |
// blah blah blah | |
} | |
// The unit test code... | |
describe('authentication', function() { | |
// For some testing frameworks like Jasmine, be sure to include the `done` | |
// callback (this is not needed when using Jasmine with Karma for AngularJS) | |
it('should provide user data', function(done) { | |
// Make a promise from scratch | |
var deferred = Q.defer(); | |
var promise = deferred.promise; | |
authenticate(someData, function(userData) { | |
deferred.resolve(userData); | |
}); | |
// This `then` pattern doesn't seem more useful than ordinary callbacks | |
// in this example, but imagine how useful it would be if you were to | |
// replace several nested callbacks with a flat promise chain. | |
promise.then(function(userData) { | |
expect(userData).toBe(xyz); | |
done(); | |
}); | |
}); | |
}); | |
// ============================================================================= | |
// Another style: Wrap non-promise async functions in your own promise-enabled functions | |
function authenticateWithPromise(dataToSubmit) { | |
var deferred = Q.defer(); | |
authenticate(dataToSubmit, function(userData) { | |
deferred.resolve(userData); | |
}); | |
return deferred.promise; | |
} | |
authenticateWithPromise(someData).then(function(userData) { | |
// do stuff after `authenciate()` has finished (aka after it has "resolved" the promise); | |
}); | |
// ============================================================================= | |
// Example of chaining promises... | |
function foo(input) { /* ... */ } // Pretend all these functions do somtething async & return a promise | |
function bar(input) { /* ... */ } // bar depends on input that is defined by the result of foo | |
function baz(input) { /* ... */ } // baz depends on input that is defined by the result of bar | |
function runPromiseChain(inputForFoo) { | |
var fooOutput; | |
var barOutput; | |
var bazOutput; | |
return foo(inputForFoo).then(function(result) { | |
fooOutput = result; | |
return bar(fooOutput); | |
}).then(function(result) { | |
barOutput = result; | |
return baz(barOutput); | |
}).then(function(result) { | |
bazOutput = result; | |
return { | |
fooStuff: fooOutput, | |
barStuff: barOutput, | |
bazStuff: bazOutput | |
}; | |
}); | |
} | |
runPromiseChain(someInput).then(function(result) { | |
// do stuff with result.fooStuff, result.barStuff, and result.bazStuff | |
}, function(err) { | |
// handle errors | |
console.log('Promise chain rejected because:', err); | |
}); | |
// ============================================================================= | |
// Example of spreading promises... | |
function foo() { /* ... */ } // Pretend all these functions do somtething async | |
function bar() { /* ... */ } // and return a promise. Also, assume they can run | |
function baz() { /* ... */ } // independently of one another. | |
var promises = [ | |
foo(), | |
bar(), | |
baz() | |
]; | |
// Option 1: Do stuff with array of results | |
Q.all(promises).then(function(arrayOfResults) { | |
console.log(arrayOfResults[0]); // resolve value from foo | |
console.log(arrayOfResults[1]); // resolve value from bar | |
console.log(arrayOfResults[2]); // resolve value from baz | |
}); | |
// Option 2: Do stuff with results separated into individual variables via `spread()` | |
Q.all(promises).spread(function(fooResult, barResult, bazResult) { | |
// do stuff with the 3 arguments | |
}); |
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