function declarations and function expressions in JavaScript

There are two ways to define a function in JavaScript, one is a function declaration and the other is a function expression. There are a few subtle differences between the two ways of defining.

1. function declaration:

function keyword + function name + function Body constitute a functional declaration, the following form:

function functionname (arg0, arg1, arg2) {//function body}

Firefox, Safari, Chrome, and Opera all define a nonstandard Name property for the function , which gives you access to the name specified by the function:

alert (functionname.name);//"functionname"

an important feature of function declarations is the promotion of function declarations ( function declaration Hoisting ) , which means that the function declaration is read before the code executes, that is, the function can be called before the function declaration:

SayHello ();//"Hello" function SayHello () {alert ("hello~");}

2. function Expressions:

a function expression is a bit like a regular variable assignment statement, in which case the function created is called an anonymous function . (anonymous function) , the specific form is as follows:

var functionname = function (arg0, arg1, arg2) {//function body};

a function expression differs from a function declaration in a two-point way because the function expression is defined as an anonymous function, so the Name property of the variable is an empty string:

alert (functionname.name);//""

The second is that the function expression does not have a function declaration promotion, so the function cannot be called before the function expression:

SayHello ();//errorvar SayHello = function () {alert ("ello~");};

Look at an example that helps us to better understand the function declaration function expression:

Code Snippet One:

if (condition) {function SayHello () {alert ("Hello version_1");  }} else {function SayHello () {alert ("Hello version_2"); }}

on the surface, the above code indicates that when condition is true , a definition of SayHello () is used , otherwise another definition is used. However, this approach is dangerous and may have unintended consequences, but if you use a function expression, there is no problem with the following code snippet:

Code fragment Two:

var sayhello;if (condition) {SayHello = function () {alert ("Hello version_1"); };}  else {SayHello = function () {alert ("Hello version_2"); };}

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function declarations and function expressions in JavaScript