The difference and internal principles between the two definition methods of JS_Basic knowledge

I believe everyone has used both methods, but not everyone knows the difference and internal principles.

Copy code The code is as follows:

// Method 1
function func1(x, y){
// your code
}
// Method 2
var func2 = function(x,y){
// your code
}

Method 1 is a typical function declaration (Function declarations).
Method 2 is function expressions, assigning an anonymous function to a variable. In other words, method 2 creates an anonymous function with formal parameters x and y, and then assigns the anonymous function to the variable func2.

The main difference is:
1, the function declaration needs to display the specified function name, here is func1; the function expression uses the anonymous function
2, method 1 is before the code is executed (interpretation period ) is loaded into the scope, method 2 needs to be loaded when the code is executed (runtime)

A simple example will help you understand the difference in their use

Copy code The code is as follows:

alert(func1); // --> func1 source code
alert(func2); // --> undefined
// Method 1
function func1(x,y){
// your code
}
// Method 2
var func2 = function(x ,y){
// your code
}

As you can see, the source code of func1 pops up the first time, but it is undefined the second time. That is, if you use method 1 (function declaration) to define a function, you can use it on top of the function code. If you use method 2 (function expression) to define a function, it cannot be used before its definition, but can only be used after its definition.

It internally involves the execution context (Execution context) and the activation object (Activation object). If you want to know more, please read the EcmaScript 5 documentation.

Recently, I have found that more and more people like to use method 2 to define functions, especially in nested functions. For example, simply defining a function is still customary method 1.