Form of programming
Process-oriented: a mechanical idea, such as for loops…
Object-oriented: JS semi-oriented processes tend to be object-oriented/Java…
//1. Typeof operator: undefined function (number Boolean string object)
example
// var num = true; // console.log(typeof(num)); Var num = [1,2]; // console.log(typeof(num)); // var num = null; // console.log(typeof(num)); // var num = function(){}; // console.log(typeof(num));Copy the code// Typeof can also add Spaces when parentheses are used
example
// var num = undefined;
// console.log(typeof num);
Copy the code2.Type conversion is unique to JS
/ parseInt int /parseFloat /Boolean / /
// number converts to a number
Example a number: NaN
// var demo = undefined;
// var num = Number(demo);
// console.log(typeof(num)+":" + num);
Copy the codeThe second case number: NaN
// var demo = "a";
// var num = Number(demo);
// console.log(typeof(num)+":" + num);
Copy the codeCase number three: NaN
// var demo = "true";
// var num = Number(demo);
// console.log(typeof(num)+":" + num);
Copy the codeNumber four: 1
// var demo = true;
// var num = Number(demo);
// console.log(typeof(num)+":" + num);
Copy the codeCase number five: NaN
// var demo = "NaN";
// var num = Number(demo);
// console.log(typeof(num)+":" + num);
Copy the code// parseInt is converted to an integer, regardless of non-numbers such as true,undefined– >NaN attribute :number
Example a number: NaN
// var demo = true;
// var num = parseInt(demo);
// console.log(typeof(num)+":"+num);
Copy the codeTwo cases number: 2833
// var demo = "b11"; // var num = parseInt(demo,16); Converted to integer 16 is "a" converted to decimal based on hexadecimal; Radix2-36 // console.log(typeof(num)+":" +num);Copy the code100 cases of three
// var demo = "100px";
// var num = parseInt(demo);
// console.log(typeof(num)+":"+num);
Copy the code// parseFloat is converted to a normal decimal number with the property number
// var demo = "100.2.3 ABC "; // var num = parseFloat(demo); // console.log(typeof(num)+":"+num);Copy the code// String is converted to the string attribute string
// to string
Case a string: 12
// var demo = 12;
// var num = demo.toString();
// console.log(typeof(num)+":"+num);
Copy the codeUndefined and null cannot use toString
// var demo = null;
// var num = demo.toString();
// console.log(typeof(num)+":"+num);
Copy the codeExample 3 to base 8 ==string:12
// var demo = 10;
// var num = demo.toString(8);
// console.log(typeof(num)+":"+num);
Copy the codeExample 4 converts a binary number to hexadecimal
// var num = 1000; // var test = parseInt(num,2); // console.log(test.tostring (16)); // Convert decimal to hexadecimalCopy the codeImplicit type conversions implicitly call explicit methods
Implicit type conversion
/ / example 0
// var num = 1 -"1"; // console.log(typeof(num) +":" + num); // Attribute // valueCopy the codeExample 1 isNaN is a non-number, check whether it is true or false —–> corresponding Number
// console.log(isNaN("abc"));
Copy the code// // The implicit conversion process – put the Number inside the Number(” ABC “), which is converted to NaN <–> NaN is true
// console.log(isNaN(null)); false
console.log(isNaN(undefined)); true
Copy the code// null is 0 in Number, and undefined is NaN
// var num = null;
// var demo = Number(num);
// console.log(typeof(demo)+":"+demo);
Copy the codeThe following implicit call to the type property becomes number
++
// var a = "123"; // a++; // select * from '123' where '123' = '123'; =124 // var a = "abs" // a ++; // "abs" is not a number, resulting in NaN, which is implicitly converted to typeof(a):numberCopy the codePlus minus one dollar plus or minus
// var a = +"abc"; // console.log(typeof(a)+":"+a); Implicitly converted to number:NaNCopy the codePlus plus example
// var a = a + 1; // console.log( typeof(a)+ ":" +a); Results: the number: NaNCopy the codeThe difference **—— + + if one of the two sides is a string, both end up as string**
// var a = "a" + 1; // console.log( typeof(a)+ ":" +a); Results: the string: a1Copy the codeMultiplication sign example
// var a = "1" * 1; // console.log(typeof(a)+":"+a); Results: the number: 1Copy the codeExample of comparison strings and numbers are converted to numbers for comparison.
// var a = "3">2; // console.log(typeof(a)+":"+a); Results: the Boolean: true,Copy the codeBut strings and strings compare without type conversions to ASC codes
// var a = "3">"2";
// console.log(typeof(a)+":"+a);
Copy the codeIs equal to the = =And the range! An example of =
// var a = 1 == "1";
// console.log(typeof(a)+":"+a);
Copy the code// var a = 1 ! = true; // console.log(typeof(a)+":"+a);Copy the code* * = = = and! ==** No type conversions occur, left and right must be equal // Special case NaN === NaN– > false
An error occurs if a variable is not defined
// console.log(a); // console.log(typeof(a)); // Console. log(typeof(a));Copy the codeSpecial case returns string undefined… typeof(typeof(a))=”string”—typeof(a)=”undefined”
// console.log(typeof(typeof(a)));
// console.log(typeof(typeof(a)+":"+(typeof(a))));
Copy the codetest
// alert(typeof(a)); // alert(typeof(undefined)); // undefined pops up. In the console typeof(a) returns undefined as a stringCopy the code// alert(typeof(NaN)); // Return number as a stringCopy the code// alert(typeof(null)); // Return object as a stringCopy the code// var a = "123abc"; // alert(typeof(+a)); // Return number as a stringCopy the code// alert(typeof(!! a)); // Return string type BooleanCopy the code// alert(typeof(a+"")); / / display the stringCopy the code // alert(1=="1");
Copy the code// alert(NaN == NaN);
Copy the code // alert(NaN == undefined);
Copy the code// alert("11"+11); // both become the string 1111Copy the code// alert(1 === "1");
Copy the code // alert(parseInt("123abc"));
Copy the code// var num = 123123.345789; // alert(num.toFixed(3)); Keep three decimal placesCopy the code// typeof(typeof(a)); // typeof(a) returns "undefined", typeof(a) returns "string";Copy the code