Question

Back to the weresquirrel. A set of daily log entries can be represented as an array. But the entries do not consist of just a number or a string—each entry needs to store a list of activities and a Boolean value that indicates whether Jacques turned into a squirrel. Ideally, we would like to group these values together into a single value and then put these grouped values into an array of log entries.

Values of the type object are arbitrary collections of properties, and we can add or remove these properties as we please. One way to create an object is by using a curly brace notation.

var day1 = {
  squirrel: false,
  events: ["work", "touched tree", "pizza", "running",
           "television"]
};
console.log(day1.squirrel);
// → false
console.log(day1.wolf);
// → undefined
day1.wolf = false;
console.log(day1.wolf);
// → false

Inside the curly braces, we can give a list of properties separated by commas. Each property is written as a name, followed by a colon, followed by an expression that provides a value for the property. Spaces and line breaks are not significant. When an object spans multiple lines, indenting it like in the previous example improves readability. Properties whose names are not valid variable names or valid numbers have to be quoted.

var descriptions = {
  work: "Went to work",
  "touched tree": "Touched a tree"
};

This means that curly braces have two meanings in JavaScript. At the start of a statement, they start a block of statements. In any other position, they describe an object. Fortunately, it is almost never useful to start a statement with a curly-brace object, and in typical programs, there is no ambiguity between these two uses.

Reading a property that doesn’t exist will produce the value undefined , which happens the first time we try to read the wolf property in the previous example.

It is possible to assign a value to a property expression with the = operator. This will replace the property’s value if it already existed or create a new property on the object if it didn’t.

To briefly return to our tentacle model of variable bindings—property bindings are similar. They grasp values, but other variables and properties might be holding onto those same values. You may think of objects as octopuses with any number of tentacles, each of which has a name inscribed on it.

The delete operator cuts off a tentacle from such an octopus. It is a unary operator that, when applied to a property access expression, will remove the named property from the object. This is not a common thing to do, but it is possible.

var anObject = {left: 1, right: 2};
console.log(anObject.left);
// → 1
delete anObject.left;
console.log(anObject.left);
// → undefined
console.log("left" in anObject);
// → false
console.log("right" in anObject);
// → true

The binary in operator, when applied to a string and an object, returns a Boolean value that indicates whether that object has that property. The difference between setting a property to undefined and actually deleting it is that, in the first case, the object still has the property (it just doesn’t have a very interesting value), whereas in the second case the property is no longer present and in will return false .

Arrays, then, are just a kind of object specialized for storing sequences of things. If you evaluate typeof [1, 2] , this produces "object" . You can see them as long, flat octopuses with all their arms in a neat row, labeled with numbers.

So we can represent Jacques’ journal as an array of objects.

var journal = [
  {events: ["work", "touched tree", "pizza",
            "running", "television"],
   squirrel: false},
  {events: ["work", "ice cream", "cauliflower",
            "lasagna", "touched tree", "brushed teeth"],
   squirrel: false},
  {events: ["weekend", "cycling", "break",
            "peanuts", "beer"],
   squirrel: true},
  /* and so on... */
];

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.