let arr = [1,2,3];
let empArr =[];
for(var i =0; i< arr.length;i++){
console.log('First',typeof empArr);
empArr+=arr[i];
console.log('Second',typeof empArr)
}
The above code gives this output
First object
Second string
First string
Second string
First string
Second string
Could anyone explain how in first iteration type was Array Object then after that it became string.How Javascript Engine works here?
If we run typeof empArr, we will see empArr an object. No matter if we declare it as an array, internally, it is an object. Further, typeof arr[i] shows arr[i] is a number. Therefore, empArr+=arr[i] means we are trying to add an object and a number. Since we are trying to add two different types, it can happen with the help of coercion, implicitly. Coercion means, converting a value of one type to another. JavaScript performs implicit coercion as per the following rules:
operand + operand = result
Note that the primitive value of an array or object is a string.
In our case, empArr is of type object and by rule 1, it is coerced as a string. Now by rule 2, the arr[i] which is a number, is coerced to a string as well and get assigned to empArr.
For more details:
According to javascript,
typeof [] is "object" ,i.e., every array is actually an object.
if you append anything to a string it will become a string
"1"+1 will equal "11"
It's because of auto type conversion.
The += is a not an array operator, and as the second operand is string - the 1st is converted to string.
Use empArr.push(arr[i])
empArr.push(arr[i])to have it stay an array+=to push to arrays, that is a string append operation.+operator is not defined for arrays, Javascript will convert arrays into strings and concatenate those