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]; 
}

• xanatos: You know that there are 3x lab and 4x val?
• Mujtaba Haider: yes i know, its only here mistakenly.. thanks for informing me

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);

• quirimmo: If in map you just make assignments, use forEach instead
• its4zahoor: I guess, i used map then because I didn't know the i can also be get in forEach.
• its4zahoor: I edited to include both the ways, map() is concise.