The six types are String String Number Number Boolean value Null Null value undefined Object Object
The first five are basic data types, and Object is a reference data type
First, take a look at typeof
The typeof operator returns a string representing the data typeof the expression.
Typeof can return the following types: number, string, Boolean, undefined, NULL, object, function, and the new symbol (see MDN)
Developer.mozilla.org/zh-CN/docs/…
Common return values
// The current browser is Chrome; var num = null ; console.log(typeof num); //object var num = /s/ ; console.log(typeof num); //object var num = "a" ; // "123" console.log(typeof num); //string var num = a; console.log(typeof (num)); Var num = 123; console.log(typeof num); //number var num = NaN; console.log(typeof num); //number var num = undefined ; console.log(typeof num); //undefined var num; console.log(typeof num); / / undefined var num = [1, 3]. console.log(typeof num); //object var num = true; console.log(typeof num); //boolean var num = new Array(); console.log(typeof num); //object var num =function(){}; //var num =function; Console. log(typeof num); //function var num =new Object(); console.log(typeof num); //object function Kkb(name){ this.name =name; } var num = new Kkb(); console.log(typeof num); //object function Kkb(name){ this.name =name; } var num = new Person(); console.log(typeof num.name); //undefined function Kkb(name){ this.name =name; } var num = new Kkb("hello"); //var num = new Person(123); -->number console.log(typeof num.name); //stringCopy the code2. Cast
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Casting a data type to another data type
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Conversion basically means converting other data types. Convert to String, Number, Boolean
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2.1 Convert to String
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- toString()
A. Call toString() on the converted data type;
B. This method will not affect the original variable, it will return the conversion result;
C, note that null and undefined do not have toString() methods
- String()
A. Call String() and pass the loaded data to the function as an argument;
B. This method will not affect the original variable, it will return the conversion result;
C. When using String(), null and undefined can be converted to String;
D, toString() for Number, Boolean; Null and undefined will not be called toString(), but directly converted to null and undefined.
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2.2 Convert to Nuber
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- Number()
A. If the string is a pure number, it is directly converted to a number.
B. If the string contains non-numeric content or is undefined, it is converted to NaN
C. If the string is an empty string or space, it is converted to 0
D, if Boolean; True converts to 1, false converts to 0;
E. If the value is Null, the value is 0
- parseInt() / parseFloat()
A, parseInt() converts a string to an integer
B, parseInt() : read from left to right, take out the valid integer content, stop reading on the first non-numeric content, if the first character is non-numeric conversion NaN; C, parseFloat() converts a string to a float, just like parseInt(), except that it takes a valid decimal;
Var a = '111.1111.11' var b = kkb111.11 parseFloat(a) // 111.111 parseFloat(b) // NaNCopy the codeParseInt () or parseFloat(), for non-strings, it will convert it to String first and then operate on it;
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2.3 Convert to Boolean
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- Boolean()
A, numeric Boolean values, all except 0 and NaN are true;
B, string to Boolean, except empty string true;
C, null, and undefined convert to Boolean values, all false;
D) Objects are also converted to true;
var KKB=100;
var KKBS='100';
console.log(typeof KKB); //number
console.log(typeof KKB); //string
console.log(typeof String(KKB)); //string
console.log(typeof Number(KKBS)); //number
Copy the codeImplicit conversion
1. Convert to String
Algorithm operator (+), any value and string +, will be converted to the string, and then the string is combined; For multiple values +, work from left to right, paying attention to numeric types;
You can use this feature to convert any data type to String by simply (any String + “”); This is an implicit conversion, done automatically by the browser, which actually calls the String() function;
2. Convert to Number
Algorithm operator (- * / %), any value of – * /, will be converted to Number, and then the operation; You can use this feature to convert any data type to Number by simply converting (any string -0) (any string * 1) (any string / 1) to Number. This is an implicit conversion that the browser does automatically, essentially calling Number();
The unary operator (+(plus sign)), which converts non-number values to Number and then evaluates; An implicit type conversion works the same way as the number () function.
3. Convert to Boolean
Logical operators (! Than),! Invert a Boolean value; If a non-Boolean value is negated, it is converted to a Boolean value and negated; It can be used to invert other data types twice and convert them to Boolean values. Same principle as Boolean();
4. Add a string to a number, and the number becomes a string.
5. Subtract a number from a string, and the string becomes a number. NaN is converted if the string is not a pure number. Same thing with strings minus numbers. Subtracting two strings is also converted to numbers first.
6. Multiplication, division, greater than, less than and subtraction are the same.
``` console.log(1+'2') //12 string console.log(1-'2')//-1 number console.log(10-'kkb') //NaN console.log(10-'10a') //NaN console.log(10*'20') //200 number console.log('10'*'20') //200 number console.log(2/'1') //2 number console.log('2'/'1') //2 number console.log('2'/'kkb') //NaN ```Copy the code7, == implicit conversion ()
A. defined equals to null b. When comparing strings with numbers, the string is changed to number C. D. When a string is compared with a Boolean, Log ('0'==0) // True string to number console.log(0==false) //true Boolean to number Console. log('0'==false) // both convert to numbers console.log(null==false) //false console.log(undefined==false)//false ' 'Copy the codeOk, that’s all, sharing is a virtue, heh heh!