javascript

Chapter 5: Reference Types
In this example, it ’ s clear that declaring two functions with the same name always results in the last
function overwriting the previous one. This code is almost exactly equivalent to the following:
var addSomeNumber = function (num){
return num + 100;
}
addSomeNumber = function (num) {
return num + 200;
}
var result = addSomeNumber(100); //300
In this rewritten code, it ’ s much easier to see exactly what is going on. The variable addSomeNumber is
simply being overwritten when the second function is created.
Function Declarations vs. Function Expressions
Throughout this section, the function declaration and function expression have been referred to as being
almost equivalent. This hedging is due to one major difference in the way that an interpreter 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:
alert(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. Changing the function declaration to an initialization, as in the following
example, will cause an error during execution:
alert(sum(10,10));
var sum = function(num1, num2){
return num1 + num2;
};
This updated code will cause an error because the function is part of an initialization statement, not part
of a function declaration. That means the function isn ’ t available in the variable sum until the
highlighted line has been executed, which won ’ t happen, because the first line causes an “ unexpected
identifier ” error.
Aside from this difference in when the function is available by the given name, the two syntaxes are
equivalent.
It is possible to use function declaration and initialization together, such as
var sum = function sum() {} . However this syntax will cause an error in Safari.
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