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return total;
Listing 5.29 shows two test cases for this method. The first test sums a series of
numbers by calling the function with parentheses, whereas the second test sums an
array of numbers via apply .
Listing 5.29 Summing numbers with apply
TestCase("SumTest", {
"test should sum numbers": function () {
assertEquals(15, sum(1, 2, 3, 4, 5));
assertEquals(15, sum.apply(null, [1, 2, 3, 4, 5]));
Remember, passing null as the first argument causes this to implicitly bind
to the global object, which is also the case when the function is called as in the first
test. ECMAScript 5 does not implicitly bind the global object, causing this to be
undefined in the first call and null in the second.
call and apply are invaluable tools when passing methods as callbacks to
other functions. In the next chapter we will implement a companion method,
Function.prototype.bind , which can bind an object as this to a given
function without calling it immediately.
5.5 Summary
In this chapter we have covered the theoretical basics of JavaScript functions. We
have seen how to create functions, how to use them as objects, how to call them,
and how to manipulate arguments and the this value.
JavaScript functions differ from functions or methods in many other languages
in that they are first class objects, and in the way the execution context and scope
chain work. Also, controlling the this value from the caller may be an unfamiliar
way to work with functions, but as we'll see throughout this topic, can be very
In the next chapter we will continue our look at functions and study some more
interesting use cases as we dive into the concept known as closures.
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