javascript

While waiting for the twits to give me an api key so I can try out tweepy etc, let's check out a re-introduction to javascript, starting with https://bit.ly/mozilla-js-reintro-tutorial Note: At Github this file is 'marked up' i.e. easier to read. You can also read it in a markdown viewer.

Basic syntax

Super-similar to C/Java:

• same control structures (for/while/do,switch/if/else if/else)
  • for (let i=0; i<5; i++) {...}
  • but adds 2 kinds of foreach: for( let val of array) and for( let prop in object)
  • also adds a forEach( callbackFunc ) member on arrays
  • and can switch on a string (handy)
• same operators: arithmetic (+ - * / %) and conditional (&& || !) (inc short circuit behaviour)
  • adds extra === operator (does not promote operands):
    • 123 == '123' //true
    • 123 === '123' //false
• same null (a la java)
  • adds special: undefined ('variable not assigned yet')

Declarations

• let x = 1; // like C++/Java, x scoped to end of nearest block
• const PI = 3.14; // like C/C++ (or final in Java)
• var globalVar = 7; //Danger: this leaks globalVar name

Basic types:

Number

- floating point doubles, so beware integer arithmetic
- builtin Math obj has methods like Java Math package
- builtin parseInt(), parseFloat('value', radix) methods 
  - happily convert numeric prefix in a string eg 10abc -> 10
- short cut: prefix unary + operator converts a string arg to number
  - but it tries to consume whole string and returns NaN if extras present
  - nb NaN is 'infectious' (article says 'toxic' :-)
- taste a value with isNan() or isFinite() before consuming
  - isFinite() checks for +/-Infinity and NaN, so safer

String

- single or double quotes 
- strings are unicode, each char is 1 or 2 'code points' (2-byte words)
- length attribute on strings, returns num code points NB not bytes or chars
- Java-like methods on strings: 
    - s.charAt(n) 
    - s.toUppercase()
    - s.replace('foo', 'bar')

Boolean

• values: true, false
• explicitly convert: Boolean( myInt )
• implicitly: these falsey values evaluate to false:
  • 0
  • empty strings
  • null
  • NaN and
  • undefined
• all other values evaluate true
• allows some nice/concise idioms:
  • var name = x && x.getName() // avoids call to getName() unless x exists
  • var name = cached || (cached=getNameSlowMethod()) //use cached copy if exists

Object

Objects are maps or collections of name,value pairs. Names are strings, values can be any javascript value.

• Constructor syntax: var x = new Object();
• Literal syntax (JSON): var x = {};
• Can initialise with structure :

  var obj = {
    name: 'Carrot',
    age: 8,
    details: {
      color: 'orange',
      size: 12
    }
  };

• Reference fields via named subscripts / sub object notation:
  • obj['details']['size'] //12 -- flexible: variables for subscripts
  • obj.details.color //'orange' -- easier to read, but hard coded names
• Classes can be defined by a prototype function :

function Student(name, age) {
  this.name = name;
  this.age = age;
}
var you = new Student('Bob', 24);  //note keyword new

• Classes can also be defined by a factory function.. here includes a method fullName:

function makeStudent(name, age) {
  return{
    name: name,
    age: age,
    describe: function() {
      return this.name + ' is ' + this.age + ' yrs old';
    }
  }
}
var me = makeStudent('Fred', 24);  //no new
let desc = me.describe(); // 'Fred is 24yrs old'

Modules

Javascript files that export functions, data, classes etc can be used by other code ([MDN Javascript guide][https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules#Exporting_module_features]).

• Decorate top-level artefacts to expose, with export keyword:

export function makeStudent(name, age) {
  return{
    name: name,
    age: age,
    describe: function() {
      return this.name + ' is ' + this.age + ' yrs old';
    }
  }
}
export var fred = makeStudent('Fred', 24);  
export let desc = fred.describe(); // 'Fred is 24yrs old'

• Or, export a list of names at end of module, in {...}:

...
export {makeStudent, fred, desc};

Node.js

Turns out Node is basically like a Java VM for Javascript, runs your javascript for you on a server rather than just in a browser.

• Strength is event-driven single threaded apparently services huge load
• Weakness is ditto - i.e. don't use it for compute intensive services
• Good for feeding into queue to update 'eventually consistent' database

Node modules

Javascript files that export functions, classes etc can be used by other code ([Node reference docs][https://nodejs.org/api/modules.html]).