Best Practice: Comparisons in JavaScript

Make your Comparisons Carefully

Make sure that your comparison expression will always be evaluated to what you expect.

Usage of == might be dangerous, when you only expect strict Boolean values.

Here are few points how to start doing better:

• use triple-equals where the data might have multiple types;
• use instanceof to identify the object type;
• ensure that an object has the property (use hasOwnProperty()) before access to it;

##Strict Equality Triple equals `===` compares both type & value, whereas `==` drops the type: '7' == 7 //true '7' === 7 //false Another example: 1 == true //true false == 0 //true 1 === true //false 0 === false //false So, the using triple equals is the best practice in the most unknown situations. ---- ##Not Strict Equality When you use double equals `==` or short boolean examinations, variables which have some value will always be evaluated to `true`, whereas the ones with no value will always be evaluated to `false`: * empty values *(equal to `false`)*: 0 //false null //false undefined //false NaN //false '' //false * not empty values **(equal to `true`): 3 //true {} //true [] //true "A" //true ---- ##Use `instanceof` to verify type of object You can check entire inheritance chain via using `instanceof`: ChildA.prototype = Object.create(Child.prototype); ChildB.prototype = Object.create(Child.prototype); childA instanceof Child //true childB instanceof Child //true childA instanceof ChildA //true childB instanceof ChildB //true childA instanceof ChildB //false childB instanceof ChildA //false Let's remove all objects of type Child from the customers list. for (var i = 0, len = customers.length; i < len; i++) { if (customers[i] instanceof Child) { console.log("removed: " + customers.splice(i, 1)[0]); i--; } } ---- ##Use `hasOwnProperty()` to prevent an error Before accessing some object or property it's better to ensure that such one exists. Let's create some object: var book = {}; book.author.name; If you'll try to take the property of the `undefined` object you'll face with the following error: Uncaught TypeError: Cannot read property 'name' of undefined(…) To make sure it won't happen anymore check whether the `author` exists before collaboration with him: if (book.hasOwnProperty("author")) { console.log(book.author.name); } ---- ## see Also * [Best Practice: Executions & Memory Control in JavaScript](http://bloomlab.blogspot.com.ee/2015/11/best-practice-executions-memory-control.html) * [Best Practice: Loops Optimization](http://bloomlab.blogspot.com.ee/2015/11/best-practice-loops-optimization-in.html) * [Fun with an Algorithm complexity](http://bloomlab.blogspot.com.ee/2014/07/fun-with-algorithm-complexity.html) * [JS - The Right Way](http://jstherightway.org/) * [Tutorials Point - Best Practices](http://www.tutorialspoint.com/developers_best_practices/) * [Keynote: Top JS frmeworks 2015](http://bloomlab.blogspot.com.ee/2015/03/top-javascript-frameworks-2015.html)