Parsing JavaScript Error objects

This article mainly introduces the analysis of JavaScript Error objects, which has certain reference value. Now I share it with everyone. Friends in need can refer to it

Detailed explanation of JavaScript Error objects

1. Concept

error refers to the abnormal running state in the program, which is called "exception" or "error" in other programming languages. The interpreter creates and throws an Error object for each error situation, which contains error description information.

ECMAScript defines six types of errors. In addition, you can also use the Error constructor to create a custom Error object and throw the object using the throw statement.

Six kinds of errors:

• ReferenceError: When the object cannot be found

• TypeError: The wrong type or object is used When using the method

• RangeError: When using the method of the built-in object, the parameter is out of range

• SyntaxError: The syntax is wrong

• EvalError: Wrong use of Eval

• URIError: URI error

2. Error handling

Even if an error occurs in the program, it is a mechanism that ensures no abnormal interruption.

try{
    可能发生错误的代码
}catch(err){
    只有发生错误时才执行的代码
}finally{
    无论是否出错，肯定都要执行的代码
}

Usage points:

• Even if there is no error in the code wrapped with try, the efficiency is lower than the code without try wrapping;

• In try, contain as little code as possible that may go wrong;

• Errors of the wrong type cannot be predicted in advance and must be captured with try catch;

• finally can be omitted;

3. Throw a custom error

When: If the definer of the function needs to inform the caller during use Error;

How to: throw new Error("prompt text");

// 程序猿甲:定义函数的人
function round(num, d){
    if(!isNaN(num) && !isNaN(d)){
        num *= Math.pow(10, d);
        return num;
    }else{
        // 抛出自定义错误
        throw new Error("参数必须是数字");  
    }
}

// 程序猿乙:调用函数的人
var d = parseInt(prompt('请输入数字'));
try{
    alert(round(123.456, d));
}catch(err){
    alert(err.message);  // 接住抛出的自定义错误
}

4. Example exercises

var n = 1;
function fun(){
    try{
        n++;
        return n;
    }catch(err){    
        // 没错误，catch不执行
        n++;
        return n;    
    }finally{
        n++;
        return n;
    }
}

console.log(fun());// 3  return为finally的
console.log(n);// 3

var n = 1;
function fun(){
    try{
        n++;
        return n;
    }catch(err){
        // 没错误，catch不执行
        n++;
        return n;
    }finally{
        n++;
    }
}
console.log(fun()); // 2  return为try的
console.log(n); // 3

var n = 1;
function fun(){
    try{
        n++;
        n += m;  //出错
        return n;   
    }catch(err){
        n++;
        return n;
    }finally{
        n++;
    }
}
console.log(fun());    // 3 return为catch的
console.log(n);        // 4

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