how to add values to an array of objects dynamically in javascript?

Question

This is an array of objects,

var data = [
      {"label" : "1", "value" : 12},
      {"label" : "1", "value" : 12},
      {"label" : "1", "value" : 12},
      {"label" : "1", "value" : 12}
    ];

How can I add values to these dynamically? iItried the below code but no success:

var lab =["1","2","3", "4"];
var val = [42,55,51,22];
var data = new Array();
for(var i=0; i<4; i++){
   data[i].label = lab[i];
   data[i].value = val[i];    
}

Answer 1

You have to instantiate the object first. The simplest way is:

var lab =["1","2","3"];
var val = [42,55,51,22];
var data = [];
for(var i=0; i<4; i++)  {
    data.push({label: lab[i], value: val[i]});
}

Or an other, less concise way, but closer to your original code:

for(var i=0; i<4; i++)  {
   data[i] = {};              // creates a new object
   data[i].label = lab[i];
   data[i].value = val[i];    
}

array() will not create a new array (unless you defined that function). Either Array() or new Array() or just [] .

I recommend to read the MDN JavaScript Guide .

Answer 2

In Year 2019, we can use Javascript's ES6 Spread syntax to do it concisely and efficiently

data = [...data, {"label": 2, "value": 13}]

Examples

 var data = [ {"label" : "1", "value" : 12}, {"label" : "1", "value" : 12}, {"label" : "1", "value" : 12}, ]; data = [...data, {"label" : "2", "value" : 14}] console.log(data) 

For your case (i know it was in 2011), we can do it with map() & forEach() like below

 var lab = ["1","2","3","4"]; var val = [42,55,51,22]; //Using forEach() var data = []; val.forEach((v,i) => data= [...data, {"label": lab[i], "value":v}] ) //Using map() var dataMap = val.map((v,i) => ({"label": lab[i], "value":v}) ) console.log('data: ', data); console.log('dataMap : ', dataMap); 

Answer 3

const [alarmData, setAlarmData] = useState([])
 
const handleConfirm = (date) => {
        console.log("A date has been picked: ", date);  
       // received new Date alarm time every time
      
       const formatedTime =  moment(date).format('hh:mm A')     

        const myData = [...alarmData]
        myData.push(""+formatedTime);
        setAlarmData(myData);    
    };