Question

One possible way is to store all the correlations in an array, using objects with name and value properties. But that makes looking up the correlation for a given event somewhat cumbersome: you’d have to loop over the whole array to find the object with the right name . We could wrap this lookup process in a function, but we would still be writing more code, and the computer would be doing more work than necessary.

A better way is to use object properties named after the event types. We can use the square bracket access notation to create and read the properties and can use the in operator to test whether a given property exists.

var map = {};
function storePhi(event, phi) {
  map[event] = phi;
}

storePhi("pizza", 0.069);
storePhi("touched tree", -0.081);
console.log("pizza" in map);
// → true
console.log(map["touched tree"]);
// → -0.081

A map is a way to go from values in one domain (in this case, event names) to corresponding values in another domain (in this case, ϕ coefficients).

There are a few potential problems with using objects like this, which we will discuss in Chapter 6 , but for the time being, we won’t worry about those.

What if we want to find all the events for which we have stored a coefficient? The properties don’t form a predictable series, like they would in an array, so we cannot use a normal for loop. JavaScript provides a loop construct specifically for going over the properties of an object. It looks a little like a normal for loop but distinguishes itself by the use of the word in .

for (var event in map)
  console.log("The correlation for '" + event +
              "' is " + map[event]);
// → The correlation for 'pizza' is 0.069
// → The correlation for 'touched tree' is -0.081

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.