Monday, 16 March 2015

Javascript "let" keyword vs "var" keyword

One of the differences between JavaScript and Java, is regarding variable scope.

In JavaScript there are two kinds of scope, global scope and function scope. Any local variable declared in a function, is visible everywhere in the function.

In the example above, both secondCarName and thirdCarName are local variables, only accessable within the function.

Note that, though the thirdCarName is declared inside an if-then block, this matters naught.

A gobal variable is visible everywhere. In the example above, this would be firstCarName.

Java

In contrast, in Java, scope can be within a method (sort of the equivalent of function), but also in block scope.

Therefore, netherlands is only available within the if statement.

let vs. var

Apparently, ECMAScript saw fit to provide JavaScript with a way to also support the Java-way of scope using the new let keyword.

But be wary, there will probably always be situations where Java and JavaScript will display different behaviour.

With let, it is possible to create a variable that obscures the variable in the higher block. This is in contrast with Java, where a error is thrown during compiling.

Disclaimer

My experience with JavaScript is spotty at best. The intricacies of scope in Java and JavaScript are more subtle than the isolated examples described here. And the new let keyword makes the scope in JavaScript even more subtle.

References

Stackoverflow - Javascript - “let” keyword vs “var” keyword
http://stackoverflow.com/questions/762011/javascript-let-keyword-vs-var-keyword
JavaScript|MDN - let keyword
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let
Hangar.Runway 7 - The Javascript Guide to Objects, Functions, Scope, Prototypes and Closures
http://hangar.runway7.net/javascript/guide

Thursday, 12 March 2015

Enhanced For Loop in JavaScript

I am no expert in JavaScript, but I feel I should mention the possible flavours of for loops in here.

The forEach method is explained in [4].

The latter example is new in ECMAScript 6 (which should be out already).3

You can test it out in [5].

References

[1] Mozilla Developer Network - for...of
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of
[2] Stackoverflow - For-each over an array in JavaScript?
http://stackoverflow.com/questions/9329446/for-each-over-an-array-in-javascript
[3] ECME-262 6h Edition, The 2015 ECMAScript Language Specification ("Harmony")
http://wiki.ecmascript.org/doku.php?id=harmony:specification_drafts
[4] forEach
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach
[5] JSFiddle - example in action
https://jsfiddle.net/maarten_l/8vc44u4b/