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Share JavaScript simple error summary error debugging

PHPz

Release： 2017-04-04 14:08:00

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Share JavaScript simple error summary error debugging, which is a summary of the notes I made during the learning process

Error summary

When judging statements if (x=0) It is different from (x==0) and (x===0)
x=0 is a negative value, 0 is false, and if x=10, it is true.
x==0 is judged, but it will automatically convert the type. x='10' is the same as x=10.
x===10 must be of the same type to judge.
Addition and connection operators are different
x='10' y=5;
x+y is equal to 105 at this time;
String branch
```
 var x = "Hello
 World!";
```
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The above branch will report an error
```
 var x = "HelloWorld!";
```
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This is correct
You can also do this, add a backslash
```
 var x = "Hello\
 World!";
```
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Points Forgot to write the number or write it wrong
return After the line break, the result has no return value
The name is used as an index in the array, and the array can only use numbers. To do indexing
Define the array object. You cannot add a comma at the end
Undefined is not Null
program Block scope needs attention

JavaScript errors

When the JavaScript engine executes JavaScript code, various errors can occur.
may be a syntax error, usually a coding error or typo made by the programmer.
could be a spelling mistake or a missing feature in the language (possibly due to browser differences).
The error may be caused by incorrect output from the server or user.
Of course, it may also be due to many other unpredictable factors.

JavaScript throws errors

When an error occurs, when something goes wrong, the JavaScript engine usually stops and generates an error message.
The technical term to describe this situation is: JavaScript will throw an error. The

JavaScript try and catch

try statements allow us to define blocks of code that are tested for errors when executed. The
catch statement allows us to define the code block that is executed when an error occurs in the try code block.
JavaScript statements try and catch appear in pairs.

try {
```
 //在这里运行代码
```
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} catch(err) {
```
 //在这里处理错误
```
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}
chestnut

ar txt="" ;
function message()
{
```
   try {
       adddlert("Welcome guest!");
   } catch(err) {
       txt="本页有一个错误。\n\n";
       txt+="错误描述：" + err.message + "\n\n";
       txt+="点击确定继续。\n\n";
       alert(txt);
   }
```
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}
message();

Share JavaScript simple error summary error debugging

Result graph

Throw statement

The throw statement allows us to create custom errors.
The correct technical term is: create or throw an exception (exception).
If you use throw with try and catch, you can control the program flow and generate custom error messages.
Syntax

 throw *exception*

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Exceptions can be JavaScript strings, numbers, logical values, or objects.

    <!doctype html>
    <html>
    <head>
    <meta charset=&#39;UTF-8&#39; />
    <title>js异常</title>
    </head>
    <body>
        <input type="text" id="id"/><br/><button onclick="myfunc()">判断</button>
        <p id="mess"></p>
    <script type="text/javascript">
    function myfunc(){
     try {
         var x=document.getElementById('id').value;
        if (x=="") throw"值为空";
            if (isNaN(x)) throw"值不是数字";
                if (x<100) throw"值太小";
                    if (x>200) throw"值太大";
     } catch (e) {
         var y=document.getElementById('mess');
        y.innerHTML="错误:"+e+".";
     } finally {
         alert("这个一定会触发，不管有没有错，catch与finally可以任选一个写也可以全写，不能不写");
     }
    }
    </script>
    </body>
    </html>

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Share JavaScript simple error summary error debugging

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