JavaScript Study Notes: Function Object_Basic Knowledge

In JavaScript, functions are objects.

There is no method overloading in JS

In JavaScript, there is no concept of method (function) overloading.

Example:

Copy code The code is as follows: function add(number) { alert(number 20); } function add(number, number1) { alert(number 30); add(10);

The pop-up box on the web page displays 40. Indicates that although the second method has two parameters, it is still called.

After exchanging the order of the two methods, the pop-up box displays 30. It can be seen that regardless of the number of parameters, the subsequent method with the same name is called.

How to explain this phenomenon?

This is because the function declaration actually creates an object:

Copy code The code is as follows: function add(number) { alert(number 20); } /* The above function is equivalent to: var add = function(number) { alert(number 20); } */ function add(number, number1) { alert(number 30); } /* The above function is equivalent to: var add = function(number, number1) { alert(number 30); */ add(10);

In this way, add actually points to the following object, and the parameters assigned when the method is called will be assigned to the formal parameters of the method in order. The parameters that are not assigned later are undefined.

There is no strict check on the number of parameters when a JavaScript function is called. It is okay for the number of actual parameters to be less than the number of formal parameters. Formal parameters that are not assigned a value are undefined.

It is also possible that the number of actual parameters is greater than the number of formal parameters, so that only the previous actual parameters will be used, and the extra actual parameters will not be used.

Function object

There is a Function object in JavaScript, and all custom functions are of Function object type.

All parameters received by the Function object are of string type, the last parameter is the function body to be executed, and the previous parameters are the parameters that the function really needs to receive.

Example:

Copy code The code is as follows: var add = new Function("number", "number1", "alert(number number1);"); var add = new Function("number", "alert(number 20);"); add(10, 30);

Implicit object arguments

In JavaScript, each function has an implicit object arguments, which represents the actual parameters passed to the function.

Arguments has nothing to do with the formal parameters of the function and their number.

Arguments has a useful attribute length, which represents the length of the actual parameter. You can use this to simulate function overloading:

Practice example:

Copy code The code is as follows: function add(number1, number2) { alert(arguments.length); alert(arguments[0]); alert(arguments[1]); alert(arguments[2]); } //add(2, 3, 4); function add2() { if(1 == arguments.length) { alert(arguments[0]); } else if(2 == arguments.length) { alert(arguments[0] arguments[1]); } else if(3 == arguments.length) { alert(arguments[0] arguments[1] arguments[2]); } } add2(3); add2(3, 4); add2(3, 4, 5); 每一个函数对象都有一个length属性，表示该函数期望接收的参数格式。

它与函数的arguments不同，arguments.length表示函数实际接收的参数个数。

例子:

复制代码 代码如下: var add = function(num, num2, num3) { alert(num num2 num3); } alert(add.length); //输出3 add(1, 2, 3); var add2 = function() { } alert(add2.length); //输出0

JavaScript Study Notes: Function Object_Basic Knowledge

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