Javascript wrinkles

Javascript notes and quirks

The plus operator: +

strings containing numbers work in calculations if prefixed with a + sign e.g. add these: +"1.1" + +"1.1".

Check existence

var myArray = new Array(); if (!myArray[0]) {console.log("Not defined")} uses false to print the statement.

Variable scope

local to the code a block resides within (not block statement scope). They are also 'hoisted' as if at the top of a function/statement.
Try this function Scope() { if (true) { var x = 5 }; alert("x: " + x + ", y: " + y); }; var y = 6;

Constants

Declared with the const keyword, e.g. const prefix = '212';z = prefix + 424;alert(prefix);.
Constants cannot share a name with a function or variable in the same scope, or be called directly e.g. alert(prefix);.

Literals

Array literals (literals have fixed values) use square brackets: var mynums = ["one", "two", "three"]; alert(mynums[0]).

Avoid spaces after \ in multiline string literals

Always add a return immediately after \ as '\ ' will produce an error. This multiline string is broken before 'another'.

Return the entire code for a function

Simply omit the brackets alert(IsDefined)

Retrieving values from an object

value by dot notation: flight.departure.IATA
value by name: flight['number'] (Douglas Crockford: Javascript: the Good Parts, or the video or a series)
value of array element in object: employees.accounting[1].firstName (from Mastering JSON)

Using the file api

HTML5 allows interaction with local files via its File API specification - test your browser's file api here.

Javascript good practice

Don't use ++ or --

Use += and -= instead, as ++ and -- are too tight, too tricky, too cryptic (Crockford)

Use === and !== for comparison

This will ensure that: 0, '', undefined, null, false, true all return the expected result. Both == and != do type coercion before comparing. This causes \f\r\n\t == 0 to be true. If you want this, use the shorthand (foo) and (!foo) instead of (foo != 0) and (foo == 0) (Crockford)

Avoid using new

Don't use new to create a Boolean, Number or String; and use {} and [] to create a new Object or Array

Use object.create, but not for arrays

because object produced will inherit the array’s values and methods, but lack the length property.

Declare functions before you call them

and declare variables at the top of functions, avoiding for(var i...

Name constructor functions with an initial capital

to help spot instances where they're called without new. Better: use object.create (instead of new) for a new instance of a specific object.