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Typecasting in Javascript

By: Emiley J. in Javascript Tutorials on 2008年08月15日 [フレーム]

It's also possible to convert values using a process called type casting. Type casting allows you to access a specific value as if it were of a different type. Three type casts are available in JavaScript:

Boolean(value) – casts the given value as a Boolean
Number(value) – casts the given value as a number (either integer or floating-point)
String(value) – casts the given value a string

Casting a value using one of these three functions creates a new value that is a direct conversion of the original. This can lead to some unexpected results. The Boolean() type cast returns true when the value is a string with at least one character, a number other than 0, or an object (discussed in the next section); it returns false when the value is an empty string, the number 0, undefined , or null . The following code snippet can be used to test type casting as a Boolean:

var b1 = Boolean(""); //false – empty string
var b2 = Boolean("hi"); //true – non-empty string
var b3 = Boolean(100); //true – non-zero number
var b4 = Boolean(null); //false - null
var b5 = Boolean(0); //false - zero
var b6 = Boolean(new Object()); //true – object

The Number() type cast works in a manner similar to parseInt() and parseFloat() , except that it converts the entire value, not just part of it. Remember that parseInt() and parseFloat() only convert up to the first invalid character (in strings), so "4.5.6" becomes "4.5" . Using the Number() type cast, "4.5.6" becomes NaN because the entire string value cannot be converted into a number. If a string value can be converted entirely, Number() decides whether to use parseInt() or parseFloat() . The following table illustrates what happens when Number() is used on various values:

Usage Result

Number(false) 0
Number(true) 1
Number(undefined) NaN
Number(null) 0
Number("5.5") 5.5
Number("56") 56
Number("5.6.7") NaN
Number(new Object()) NaN
Number(100) 100

The last type cast, String() , is the simplest because it can accurately convert any value to a string value. To execute the type cast, it simply calls the toString() method of the value that was passed in, which converts 1 to "1", true to "true", false to "false", and so on. The only difference between type casting as a string and using toString() is that the type cast can produce a string for a null or undefined value without error:

var s1 = String(null); //"null"
var oNull = null;
var s2 = oNull.toString(); //won’t work, causes an error

Type casting is very helpful when dealing with the loosely typed nature of JavaScript, although you should ensure that only proper values are used.




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