Js Core Notes Day 7 (I)

Error Error object

1. Browsers come with four types of error objects

SyntaxError: SyntaxError — usually a symbol written wrong somewhere
```
 while() {// No parentheses
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```

ReferenceError — used without being created

 console.log(a) // A has not been created yet
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TypeError: TypeError – used for a method or property that is not your own

 const str = 'asdd'
 str.pop() //pop is an array method
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RangeError: RangeError – only encountered in some individual places

 const num = 123.456
 num.toFixed(101) // The number of digits reserved for decimals is between 0 and 100
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If an error occurs, an error will be reported, and the subsequent code will not run, so you need to handle the error. Sometimes we don’t want it to run after an error, but to give a prompt, we can use try…. catch….. grammar

 try{error code; }catch(err){execute only if an error is reported; }console.log("Subsequent code")
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 const num = 123.456
 
 try{
     num.toFixed(101);
 }catch(err){    
     console.log('You're out of range.');
 }
 console.log("Subsequent code")
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try… catch… Performance is so poor that almost all of the code in it is minimized — it can be branched instead

 const num = parseFloat(prompt("Please enter a number."));
 const d = parseInt(prompt("Please enter the number of decimals you want to save."));
 
 if(d >= 0 && d <= 100) {console.log(num.toFixed(d));
 }else{
     console.log("The reserved decimal number must be between 0 and 100.");
 }
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2. Throw custom errors

 throw new Error("Custom error text"); // If an error is reported, subsequent code will be blocked
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The Function object

1. Creation method

A declarative way

functionThe function name (Parameter,...){function body;returnResults; }Copy the code

Direct quantity mode

constThe function name =function(Parameter,...){function body;returnResults; }Copy the code

Constructor mode

When the function body is not fixed, it can be used to concatenate it with strings

constThe function name =new Function(Parameter "1"."Parameter 2". ."Function body return result");
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2. Scope

Global scope – can be used anywhere

Function scope – members are only available internally when a function is called

Use local first, then find global, no more error

const a = 10

function(){
    const b = 20
    console.log(a) / / 10
    console.log(b) / / 20
    console.log(c) / / an error
}
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3. Hoist (hoist)

Before the program executes, the browser precompiles the variables declared by var and the set of functions declared by function to the top of the current scope, leaving the assignments where they are, the function being heavier than the variable, and the function under the variable. (As explained in the previous notes, I won’t post the code here.)

4. pass by value — an assignment between two variables

If the type is original, the two copies are made to each other

If it is a reference type, the two affect each other and use the same address value

/ / the original
let a = 10
const b = a
console.log(a) / / 10
console.log(b) / / 10
a = 30
console.log(a) / / 30
console.log(b) / / 10


// Reference type
const obj = {a:10}
const newObj = obj
console.log(obj.a) / / 10
console.log(newObj.a) / / 10


obj.a = 20
console.log(obj.a) / / 20
console.log(newObj.a) // 20 -- Reference the same address, one change will change the other
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5, overloading

Overloading refers to the same function name, depending on the argument passed in, you can choose to execute the corresponding function call execution, but JS does not support overloading, JS does not allow multiple functions with the same name to exist, the last function will override all previous functions with the same name

Arguments Inside a function there is a arguments object that can only be used internally during a function call. The object is automatically created with an array of classes that takes all arguments passed in.

The arguments function can be used to determine what action should be performed based on its length, type, etc

 // Sum, pass a few parameters we can find the sum of several parameters
 function add(){
     let sum = 0
     for(let i = 0; i < arguments.length; i++) {
         sum += arguments[i]
     }
     return sum
 }
 
 add(1.2.3.4)  / / 10
 add(1.2.3.4.5)  / / 15
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6. Anonymous functions

Self-tuning of anonymous functions – can replace global scope

 (function(){
     const a = 10
     console.log(a) / / 10}) ()console.log(a) / / an error
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Anonymous function callback: Pass an anonymous function as an argument to another function call
```
 arr.sort((a, b) = > a - b)
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```
Which functions are callbacks?

A: Anonymous functions, either auto-callback or callback
```
 xxx.onclick = function(){} // This is a callback
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```