Promise summary

promise It is an important knowledge point in the front end , Here's a brief summary .

First of all promise Implementation form , The grammatical structure is

const Variable =new Promise((resolve,reject)=>{

        resolve()    // A successful call resolve

        reject()     // Failed call reject

})

Variable .then((value)=>{

        The result you want when you succeed

},(reason)=>{

        The result you want when you fail

})

Give me an example ：

Now write a story about promise Example , It is required to customize a number , When the number is greater than 30 When , It outputs “ I'm sorry ”, When the number is less than 30 I'll output congratulations when you ！

            const pro=new Promise((resovle,reject)=>{
                let n=98
                if(n<=30){
                    resovle(n)
                }else{
                    reject(n)
                }
            })

            pro.then((value)=>{
                alert(" congratulations !!!"+value)
            },(reason)=>{
                alert(" I'm sorry ！！"+reason)
            })

Because when defining numbers 98, Greater than 30, So the output is sorry