How to modify values in object lightning web components

Question

@track defaultRecordWith4Field = {
        recId : 0,
        fieldList : [
            {
                name:"description__c",
                value :null
            },{

                name:"type__c",
                value :null
            },{
                name:"property__c",
                value :null

            },{
                name:"Direction_property__c",
                value :null

            }
        ]
    };

I'm trying to modifying recId in the object declared above to add one more row I'm not getting how to do that

Trying to add new value to "recId"

handleAdd(){
        console.log(object.key(this.defaultRecordWith4Field));
        this.defaultRecordWith4Field.recId=this.records.length++;// I'm trying here
        var newRecord = Object.assign({}, this.defaultRecordWith4Field);
        this.records.push(newRecord); 
        console.log("this.records : " + JSON.stringify(this.records));
    }

Answer 1

    this.defaultRecordWith4Field.recId=this.records.length++;// I'm trying here

The x++ operator returns a variable value, then increases it by 1 (you can also ++x, which increases by 1, then returns the newly increased variable value). In JavaScript, this actually increases the associated array's size by one (a new null entry is added at the end).

What you probably meant to do is to add 1 to the value:

    this.defaultRecordWith4Field.recId=this.records.length+1;

Which does not modify the variable being used.