JS is a weakly typed language1. When declaring a variable, you do not need to specify the data type stored by the variable2. Dynamically determine the type of data when assigning a valueThe same variable can hold different types of values successively3.js will automatically convert the type of data when needed two major types of conversions: implicit conversion and casting
Implicit conversionsNo programmer intervention required, JS auto-complete type conversionIn arithmetic calculations, all data is converted by default to a number, and then in the + operation, when the string is encountered, + becomes a string concatenation, the other is not a string of data, is automatically converted to a string special case: If the data cannot be converted to a numeric type, it will be converted to Nan, Represents a number that is not a number (not a valid number) Nan does not equal, is not greater than, not less than, any value, including Nan Nan participates in any arithmetic result that is always a Nan expression: a formula that consists of variables, data, and operators, by default, from Left to right, 22 operation, any expression has an operation result any expression can use a numeric + string as a value: string number + Boolean: Number (true=1,false=0) string + Boolean: String (boolean value =true/ False) Boolean value + Boolean: Number (true=1,false=0)
Example:var n1=2, n2=3, s1= "2", s2= "3", B1=true, B2=false, Str;console.log (N1+N2); 5
Console.log (N1+S2); "23"
Console.log (S2-S1); 1
Console.log (S1-N2); -1
Console.log (N1+N2+S2); "53"
Console.log (N1+S2+N2); "233" Console.log (S2-B1); 2
Console.log (S2+B1); "3true"
Console.log (S1*B2); 0
Console.log (S1+STR); "2undefined"
Console.log (S2-STR); NaN
Console.log (N1*STR); NaN
Forced Conversionsprogrammers actively invoke specialized functions to perform conversionsWhen to use: the type of automatic conversion, not the type you wantconvert any type to a string type
x.tostring (); X is not undefined and null can be usedString (x); equivalent to an implicit conversion converts a non-string type to a numeric typeNumber (x)To convert a non-string to a number, rather than an implicit conversion, to convert a string type to a numeric typeparseint (str); starts with the first character, reads each number sequentially, stops automatically skipping the empty character at the beginning of the first non-numeric character, and does not recognize when the decimal point is used: Converts a string to an integerparsefloat (str); recognize when the first decimal point is used: Convert a string to a decimal to convert any type to a Boolean typeBoolean (x); equivalent to an implicit conversion only 5 values are converted to false, the rest is true "" NaN undefined null 0 fast conversion: Any type to STRING:X + "" Any type of turn boolean:!! X prompt ();Hint information var salary = prompt ("Please enter your salary");//Enter 5000var mysalary = Salary+1000;//console.log ("Your salary after salary is (error):" + mysalary);// 50001000var Mysalary2 = number (Salary) +1000;console.log ("Your salary after salary is (correct):" + mysalary2);//6000 * * * * All data fetched (returned) from the page is a string, Must first be converted in the calculation
JavaScript Data type conversions
