Arrays

Javascript only has array-like objects which are slower than 'real' arrays.

Retrieval and updating of properties works the same as with an object except with integer property names.

Arrays have their own literal format and their own set of methods

Array Literals

• An array literal is a pair of square brackets surrounding zero or more comma-seperated values [a, b, c, etc]
  • The first value will get the property name '0', the second will be '1' and so on
• Javascript allows an array to contain any mixture of values

Length

• If you add to the array, the length property will increase to contain the new element - it will not give an error
• If you set the .length to a smaller number than the current length of the array, it will delete any properties with a subscipt >= the new length
• The push() method is sometimes useful to add an element to the end of an array numbers.push('go') //adds the element 'go' to the end of the numbers array

Delete

• Elements can be deleted from the array object using delete but this leaves a hole in the array
• Use array.splice(keyInArray, howManyElementsToDelete) which changes the keys for the remaining values in the array so there is no hole left
  • May be slow

Enumeration

• A for statement can be used to iterate over all the properties of an array (as it is an object)
• Do not us for in as it does not iterate through the properties in order and sometimes pulls in from furhter up the prototype chain

Confusion

The rule is simple: when the property names [keys] are small sequential integers, you should use an array. Otherwise, use an object.

• Arrays are most useful when property names are integers but they can also accept strings as property names
• Javascript doesn't have a good way of telling an object from an array as typeof array === object
• To accurately detect arrays, have to define our own function:

  var is_array = function (value) {
    return Object.prototype.toString.apply(value) === '[object Array]';
    //apply(value) binds `value` to `this` & returns true if `this` is an array
  }
  

Methods

• Array methods are stored in Array.prototype which can be augmented using the format:

  //capital A in Array means this refers to the prototype
  Array.method('reduce', function (parameters){
    //define variables and function
    //return a value
  });
  

• Remember, every array inherits and can use the methods you add to Array.prototype
• You can also add methods directly to an array because they are objects
  • myArray.total = function () { //statements to execute; } adds a 'total' function to the array myArray
• DO NOT USE: Object.create() will create an object - lacking the length property - not an array.

Dimensions

• Using [] will create an empty array as they are not initialized in JavaScript
• Accessing a missing element will give you undefined
• If you have an algorithm that relies on the array not being empty and not having undefined values, you can write a function that will prep your array to have a certain number of defined values, essentially initializing it with certain values in place
  • An Array.dim function is oulined on page 63 which will allow var myArray = Array.dim(10,0) to make an array with 10 zeroes starting from the first position in the array(0)
• Javascript only has one dimensional arrays but can have arrays of arrays
• Two dimensional arrays (matricess) will have to be set up by the programmer
  • page 63 gives a method for this and for explicitly setting cell values so as not to have an empty matrix