#core functions

[obscurejavascript]

Reflect in JavaScript

Many of the functions for general operations are distributed across a bunch of objects and keywords in JavaScript. This increases the cognitive load and increases the number of times a language operation needs to be looked up. Here are a few quick examples:

const obj = { property1: 'value1', property2: 'value2', }; delete obj.property1; Object.preventExtensions(obj); try { obj.property3 = 'value3' } catch(e) { console.log(e.message); } console.log(obj); // { property2: 'value2' }

Reflect simplifies this by providing a consistent API for all of the operations:

const obj2 = { property1: 'value1', property2: 'value2', }; Reflect.deleteProperty(obj2, 'property1'); Reflect.preventExtensions(obj2); const succeeded = Reflect.set(obj2, 'property3', 'value3'); if (succeeded !== true) { console.log('Failed to set "property3", obj is not extensible'); } console.log(obj2); // { property2: 'value2' }

Notice how Reflect.set does not return an error and returns if the value was set instead. Even for basic operations, reflect can help clarify the code more

Those were just a few examples, you can view the full list here. Since Reflect provides a consistent API for many operations, it should generally be used instead of all the separate interfaces when it is available. There are exceptions when the syntax is more clear though; except for cases like the above, setting variables with obj.a = 'value' is usually clearer than Reflect.set(obj, 'a', 'value'). It is supported in all browsers including MS Edge, but not IE 11.

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2017/reflect.js

[obscurejavascript]

JavaScript’s Map Object

Often lookup tables/hash maps are implemented as objects in JavaScript. But the Map object is better since it is more specialized:

Maps are not inherited from Object, so there are no default keys to accidentally override.

It has a size property. Objects keys must be manually counted to get the size.

The map key can be any value including an object (object keys auto-convert to strings)

Comparisons are done using ===, so using non-primitive values is not very useful since the original object would have to be used. Though not converting every key to a string does have advantages. For example, this simulates adding to a map over time which a sort by date:

// Rough launch dates in MM/DD/YYYY const cars = new Map(); cars.set(new Date('09/01/2013'), '4C'); cars.set(new Date('07/04/1989'), 'miata'); cars.set(new Date('09/01/1996'), 'elise'); console.log(cars.size); // 3 // later in the code const byDate = []; cars.forEach(function(value, key) { const result = { name: value, date: key }; if (byDate.length === 0 || key <= byDate[byDate.length - 1].date) { byDate.push(result); } else { byDate.splice(byDate.length - 1, 0, result); } }); console.info(byDate); // [ { name: '4C', date: 2013-09-01T04:00:00.000Z }, // { name: 'elise', date: 1996-09-01T04:00:00.000Z }, // { name: 'miata', date: 1989-07-04T04:00:00.000Z } ]

Notice how a normal forEach loop can be added. The loop is also gauranteed to happen in order of insertion while object property iteration order varies per environment. Doing this with an object based hash map would require a strategy like saving each data as a string of its timestamp (making the keys less readable). Then extracting and converting them which is a cumbersome process.

[obscurejavascript]

Set in JavaScript

Sets allow you store unique values. So instead of filtering lists for duplicates, items can just be inserted into a set then extracted from. For example, to unify to sets of asynchronous data with no duplicates:

const getFastCars = function(callback) { // Simulate async API setTimeout(function() { callback([ { name: 'charger', hp: '707' }, { name: 'elise', hp: '217' }, ]); }, 500); } const getGoodHandlingCars = function(callback) { // Simulate async API setTimeout(function() { callback([ { name: 'miata', hp: '155' }, { name: 'elise', hp: '217' }, ]); }, 300); } const getGoodPerformanceCars = function(callback) { let totalResults = new Set(), totalCalls = 0; const onCallComplete = function(results) { results.forEach(function(result) { totalResults.add(result.name); }); totalCalls += 1; if (totalCalls >= 2) { // Note that Array.from has no IE support, only MS Edge callback(Array.from(totalResults)); } } getFastCars(onCallComplete); getGoodHandlingCars(onCallComplete); } getGoodPerformanceCars(function(results) { console.info(results); }); // [ 'miata', 'elise', 'charger' ]

Which requires much less setup to get the unique parts. Normally, some kind of map would have to be used, then its keys would have to be extracted.

Unfortunately, sets track uniqueness in objects by reference and not value. So just inserting objects will not ensure they have they are unique in most cases. But for primitives (like Number), they work very well.

