Type conversion of JavaScript objects

In JavaScript, the toString() method and alueOf() method are used to convert objects into basic data types or strings.



When converting an object into a string:

1. Call the toString() method of the object to convert the object into the returned string. If the method returns other basic data types, it will be automatically converted into a string;

2. If the object does not have a toString() method, or the method returns is not a basic data type, then call the valueOf() method in the same way;

3. Similarly, if the valueOf() method does not exist or the return value is not a basic data type , an error will be prompted (IE may not report an error);

Js code
//Custom function (class)
function myObject(objectName)
{
This.objectName = objectName;
}
var myObj = new myObject(" MyObj");

//Normally, first call the toString() method
myObject.prototype.toString = function(){return 123;};
myObject.prototype.valueOf = function(){return 321;};
alert(myObj); //The result is 123

//The toString() method returns an incorrect type, 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); // Prompt error (no error is reported in IE8 and prompt [Object])

//Custom function ( class)
function myObject(objectName)
{
This.objectName = objectName;
}
var myObj = new myObject("MyObj");

//Normally, first call the toString() method
myObject.prototype.toString = function(){return 123;};
myObject.prototype.valueOf = function(){return 321;};
alert(myObj); //The result is 123

//The toString() method returns an incorrect type, Call the valueOf() method
myObject.prototype.toString = function(){return new Date();};
myObject.prototype.valueOf = function(){return 321;};
alert(myObj); //The result is 321

//toString() method does not exist, valueOf() method returns incorrect type
myObject.prototype.toString = undefined;
myObject.prototype.valueOf = function(){return new Date();};
alert (myObj); //Prompt error (no error is reported in IE8 and prompts [Object])

Similarly, the corresponding processing will be done when converting the object into a basic data type, but the valueOf() function will be called first instead of toString( ).

When converting an object into a basic data type:

1. Call the valueOf() method of the object to convert the object into the returned basic data type;

2. If the object does not have a valueOf() method, or the method returns something other than Basic data type, then call the toString() method in the same way;

3. Similarly, if the toString() method does not exist or the return value is not a basic data type, an error will be prompted;



Js code
//Customized Function (class)
function myObject(objectName)
{
this.objectName = objectName;
}
var myObj = new myObject("MyObj");

//Normally, first call the valueOf() method
myObject.prototype .toString = function(){return 123;};
myObject.prototype.valueOf = function(){return 321;};
alert(myObj*2); //The result is 642

//The valueOf() method returns Incorrect type, call toString() method
myObject.prototype.toString = function(){return 123;};
myObject.prototype.valueOf = function(){return new Date();};
alert(myObj*2 ; );};
alert(myObj*2); "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);                                                                                                                                                                               . prototype.toString = undefined;
myObject.prototype.valueOf = function(){return new Date();};
alert(myObj*2); Pay special attention to this, because "+" can perform both arithmetic operations and string concatenation.

1. When the two operands of "+" have object types, first convert the objects into basic data types according to the above rules;

2. If one of the two converted operands is of string type , then convert the other operand into a string;

3. Otherwise, convert both operands into numeric types (or NaN) and perform the addition operation.



Js code
//Custom function (class)
function myObject(objectName)
{
this.objectName = objectName;
}
var myObj = new myObject("MyObj");

//Normal situation , first call the valueOf() method and return the number
myObject.prototype.toString = function(){return 123;};
myObject.prototype.valueOf = function(){return 321;};
alert(myObj + 1); //The result is 322 (added values)
alert(myObj + "1"); 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, calls the toString() method, and returns 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 splicing)

//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); No error is reported, and prompts undefined)

//Custom function (class)
function myObject(objectName)
{
This.objectName = objectName;
}
var myObj = new myObject("MyObj");
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//Under normal circumstances, first 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 (added values)
alert(myObj + "1"); , return the string
myObject.prototype.toString = function(){return 123;};
myObject.prototype.valueOf = function(){return "321";};
alert(myObj + 1); 3211 (string concatenation)
alert(myObj + "1");                                    //The result is 3211 (string concatenation)
 
//The valueOf() method returns an incorrect type. Call the toString() method 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"); () method does not exist, valueOf() method returns incorrect type
myObject.prototype.toString = undefined;
myObject.prototype.valueOf = function(){return new Date();};
alert(myObj + 1); //Prompt an error (IE8 does not report an error, and prompts undefined)
alert(myObj + "1");