Introduction to Working with TypeScript Callbacks

Working with TypeScript Callbacks

In programming, callbacks are common as they allow you to create code which can react to events that occur within the program. Callbacks are popular in TypeScript, too, and the language provides several ways to work with them. Let’s take a look at how we can use callbacks in TypeScript.

Defining a Callback

The first step in working with callbacks is defining one. To do so, you define a function with the exact parameters and return type that the calling code expects. You might define a parameter as an array of data that can be passed into the callback:

function myCallback(data: Array) {
    // your code here
}

Once you have defined the callback, you can use it in your code by passing it as an argument when calling the function that needs it. For example, if you were using the Array.sort() method, you could specify the callback as the second argument:

data.sort(myCallback);

Passing Parameters to the Callback

Often, you may want to pass some data into the callback. This allows the callback to use the passed-in data to take an action, such as changing a variable’s value. To do this, you must specify additional parameters for the callback, such as this one which takes a number parameter:

function myCallback(data: Array, num: number) {
    // your code here
}

When calling the function with the callback, you must then also supply the parameters needed by the callback. In this example, we can pass the number parameter when calling Array.sort():

data.sort(myCallback, 5);

Using an Anonymous Function as a Callback

TypeScript also allows you to use an anonymous function as a callback. This allows you to write the callback code directly in your calling code rather than having to define a separate callback function. To do this, you can use a lambda expression (also known as an arrow function):

data.sort((a, b) => {
    // your code here
});

The benefit of using anonymous functions is that it makes your calling code more self-contained and easier to read. However, if you find yourself using the same callback multiple times, you should certainly consider defining a separate function for the callback.

Conclusion

Callbacks are a powerful pattern for working with events in TypeScript. With TypeScript’s support for defining callbacks, parameter passing, and anonymous functions, you can easily work with callbacks in your code. As always, keep in mind that overusing callbacks can make your code difficult to read, so try to use them only when necessary.