JavaScript display and implicit type conversion

1. Convert to String

Most JavaScript hosting environments (such as node. js and chrome) provide global functions toString , while Object.prototype methods are defined toString so that all objects have the ability to convert to strings.

For example Number , a conversion to String :

var n = 1; n.tostring ();    // ' 1 '

toStringAccepts a parameter specifying the binary, which defaults to 10. You can use this parameter to generate a random string that includes letters and numbers:

Math.random (). toString (+). substr (2);

randomGenerates a random number from 0 to 1, and the character set of the 36 binary is [0-9a-z] (36), which is substr used to intercept the start "0." . It Object.prototype.toString can also be used to detect the types of JavaScript objects:

varToString =Object.prototype.tostring;tostring.call (NewDate);//[Object Date]Tostring.call (NewString);//[Object String]Tostring.call (Math);//[Object Math]//Since JavaScript 1.8.5Tostring.call (undefined);//[Object Undefined]Tostring.call (NULL);//[Object Null]//Custom TypesTostring.call (NewMyClass);//[Object Object]

2. Convert to Digital

The conversion of strings to numbers is also a common requirement, often used to get a user input or a file to Number， be used directly in JavaScript parseInt parseFloat . For example:

var iNum1 = parseint ("12345red");   // back to 12345 var iNum1 = parseint ("0xA");    // return ten var iNum1 = parseint ("56.9");   // back to var iNum1 = parseint ("red");    // return NaN var // back to 11.22

Note that NaN it is the only value in JavaScript that does not equal itself. (NaN == NaN) === false! If an illegal character is encountered, parseInt and parseFloat all content after it is ignored.

parseFloatA string that accepts only decimal digits, and parseInt optionally a second argument that specifies a string to represent the binary of the number:

var iNum1 = parseint ("Ten", 2);  // returns 2 var iNum2 = parseint ("Ten", 8);  // returns 8 var // return ten

3. Forcing type conversions

Boolean (0)                //  = false-0 Boolean (new object ())   //  = = True-Object 
    number (undefined)       // = =   NaNnumber (null)              //  = > 0String (null)              //  = "NULL"

4. Implicit type conversions

Implicit type conversions are the most hidden places, and it's easy to make mistakes at this point without noticing, and the mastery of this is also a reflection of the JavaScript Programmer's experience. JavaScript automatically converts the type of an object in an expression to complete an expression evaluation.

Arithmetic

The addition operator + is a binocular operator, so long as one is a String type, the value of the expression is one String .

For other arithmetic, only one of them is a Number type, and the value of the expression is one Number .

The case for illegal characters is usually returned NaN :

' 1 ' * ' a '     //  = = Nan, because the parseint (a) value is nan,1 * nan or nan

Judgment statement

The judging condition in the judgment statement needs to be a Boolean type, so the conditional expression is implicitly converted to Boolean . The same Boolean constructor as the transformation rule. Like what:

var obj = {}; if (obj) {    while (obj);}

Native code calls

The JavaScript hosting environment provides a large number of objects that are often implemented by JavaScript. The parameters that JavaScript passes to these functions are also implicitly converted. For example, a BOM provides a alert method that accepts a String type of parameter:

Alert ({a:1});    // = = [Object Object]

JavaScript display and implicit type conversion