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2017/setObject.js

[obscurejavascript]

Array.from in JavaScript

Array.from simplifies array conversion by providing one simple interface to turn objects that can be iterated through to arrays. Normally, separate operations are required for different types:

const str = 'test'; const strArray = str.split(''); const printArguments = function() { console.log([].slice.call(arguments)); } console.log(strArray); // [ 't', 'e', 's', 't' ] printArguments('test1', 'test2', 'test3'); // [ 'test1', 'test2', 'test3' ]

This is simplified with Array.from:

const strArray2 = Array.from(str); const printArguments2 = function() { console.log(Array.from(arguments)); } console.log(strArray); // [ 't', 'e', 's', 't' ] printArguments('test1', 'test2', 'test3'); // [ 'test1', 'test2', 'test3' ]

It is also useful for new ES6 Objects like Set and Map:

const map1 = new Map([['property1', 'value1'], ['property2', 'value2']]); const set1 = new Set(['test1', 'test2', 'test1', 'test3']); const map1Str = Array.from(map1); const set1Str = Array.from(set1); console.log(map1Str); // [ [ 'property1', 'value1' ], [ 'property2', 'value2' ] ] console.log(set1Str); // [ 'test1', 'test2', 'test3' ]

Browser support is good other than there is no IE support, only MS Edge. MDN has a polyfill that can help in that case.

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2017/arrayFrom.js

[obscurejavascript]

JavaScript Split And the Limit Property

One of the little used and known properties in JavaScript is the limit property in split. Limit returns the number of splits specified instead of all that can be found in a string. This is simpler than using regular expressions for simple operations like getting the first 2 lines of a string:

// From the command: df -h const commandResult = '' + 'Filesystem Size Used Avail Capacity iused ifree %iused Mounted on\n' + '/dev/disk1 233Gi 202Gi 30Gi 87% 2143375 4292823904 0% / \n' + 'devfs 186Ki 186Ki 0Bi 100% 644 0 100% /dev \n' + 'map -hosts 0Bi 0Bi 0Bi 100% 0 0 100% /net \n' + 'map auto_home 0Bi 0Bi 0Bi 100% 0 0 100% /home \n' + '/dev/disk0s3 620Mi 553Mi 66Mi 90% 71 4294967208 0% /Volumes/Recovery HD'; const parts = commandResult.split('\n', 2); // Print the first 2 lines console.info('Main system memory usage:\n', parts.join('\n')); // Filesystem Size Used Avail Capacity iused ifree %iused Mounted on // /dev/disk1 233Gi 202Gi 30Gi 87% 2143375 4292823904 0% /

Limit can also help with efficiency when parsing larger strings. For example, when parsing a file, it would be more efficient to extract the first 3 columns each time if a split with a limit of 3 was used:

const simulatedFileInput = '' + 'name,hp,weight,brand,drive\n' + 'miata,155,2400,mazda,RW\n' + 'elise,217,2000,lotus,RW\n' + '4C,237,2465,alpha romeo,RW'; var parseLine = function(line, index) { if (index > 0) { const parts = line.split(',', 3); return { name: parts[0], hp: parts[1], weight: parts[2] }; } return {}; } // Simulated line-by-line file stream const formattedData = simulatedFileInput.split('\n').map(parseLine); console.log(formattedData.slice(1));

The performance benefit can be seen with these JSPerf tests: https://jsperf.com/splitvslimitedsplit

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2017/splitLimit.js

[apekssolutions]

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[partitiondanburyworkersw]

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[obscurejavascript]

Getters and Setters in Javascript

Getters and setters are specific functions for objects that accept one argument for the setter and no arguments for the getter. They are called via equals and normal property accessors. For example:

var circle = { radiusSquared: 0, get area() { return Math.round(4 * Math.PI * this.radiusSquared); }, set radius(radius) { this.radiusSquared = Math.pow(radius, 2); } } circle.radius = 1.83; console.log('Area:', circle.area); // Area: 42

This actually can be thought of as a specific case of the Object.defineProperties syntax:

var circle = Object.defineProperties({}, { radiusSquared: { value: 0, writable: true }, area: { get: function() { return Math.round(4 * Math.PI * this.radiusSquared); } }, radius: { set: function(radius) { this.radiusSquared = Math.pow(radius, 2); } } }); circle.radius = 1.83; console.log('Area:', circle.area); // Area: 42

Which also provides the way to add getters and setters dynamically to an object. Finally any getters or setters can be removed with delete.

Although getters and setters provide a shorter syntax than manually adding functions such as getArea(), I find them confusing for 2 reasons:

There are no () for getters making a statement like obj.property1.property2().property3 confusing because it looks like there is just one call, but there could be 1 to 3.

Getters or setters cannot be passed into other functions directly, making functions that take functions as parameters harder to implement like Array.map.

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2016/getters.js