var vs dynamic in Dart

#dart #var #dynamic

In Dart and JavaScript we have var keyword for the variable declaration but both are not same.

There is the dynamic keyword for variable declaration in Dart which is equivalent to JavaScript var keyword.

Dart var keyword is very much similar to javascript var keyword but not the exact same. Most of Dart developer uses Dart var as JS var which is not correct. Let's understand this by some examples.

I will give 4 different examples.

Example 1

In JavaScript and Dart, we can store any kind of data in a variable which has been defined using var keyword.

Dart

// This is valid Example this will work
void main() {
     var x = 'Maths pi';
     var y = 3.14;

     print(x); // Maths pi
     print(y); // 3.14
}

JavaScript

// This is valid Example this will work
 var x = 'Maths pi';
 var y = 3.14;

 console.log(x); // Maths pi
 console.log(y); // 3.14

Example 2

In JavaScript and Dart, we can store different kind of data in the same variable.

Dart

// This is valid Example this will work
void main() {
    var x ;
    x = 'Math pi';
    print(x); // Maths pi

    x = 3.14;
    print(x); // 3.14
}

JavaScript

// This is valid Example this will work
 var x ;
 x = 'Math pi';
 console.log(x); // Maths pi

 x = 3.14;
 console.log(x); // 3.14

Example 3

Now I am going to mix Example 1 and Example 2
Now I am going to initialize the variable and trying to store different data type in the same variable. This is going to give me some error because this is not valid in Dart but its valid in JavaScript.

This is the place where `var` behaves differently in Dart

Dart

// This is invalid Example this will work
void main() {

     var x = 'Math pi';
     print(x);  // Maths pi


     x = 3.14;  // Compilation failed error because
                // you are trying to assign double to String variable.
     print(x);
}

JavaScript

// This is valid Example this will work
 var x ;
 x = 'Math pi';
 console.log(x); // Maths pi
 x = 3.14;
 console.log(x); // 3.14

Example 4

If we had used dynamic instead of var in Dart code we haven't got error because dart dynamic is same as JavaScript var.

Dart

// This is invalid Example this will not work
void main() {
     var x = 'Math pi';
     print(x); // Maths pi
     x = 3.14; 
     print(x); // expected 3.14
}

JavaScript

// This is valid Example this will work
 var x ;
 x = 'Math pi';
 console.log(x); // Maths pi
 x = 3.14;
 console.log(x); // 3.14

This is my first post here. If you like it let me so that I can be motivated to post here

Top comments (6)

Sung M. Kim • Feb 22 '19

Thanks for the post, Nitish 👍

I've never used Dart and the post was very easy to follow.
How I understood about var in Dart is that, Dart keeps track of initial variable type while dynamic.

And shouldn't the example 4 use dynamic keyword?

// This is invalid Example this will work
void main() {
     dynamic x = 'Math pi';
     print(x); // Maths pi
     x = 3.14; 
     print(x); // 3.14
}