FUNCTION DECLARATIONS VERSUS FUNCTION EXPRESSIONS

Throughout this section, the function declaration and function expression are referred to as being almost equivalent. This hedging is due to one major difference in the way that a JavaScript engine loads data into the execution context. Function declarations are read and available in an execution context before any code is executed, whereas function expressions aren't complete until the execution reaches that line of code. Consider the following:

// OK
console.log(sum(10, 10));
function sum(num1, num2) {
 return num1 + num2;
}

This code runs perfectly because function declarations are read and added to the execution context before the code begins running through a process called function declaration hoisting. As the code is being evaluated, the JavaScript engine does a first pass for function declarations and pulls them to the top of the source tree. So even though the function declaration appears after its usage in the code, the...