Basically, all JS data types have the valueof and tostring methods, except null. They solve the Javascript value calculation and display problems.
Valueof () method of JavaScript
The valueof () method returns the original value of a Boolean object.
Use booleanobject. valueof (). The return value is the original Boolean value of booleanobject. If the object that calls this method is not Boolean, an exception typeerror is thrown.
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<script type="text/javascript"> |
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var boo = new Boolean(false); |
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document.write(boo.valueOf()); |
The above script outputs false.
Tostring () method of JavaScript
The tostring () method converts a logical value to a string and returns the result.
Use booleanobject. tostring () to return a string "true" or "false" based on the original Boolean value or the value of the booleanobject ". If the object that calls this method is not Boolean, an exception typeerror is thrown.
This method is automatically called when a Boolean object is used in a string environment.
The following script creates a Boolean object and converts it to a string:
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<script type="text/javascript"> |
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var boo = new Boolean(true); |
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document.write(boo.toString()); |
Script output: True.
Let's take a look at the following example:
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valueOf: function() { return this.i+30; }, |
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toString: function() { return this.valueOf()+10; } |
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alert(aaa > 20); // true |
This is because they secretly call the valueof or tostring method. But how can we tell which method is called? We can test it using another method. Because console. log is used, please experiment in FF with firebug installed!
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toString: function() { |
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console.log('toString'); |
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console.log('valueOf'); |
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alert(bbb);// 10 toString |
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alert(+bbb); // 10 valueOf |
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alert(''+bbb); // 10 valueOf |
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alert(String(bbb)); // 10 toString |
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alert(Number(bbb)); // 10 valueOf |
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alert(bbb == '10'); // true valueOf |
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alert(bbb === '10'); // false |
At first glance, it seems that the tostring method is called when it is converted to a string, and the valueof method is called when it is converted to a value, but two of them are not harmonious. One is alert (''+ BBB), and the string combination should be to call the tostring method ...... Another operator we can understand for the time being is that the = operator does not perform implicit conversion, so they are not called. We need more rigorous experiments to investigate the truth.
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toString: function() { |
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console.log('toString'); |
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alert(aa);// 10 toString |
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alert(+aa); // 10 toString |
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alert(''+aa); // 10 toString |
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alert(String(aa)); // 10 toString |
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alert(Number(aa)); // 10 toString |
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alert(aa == '10'); // true toString |
Let's look at valueof.
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console.log('valueOf'); |
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alert(bb);// [object Object] |
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alert(+bb); // 10 valueOf |
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alert(''+bb); // 10 valueOf |
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alert(String(bb)); // [object Object] |
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alert(Number(bb)); // 10 valueOf |
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alert(bb == '10'); // true valueOf |
Is it a little different ?! It is not as unified as the tostring above. For the [object], I guess it is inherited from the object. Let's take a look at it.
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Object.prototype.toString = null; |
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console.log('valueOf'); |
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alert(cc);// 10 valueOf |
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alert(+cc); // 10 valueOf |
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alert(''+cc); // 10 valueOf |
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alert(String(cc)); // 10 valueOf |
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alert(Number(cc)); // 10 valueOf |
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alert(cc == '10'); // true valueOf |
If tostring is overwritten, the existence of valueof is ignored during object conversion. However, if you only override the valueof method, the valueof method is preferred when you want to convert it to a string. When tostring cannot be called, the valueof can only be mounted. For that strange String concatenation problem, it may be out of the operator, open the ECMA262-5 and find that there is a getvalue operation. Well, the answer should be uncovered. Rewriting will increase the optimization of their calls, while in the case of operators, valueof has a higher priority than tostring.
