Question

The bind method, which all functions have, creates a new function that will call the original function but with some of the arguments already fixed.

The following code shows an example of bind in use. It defines a function isInSet that tells us whether a person is in a given set of strings. To call filter in order to collect those person objects whose names are in a specific set, we can either write a function expression that makes a call to isInSet with our set as its first argument or partially apply the isInSet function.

var theSet = ["Carel Haverbeke", "Maria van Brussel",
              "Donald Duck"];
function isInSet(set, person) {
  return set.indexOf(person.name) > -1;
}

console.log(ancestry.filter(function(person) {
  return isInSet(theSet, person);
}));
// → [{name: "Maria van Brussel", …},
//    {name: "Carel Haverbeke", …}]
console.log(ancestry.filter(isInSet.bind(null, theSet)));
// → … same result

The call to bind returns a function that will call isInSet with theSet as first argument, followed by any remaining arguments given to the bound function.

The first argument, where the example passes null , is used for method calls, similar to the first argument to apply . I’ll describe this in more detail in the next chapter .

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.