Type conversion of JavaScript objects

In JavaScript, The toString () method and the alueOf () method are used to convert an object to a basic data type or string.
 
 
 
When converting an object to a string:
 
1. Call the toString () method of the object to convert the object to the returned string. If the method returns other basic data types, it is automatically converted to a string;
 
2. If the object does not have the toString () method, or the method returns a non-basic data type, call the valueOf () method in the same way;
 
3. Similarly, if the valueOf () method does not exist or the returned value is not of the basic data type, an error is prompted (IE may not report an error );
 
Js Code
// Custom function (class)
Function myObject (objectName)
{
This. objectName = objectName;
}
Var myObj = new myObject ("MyObj ");
 
// Normally, call the toString () method first.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj); // The result is 123.
 
// The type returned by the toString () method is incorrect. Call the valueOf () method.
MyObject. prototype. toString = function () {return new Date ();};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj); // The result is 321.
 
// The toString () method does not exist. The valueOf () method returns an incorrect type.
MyObject. prototype. toString = undefined;
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj); // the error message is displayed. ([Object] is not reported in IE8.)
 
// Custom function (class)
Function myObject (objectName)
{
This. objectName = objectName;
}
Var myObj = new myObject ("MyObj ");
 
// Normally, call the toString () method first.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj); // The result is 123.
 
// The type returned by the toString () method is incorrect. Call the valueOf () method.
MyObject. prototype. toString = function () {return new Date ();};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj); // The result is 321.
 
// The toString () method does not exist. The valueOf () method returns an incorrect type.
MyObject. prototype. toString = undefined;
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj); // the error message is displayed. ([Object] is not reported in IE8.)
 
Similarly, when an object is converted to a basic data type, the corresponding processing is performed, but the valueOf () function is called first instead of toString ().
 
When converting an object to a basic data type:
 
1. Call the valueOf () method of the object to convert the object to the basic data type returned;
 
2. If the object does not have the valueOf () method, or the method returns a non-basic data type, call the toString () method in the same way;
 
3. Similarly, if the toString () method does not exist or the return value is not of the basic data type, an error is prompted;
 
 
 
Js Code
// Custom function (class)
Function myObject (objectName)
{
This. objectName = objectName;
}
Var myObj = new myObject ("MyObj ");
 
// Normally, call the valueOf () method first.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj * 2); // The result is 642.
 
// The valueOf () method returns an incorrect type. Call the toString () method.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj * 2); // The result is 246.
 
// The toString () method does not exist. The valueOf () method returns an incorrect type.
MyObject. prototype. toString = undefined;
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj * 2); // error message
 
// Custom function (class)
Function myObject (objectName)
{
This. objectName = objectName;
}
Var myObj = new myObject ("MyObj ");
 
// Normally, call the valueOf () method first.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj * 2); // The result is 642.
 
// The valueOf () method returns an incorrect type. Call the toString () method.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj * 2); // The result is 246.
 
// The toString () method does not exist. The valueOf () method returns an incorrect type.
MyObject. prototype. toString = undefined;
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj * 2); // error message
 
Pay special attention to the design of the "+" Operator for object operations, because "+" can both perform arithmetic operations and String concatenation operations.
 
1. When the two operands of "+" have the object type, convert the object to the basic data type according to the above rules;
 
2. If one of the converted two operands is of the string type, convert the other operand to a string;
 
3. Otherwise, convert both operands to the numeric type (or NaN) for addition.
 
 
 
Js Code
// Custom function (class)
Function myObject (objectName)
{
This. objectName = objectName;
}
Var myObj = new myObject ("MyObj ");
 
// Normally, call the valueOf () method and return a number.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj + 1); // The result is 322 (Number Addition)
Alert (myObj + "1"); // The result is 3211 (String concatenation)
 
// Normally, call the valueOf () method first to return the string
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return "321 ";};
Alert (myObj + 1); // The result is 3211 (String concatenation)
Alert (myObj + "1"); // The result is 3211 (String concatenation)
 
// The valueOf () method returns an incorrect type. Call toString () to return a string.
MyObject. prototype. toString = function () {return "123 ";};
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj + 1); // The result is 1231 (String concatenation)
Alert (myObj + "1"); // The result is 1231 (String concatenation)
 
// The toString () method does not exist. The valueOf () method returns an incorrect type.
MyObject. prototype. toString = undefined;
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj + 1); // The system prompts an error (IE8 does not report an error and the system prompts undefined)
Alert (myObj + "1"); // The system prompts an error (IE8 does not report an error and the system prompts undefined)
 
// Custom function (class)
Function myObject (objectName)
{
This. objectName = objectName;
}
Var myObj = new myObject ("MyObj ");
Www.2cto.com
// Normally, call the valueOf () method and return a number.
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return 321 ;};
Alert (myObj + 1); // The result is 322 (Number Addition)
Alert (myObj + "1"); // The result is 3211 (String concatenation)
 
// Normally, call the valueOf () method first to return the string
MyObject. prototype. toString = function () {return 123 ;};
MyObject. prototype. valueOf = function () {return "321 ";};
Alert (myObj + 1); // The result is 3211 (String concatenation)
Alert (myObj + "1"); // The result is 3211 (String concatenation)
 
// The valueOf () method returns an incorrect type. Call toString () to return a string.
MyObject. prototype. toString = function () {return "123 ";};
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj + 1); // The result is 1231 (String concatenation)
Alert (myObj + "1"); // The result is 1231 (String concatenation)
 
// The toString () method does not exist. The valueOf () method returns an incorrect type.
MyObject. prototype. toString = undefined;
MyObject. prototype. valueOf = function () {return new Date ();};
Alert (myObj + 1); // The system prompts an error (IE8 does not report an error and the system prompts undefined)
Alert (myObj + "1"); // The system prompts an error (IE8 does not report an error and the system prompts undefined)
 
From haibin369