vue 監(jiān)聽數(shù)組變化，并將數(shù)值賦值給oldArray時，oldArray提示undefined

在watch方法里，使用深度監(jiān)聽，監(jiān)聽newArray的變化，變化能監(jiān)聽到，但是將變化的值賦值給oldArray時，提示undefined

        data (){
            return {
                downloadUrl: (function () {
                    return 'http://' + document.domain + ':38010'
                })(),
                oldImgs: this.$route.query.contractInfo,
                rowkey: this.$route.query.rowkey,
                copyLocalIds: [],
                newImgs:[],
                transLocalIds:[]
            }
        },
        created (){
            if (this.oldImgs && this.oldImgs.length > 0) {
                this.oldImgs = this.oldImgs.split(';');
                if (this.oldImgs[this.oldImgs.length-1] == '') {
                    this.oldImgs.pop();//刪除最后一個元素
                }
                for (var i = 0; i < this.oldImgs.length; i++) {
                    this.oldImgs[i] = this.downloadUrl+"/contractInfo"+this.oldImgs[i];
                }
            }
        },
        watch :{
            'newImgs':{
                deep: true,
                handler: (val,oldVal)=>{
                    console.log('老圖片:'+this.oldImgs);
                    if (this.oldImgs.length>0) {
                        this.oldImgs = this.oldImgs.concat(val);
                    } else {
                        this.oldImgs = val;
                    }
                    
                }
            }
        },