#hashes

keyBy takes an array of object and a property name then creates an object map of that property name to the corresponding array element. This is useful to make checks for various properties in arrays easier and faster. After doing the keyBy direct property tests can be used instead of find loops or other tactics. For example:

const cars = [ { id: 'miata', hp: 155, weight: 2400 }, { id: '4c', hp: 237, weight: 2465 }, { id: 'elise', hp: 217, weight: 2000 } ]; const carIds = keyBy(cars, 'id'); console.log('Cars found (keyBy)'); console.log('miata:', 'miata' in carIds); console.log('elise:', 'elise' in carIds); console.log('ferrari:', 'ferrari' in carIds); // miata: true // elise: true // ferrari: false

The only disadvantage is that the array must be looped through once to create the key-value object:

function keyBy(arr, propertyName) { const obj = {}; for (const ele of arr) { obj[ele[propertyName]] = ele; } return obj; }

keyBy is available in utility libraries like Lodash: https://lodash.com/docs/4.17.15#keyBy

A simple invert function can do this. Here is how it works:

const obj = { a: 1, b: 2, c: 3 }; console.log(invert(obj)); // { '1': 'a', '2': 'b', '3': 'c' }

This is useful when there is a list of names to ids. Sometimes you will need to check if an id exists in that list. If this is done in a loop, it is usually much faster to check an inverted id to name object that is created before the loop:

function hasIds(obj, ids) { const byId = invert(obj); return ids.map((id) => byId[id] !== undefined); } const cars = { 'miata': 1, 'elise': 2, 'elan': 3 }; console.log(hasIds(cars, [1, 2, 5])); // [ true, true, false ]

The implementation for invert is simple:

function invert(obj) { const newObj = {}; for (const keyName in obj) { if (obj.hasOwnProperty(keyName)) { newObj[obj[keyName]] = keyName; } } return newObj; }

It is also available in many utility libraries like Lodash.

Conditional expressions are often used by default to process logic. For example, the following could be used to toggle between a GUI with 4 different items:

function toggleMode(oldMode) { if(oldMode === 'bar-chart') { return 'total-view'; } else if(oldMode === 'total-view') { return 'percentage-view'; } else if(oldMode === 'percentage-view') { return 'grid-view'; } else if(oldMode === 'grid-view') { return 'bar-chart'; } } console.log('Toggle mode with conditionals:'); console.info(toggleMode('bar-chart')); // total-view console.info(toggleMode('total-view')); // percentage-view console.info(toggleMode('percentage-view')); // grid-view console.info(toggleMode('grid-view')); // bar-chart

By using a hash the code is simplified and easier to modify in the future:

function toggleMode(oldMode) { const modeMap = { 'bar-chart': 'total-view', 'total-view': 'percentage-view', 'percentage-view': 'grid-view', 'grid-view': 'bar-chart', } return modeMap[oldMode]; }

Even better, the hash approach can be used to automate logic. For example, if the toggle set needs to be genericized to toggle any list, the hash can be generated then used:

function createToggle(allModes) { const modeMap = {}; allModes.forEach(function(mode, index) { const next = (index + 1) % allModes.length; modeMap[mode] = allModes[next]; }); return function(oldMode) { return modeMap[oldMode]; }; } const allModes = ['bar-chart', 'total-view', 'percentage-view', 'grid-view']; const toggleMode3 = createToggle(allModes); console.info(toggleMode3('bar-chart')); // total-view console.info(toggleMode3('total-view')); // percentage-view console.info(toggleMode3('percentage-view')); // grid-view console.info(toggleMode3('grid-view')); // bar-chart

This approach is obvious in retrospect, but hard to realize initially. By keeping this in mind, certian flows of logic can become easier to maintain. Of course, for some complex flows of logic, it may make sense to express code in individual conditionals.

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