if(this.formData[i].questionType == 3){
    userBlanksDetail:{
        answer:this.formData[i].fill
    },
}

to

userBlanksDetail:{
    answer:this.formData[i].questionType == 3?this.formData[i].fill:''
},

for(var i=0;i<this.formData.length;iPP){
        let obj ={};
        if(this.formData[i].questionType == 3){
         obj = {
            questionUid:this.formData[i].questionUid, //id
            questionType:this.formData[i].questionType, //1:23
            //
            userBlanksDetail:{
                answer:this.formData[i].fill
            },
            //
            userOptionDetail:{
                optionUid:this.formData[i].single?this.formData[i].single:this.formData[i].mul.join(',')
            }
        }
        }else{
            obj = {
            questionUid:this.formData[i].questionUid, //id
            questionType:this.formData[i].questionType, //1:23
            //
            userOptionDetail:{
                optionUid:this.formData[i].single?this.formData[i].single:this.formData[i].mul.join(',')
            }
        }
        }
        this.result.push(obj);
    }

what kind of strange phenomenon is this?

what is your strange way of writing? I'm afraid I can't understand

this is a creative way of writing. Did you invent it? But javascript engine doesn't recognize it?

what kind of strange phenomenon is this?

with all due respect, this method of writing, not to mention putting it in vue, will not succeed in any js running program of react ng jquery!

if no one writes this, there is a problem with grammar. So:

let obj = {
    1: 'value1',
    2: 'value2',
    : condition ? 'value1' : 'value2',
}

// 
if () {
    obj[''] = ''
}