A brief discussion on the difference between javascript function expressions and function declarations_javascript skills

There are two ways to declare functions in JavaScript: function declaration and function expression.

The differences are as follows:

1). For functions defined by function declaration, the function name is required, while the function name of the function expression is optional.

2). For functions defined by function declaration, the function can be called before the function declaration, while the function of the function expression can only be called after the declaration.

3). Functions defined by function declaration are not real declarations. They can only appear globally or nested in other functions, but they cannot appear in loops, conditions or try/catch/ finally in, and

Function expressions can be declared anywhere.

The functions are defined in two ways below:

 //函数声明式
 function greeting(){
    console.log("hello world"); 
 }
 
 
 //函数表达式
 var greeting = function(){
   console.log("hello world"); 
 }

An interesting javascript below:

function f() { console.log('I am outside!'); }
(function () {
 if(false) {
  // 重复声明一次函数f
  function f() { console.log('I am inside!'); }
 }
 f();
}());

What will be output? The first reaction should be "I am outside". The result is "I am inside" in chrome, IE11 directly reports an error, and the lower version of firefox outputs "I am outside"...

The results output by chrome clearly reflect the characteristics of functions declared using function declarations-the function can be called before it is declared.

IE error shows that the object is missing because the function is declared in the condition, which violates the principle of function declaration.

Scope of function expression:

If the function declared by the function expression has a function name, then the function name is equivalent to a local variable of the function and can only be called inside the function. For example:

var f = function fact(x) { 
        if (x <= 1) 
          return 1;
        else 
          return x*fact(x-1);
        };
        alert(fact());  // Uncaught ReferenceError: fact is not defined

fact() can be called inside the function, but when called outside the function, an error will be reported: fact is undefined.
fact

Let’s take a closer look

Function declaration

Function declaration sample code

The code is as follows:

function fn () {
  console.log('fn 函数执行..');
  // code..
}

In this way, we have declared a function named fn. Here is a thought. Do you think it will be executed if you call it on top of this function? Or will an error be reported?

The code is as follows: fn(); // Call the fn function we declared before function fn () { console.log('fn function execution..'); // code..}

Console output:

Yes, the fn function can be called at this time. Here is a summary of the reasons.

Summary:

1: At this time, the fn function is the result of the variable, which is stored in the variable of the global context by default (can be verified by window.function name)

　2: This method is function declaration, which is created when entering the global context phase. They are already available during the code execution phase. ps: javaScript will initialize the context environment (from global → local) every time it enters a method

　3: It can affect variable objects (only variables stored in the context)

Function expression

Function expression example code

The code is as follows:

var fn = function () {
  console.log('fn 函数【表达式】声明执行..')
  // code..
}

In this way we declare an anonymous function and point its reference to the variable fn?

Call the function declared by the expression once again above and below to see the output on the console.

The code is as follows:

// 为了清晰的看到控制台的输出，我们在各自调用前后做个标记，增加可读性。
console.log('之前调用开始..');
fn();
console.log('之前调用结束..');
var fn = function () {
  console.log('fn 函数【表达式】声明执行..')
  // code..
}
console.log('之后调用开始..');
fn();
console.log('之后调用开始..');

Console print result:

You can see that when the code is executed and the fn() function is called for the first time, it prompts: fn is not a function (fn is not a method), and the operation is terminated when an error is encountered.

This shows that when fn() is called for the first time, the var fn variable does not exist as an attribute of the global object, and the anonymous function context referenced by fn has not been initialized, so the previous call failed.

The code is as follows:

// 现在先把之前的调用逻辑给注释掉，再看下控制台的输出
//  console.log('之前调用开始..');
//  fn();
//  console.log('之前调用结束..');
  var fn = function () {
    console.log('fn 函数【表达式】声明执行..')
    // code..
  }
  console.log('之后调用开始..');
  fn(); // 在表达式之后调用
  console.log('之后调用开始..');

Console print result:

It can be seen that it is possible to call after the expression function. Let’s summarize why?

Summary:

1: First of all, the variable itself does not exist as a function, but a reference to an anonymous function (value types are not references)

2: During the code execution phase, when the global context is initialized, it does not exist as a global attribute, so it will not cause pollution of variable objects

3: This type of declaration is generally common in plug-in development, and can also be used as a call to a callback function in a closure

　So function fn () {} is not equal to var fn = function () {}, they are essentially different.

The above is the entire content of this article. The idea is very clear and the comparison is also very clear. It is a very good article. Friends, you must read it carefully