Chaining Promises

Chaining Promises.markdown

Chaining Promises

Example on how to chain Promises. We can pipe the callbacks to queue asynchronous tasks or transform values. Examples are done using expect library

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License.

script.js

'use strict';

/**
 * Increment a given value
 * @param {Number} val
 * @returns {Promise}
 */
const foo = (val) => {
	/**
	 * Return a promise.
	 * @param {Function} resolve
	 * @param {Function} reject
	 */
	return new Promise((resolve, reject) => {
		if (!val) {
			return reject(new RangeError("Value must be greater than zero"));
		}
		setTimeout(() => {
			resolve(val + 1);
		}, 0);
	});
};

const testFunctionToTest = () => {
	let result = foo(1).then((val) => {
		// chaining async call
		return foo(val);
	}).then((val) => {
		// transforming output
		return val + 2;
	}).then((val) => {
		expect(val)
		.toEqual(5);
	}).catch((err) => {
		console.error("Error caught: ", err.message);
	});
};

testFunctionToTest();

console.log('All Tests Passed!');

scripts

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