Daily javascript Summary (Basic Knowledge)
- 2020-10-23 19:59:54
- OfStack
1. Character conversion
var s1 = "01";
var s2 = "1.1";
var s3 = "z";// The letter 'z' Cannot convert to number, so or return NaN
var b = false;
var f = 1.1;
var o = {
valueOf: function() {
return -1;
}
};
s1 = -s1; //value becomes numeric -1
s2 = -s2; //value becomes numeric -1.1
s3 = -s3; //value becomes NaN
b = -b; //value becomes numeric 0
f = -f; //change to -1.1
o = -o; // 1 Execute the object first valueOf() Method returns -1,--1 for 1
o = +o;//-1 o the valueOf() A value of -1 . +-1 or -1
2. Special character operation
var result1 = 5 - true; //4 because true is converted to 1
var result2 = NaN - 1; //NaN NaN not 1 A number, and any number to do any arithmetic NaN
var result3 = 5 - 3; //2
var result4 = 5 - ""; //5 because "" is converted to 0
var result5 = 5 - "2"; //3 because "2" is converted to 2
var result6 = 5 - null; //5 because null is converted to 0
3, variable string operation
var value1 = 10;
var value2 = true;
var value3 = null;
var value4;//value4 No assignment is underfined Converting to a string is 'underfined'
alert(String(value1)); //"10"
alert(String(value2)); //"true"
alert(String(value3)); //"null"
alert(String(value4)); //"undefined"
4. Decimal conversion of Numbers
var num = 10;
alert(num.toString()); //"10" The default 10 Into the system
alert(num.toString(2)); //"1010"2 Into the system
alert(num.toString(8)); //"12"8 Into the system
alert(num.toString(10)); //"10"10 Into the system
alert(num.toString(16)); //"a"106 Into the system
5, string comparison operation
var result1 = 5 > 3; //true
var result2 = 5 < 3; //false
var result3 = "Brick" < "alphabet"; //true String comparisons are performed alphabetically Lowercase letters follow uppercase letters
var result4 = "Brick".toLowerCase() < "alphabet".toLowerCase(); //false Alphabetical comparison
var result5 = "23" < "3"; //true character 2 Less than the characters 3
var result6 = "23" < 3; //false At this time '23' Converted to 23 the
var result7 = "a" < 3; //false because "a" becomes NaN character 'a' It cannot be converted to a number
var result8 = NaN < 3; //false NaN Compared to any number, it can't be converted into a number, so 1 Straight is false
var result9 = NaN >= 3; //false
6. Character conversion
var num1 = parseInt("AF", 16); //175 In accordance with the 16 Hexadecimal output 10 Hexadecimal data 10*16+15*1
var num2 = parseInt("AF"); //NaN No base specified, default to 10 Base conversion, because AF Not in 10 Base range, return NaN
alert(num1);
alert(num2);
7. Use of parseInt
var num1 = parseInt("1234blue"); //1234
var num2 = parseInt(""); //NaN character '' It cannot be converted to a number
var num3 = parseInt("0xA"); //10 - hexadecimal 16 Into the system A
var num4 = parseInt(22.5); //22
var num5 = parseInt("70"); //70 - decimal
var num6 = parseInt("0xf"); //15 16 Into the system for 15
8. Use of Number objects
var num1 = Number("Hello world!"); //NaN
var num2 = Number(""); //0 An empty string can be converted to 0 this parseInt() Don't 1 sample
var num3 = Number("000011"); //11
var num4 = Number(true); //1
9. NaN
alert(NaN == NaN); //false
alert(isNaN(NaN)); //true
alert(isNaN(10)); //false ?10 is a number
alert(isNaN("10")); //false ?can be converted to number 10
alert(isNaN("blue")); //true ?cannot be converted to a number
alert(isNaN(true)); //false ?can be converted to number 1
10. Maximum number of system
var result = Number.MAX_VALUE + 1;
alert(isFinite(result)); // false
11. Infinity is infinite
var result1 = 5 - true; //4 because true is converted to 1
var result2 = NaN - 1; //NaN NaN not 1 A number, and any number to do any arithmetic NaN
var result3 = 5 - 3; //2
var result4 = 5 - ""; //5 because "" is converted to 0
var result5 = 5 - "2"; //3 because "2" is converted to 2
var result6 = 5 - null; //5 because null is converted to 0
12. for in cycle
var result1 = 5 - true; //4 because true is converted to 1
var result2 = NaN - 1; //NaN NaN not 1 A number, and any number to do any arithmetic NaN
var result3 = 5 - 3; //2
var result4 = 5 - ""; //5 because "" is converted to 0
var result5 = 5 - "2"; //3 because "2" is converted to 2
var result6 = 5 - null; //5 because null is converted to 0
13. Special character comparison
var result1 = 5 - true; //4 because true is converted to 1
var result2 = NaN - 1; //NaN NaN not 1 A number, and any number to do any arithmetic NaN
var result3 = 5 - 3; //2
var result4 = 5 - ""; //5 because "" is converted to 0
var result5 = 5 - "2"; //3 because "2" is converted to 2
var result6 = 5 - null; //5 because null is converted to 0
14. Boolean object
var message = "Hello world!";
var messageAsBoolean = Boolean(message);
alert(messageAsBoolean); //true
15. for cycle and label 1 start to be used
Both break and continue statements can be used in conjunction with tagged statements to return specific locations in code.
Usually, this is done when there is a loop inside the loop, for example:
var result1 = 5 - true; //4 because true is converted to 1
var result2 = NaN - 1; //NaN NaN not 1 A number, and any number to do any arithmetic NaN
var result3 = 5 - 3; //2
var result4 = 5 - ""; //5 because "" is converted to 0
var result5 = 5 - "2"; //3 because "2" is converted to 2
var result6 = 5 - null; //5 because null is converted to 0
In the above example, the tag outermost represents the first for statement. Normally, each for statement executes a block of code 10 times, which means that num++ would normally be executed 100 times, and num should be equal to 100 when execution is complete. The break statement here has one argument, the label of the statement to jump to after the loop is stopped. In this way, the break statement can jump out not only the internal for statement (that is, the statement using the variable j), but also the external for statement (that is, the statement using the variable i). Therefore, the final value of num is 55, because the loop terminates when both i and j are equal to 5.
You can use the continue statement in the same way:
var result1 = 5 - true; //4 because true is converted to 1
var result2 = NaN - 1; //NaN NaN not 1 A number, and any number to do any arithmetic NaN
var result3 = 5 - 3; //2
var result4 = 5 - ""; //5 because "" is converted to 0
var result5 = 5 - "2"; //3 because "2" is converted to 2
var result6 = 5 - null; //5 because null is converted to 0
In the above example, the continue statement forces the loop to continue, not only the inner loop, but also the outer loop. This occurs when j is equal to 5, which means that the inner loop will be reduced by five iterations, resulting in a value of 95 for iNum.
Tip: As you can see, the tagged statements used in conjunction with break and continue are very powerful, but overusing them can cause problems in debugging code. Be sure to use declarative tags without nesting too many layers of loops.
So that's the summary of today's javascript, and we will continue to update it every day. I hope you will continue to pay attention to it.