承诺中的角度承诺？

Question

I have a function like this:

resulta = 0;
resultb = 0;
resultc = 0;
resultd = 0;
    
    var wait = new promise((resolve, reject) => {
        data.forEach((single,index,array) =>{
            if (single.a == '1'){this.resulta = '1'}
            else if (single.a=='2') {this.resultb = '2'}
            else if (single.a=='3'){this.resultc = '3'}
            else if (single.a=='4'){
                this.function(single.a).then(async (bs) => {
                    if (somecondition) {
                        await bs.forEach(b => {
                            this.ccount = 0;
                            if(condition === true){
                                this.c = 'yes';
                                this.ccount = this.ccount +1;                      
                            }
                        });
                    }
                    if(this.ccount > 0){
                        this.resultd = '4';
                    }
                    else {
                       this.resultd = 'N/A';
                    }
                }
            }
            if (index === array.length -1)resolve();
        });
    });
    wait.then(async()=>{
        console.log('count', this.count); // always is 0 but it should be something > 0
        console.log('resulta', this.resulta); // 1 as it should be
        console.log('resultb', this.resultb); // 2 as it should be
        console.log('resultc', this.resultc); // 3 as it should be
        console.log('resultd', this.resultd); // 0 and it should be or 4 or N/A
    });

so the problem here is that the first foreach ends before the 2nd foreach so its resolve the promise, how can i wait for the second foreach (inside the latest else if) to finish before it ends the first foreach?

Answer 1

You're thinking promises should work well in forEach block. No it does not happen in that way. You have to make sure that your inner promise should be return returned properly. Then you can create a promises array, and by using Promise.all you can check whether all promises are finished or not.

Code

const promises = data.map((single,index,array) =>{
    if (single.a == '1'){ return this.resulta = '1' }
    else if (single.a=='2') {return this.resultb = '2' }
    else if (single.a=='3'){ return this.resultc = '3' }
    else if (single.a=='4') {
        return this.function(single.a).then((bs) => {
            if (somecondition) {
                bs.forEach(b => {
                    this.ccount = 0;
                    if(condition === true){
                        this.c = 'yes';
                        this.ccount = this.ccount +1;                      
                    }
                });
            }
            if(this.ccount > 0){
                this.resultd = '4';
            }
            else {
               this.resultd = 'N/A';
            }
        }
    }
});

Promise.all(promises).then(async()=>{
    console.log('count', this.count);
    console.log('resulta', this.resulta);
    console.log('resultb', this.resultb);
    console.log('resultc', this.resultc);
    console.log('resultd', this.resultd);
});