What is Function Binding? 💫

When working with a regular function in JavaScript (not an arrow function), you'll find that when using the this context, you're referring to the object that invoked the function.

Sometimes, this ends up creating different results than what you'd expect.

Take the following example:

const Bob = {
  temperature: "45 degrees"
  sayNews() {
  console.log('It is ${this.temperature} outside!')
  }
}

setTimeout(Bob.sayNews, 1000) // It is undefined outside!

You'll notice that we get undefined for the temperature variable. That's because when we refer to this in our setTimeout call, we are actually referring to the Window Object itself, and NOT Bob.

Are you confused yet?
So let's break this down.

1. setTimeout called on Bob
2. Inside Bob, in the sayNews function, this was referred to.
3. Since setTimeout is a Window Object method, Javascript thinks that this is referring to the Window.

To fix this, we could do one of two things
Use .bind to specifically bind an object to a function

setTimeout(Bob.sayNews.bind(Bob) // It is 45 degrees outside!

Use arrow functions, introduced in ES6

setTimeout(() => Dug.sayHi(), 1000) // It is 45 degrees outside!

I hope this was helpful. Thanks for reading :)

Resources

• MDN, Arrow Functions
• Medium article

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