Basic content of javascipt--details that need attention_Basic knowledge

javascipt-Basics---Details that need attention:

1. Special values ​​: NaN, Infinity, isNaN(), isFinite()

NaN:

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var a=parseInt('a123 ');
window.alert(a); //Output NaN

Infinity:

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window.alert(6/0 );//Output Infinity (it’s best not to write it like this)

isNaN(): Determine whether it is a number. If it is not a number, it returns true. If it is a number, it returns false.

var a="dd";window.alert(isNaN(a)); //Return true isFinite(): Used to determine whether it is infinite. Returns false if number is NaN (not a number), or a positive or negative infinity number.

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window.alert(isFinite(6/1)); //Return truewindow.alert(isFinite(6/0)); //Return false 2. Logical operators:

In logical operations, 0, "", false, null, undefined, and NaN all represent false

(or || ) || will return the first value that is not false (objects are also acceptable), or the last value (if all are false)

This knowledge point is used a lot in JavaScript frameworks. a、

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The code is as follows:

var a=true;var b=false ;var c=b || a; window.alert(c); //Output true

b、

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The code is as follows:

var a=2;var b=0 var c= a || b; window.alert(c); //Return the first value, output 2

c、

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The code is as follows:

var a=false;var b=" ";var c =0;var d =new Object(); //Object var aa=a || b || c ||d ; //a,b,c are all false and this returns d

window.alert(aa); //returns d (object)

4. Multi-branch switch

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The code is as follows:

var flag=1; switch(flag){default:window.alert("nothing");

case 'a':
window.alert("a ");

case 'b':
window.alert("b"); //There is no break statement and no match is successful. At this time, the results are output

}

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The code is as follows:

var flag=1; switch(flag){default:window.alert("nothing");

case 'a':
window.alert("a ");

case 1:
window.alert("b"); //There is no break statement. When the match is successful, no break statement will be found. At this time, b

}

5. Function call



func.js

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The code is as follows:

function abc(val){ window.alert("abc()" val);} //Function with return value

function test(num1,num2){

var res=0;
res =num1 num2;

return res;}

//Function without return value

function noVal(num1,num2){

var res=0;

res=num1 num2;

}

Function call:

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The code is as follows:

js supports functions with variable number of parameters

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