How to Use a Variable for a Key in a JavaScript Object Literal?

When working with JavaScript and object literals, you may encounter situations where you need to use a variable as a key. However, it may not work as expected. In this article, we will explore why using a variable for a key in a JavaScript object literal may not work and how to solve this problem.

Understanding the Problem

Let's start by understanding the problem. Consider the following code:

            something.stop().animate(
                { 'top' : 10 }, 10
            );
        

In the above code, the 'top' property is being used as a key in the object literal passed to the animate function. This works as expected.

Now, let's try to use a variable as a key in the object literal:

            var thetop = 'top';
            something.stop().animate(
                { thetop : 10 }, 10
            );
        

Unfortunately, this doesn't work as expected. The variable 'thetop' is treated as a string literal instead of a variable.

Why Doesn't it Work?

The reason why using a variable as a key in a JavaScript object literal doesn't work is because the object literal expects a string literal as the key. When you use a variable as a key, it is treated as a plain string instead of evaluating the value of the variable.

In the example above, the key 'thetop' is treated as a plain string literal instead of evaluating the value of the 'thetop' variable.

Solution: Using Computed Property Names

To solve this problem, you can use computed property names in JavaScript object literals. Computed property names allow you to use variables as keys in object literals.

Here's how you can use computed property names to solve the problem:

            var thetop = 'top';
            var options = {};
            options[thetop] = 10;
        
            something.stop().animate(options, 10);
        

In the code above, we create an empty object 'options' and then assign the value of the 'top' variable as the key using computed property names. This allows the JavaScript interpreter to evaluate the value of the variable 'thetop' as the key.

By passing the 'options' object to the animate function, we can now use a variable as the key in the object literal.

Example

Let's see a complete example to understand how to use a variable for a key in a JavaScript object literal:

            // Define a variable
            var key = 'name';

            // Create an object literal with computed property names
            var obj = {
                [key]: 'John'
            };

            // Access the value using the variable as the key
            console.log(obj[key]); // Output: John
        

In the above example, we define a variable 'key' and use it as the key in the object literal. We can access the value 'John' by using the variable 'key' as the key.

Conclusion

When you need to use a variable as a key in a JavaScript object literal, you can use computed property names to solve the problem. Computed property names allow you to dynamically evaluate the value of a variable as the key in an object literal.

By using computed property names, you can pass variables as keys and access their values as expected.