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libs:

  • lodash

Function binding

When using setTimeout with object methods or passing object methods along, there's a known problem: "losing this".

Suddenly, this just stops working right. The situation is typical for novice developers, but happens with experienced ones as well.

Losing "this"

We already know that in JavaScript it's easy to lose this. Once a method is passed somewhere separately from the object -- this is lost.

Here's how it may happen with setTimeout:

let user = {
  firstName: "John",
  sayHi() {
    alert(`Hello, ${this.firstName}!`);
  }
};

*!*
setTimeout(user.sayHi, 1000); // Hello, undefined!
*/!*

As we can see, the output shows not "John" as this.firstName, but undefined!

That's because setTimeout got the function user.sayHi, separately from the object. The last line can be rewritten as:

let f = user.sayHi;
setTimeout(f, 1000); // lost user context

The method setTimeout in-browser is a little special: it sets this=window for the function call (for Node.JS, this becomes the timer object, but doesn't really matter here). So for this.firstName it tries to get window.firstName, which does not exist. In other similar cases as we'll see, usually this just becomes undefined.

The task is quite typical -- we want to pass an object method somewhere else (here -- to the scheduler) where it will be called. How to make sure that it will be called in the right context?

Solution 1: a wrapper

The simplest solution is to use an wrapping function:

let user = {
  firstName: "John",
  sayHi() {
    alert(`Hello, ${this.firstName}!`);
  }
};

*!*
setTimeout(function() {
  user.sayHi(); // Hello, John!
}, 1000);
*/!*

Now it works, because it receives user from the outer lexical environment, and then calls the method normally.

The same, but shorter:

setTimeout(() => user.sayHi(), 1000); // Hello, John!

Looks fine, but a slight vulnerability appears in our code structure.

What if before setTimeout triggers (there's one second delay!) user changes value? Then, suddenly, it will call the wrong object!

let user = {
  firstName: "John",
  sayHi() {
    alert(`Hello, ${this.firstName}!`);
  }
};

setTimeout(() => user.sayHi(), 1000);

// ...within 1 second
user = { sayHi() { alert("Another user in setTimeout!"); } };

// Another user in setTimeout?!?

The next solution guarantees that such thing won't happen.

Solution 2: bind

Functions provide a built-in method bind that allows to fix this.

The basic syntax is:

// more complex syntax will be little later
let boundFunc = func.bind(context);

The result of func.bind(context) is a special function-like "exotic object", that is callable as function and transparently passes the call to func setting this=context.

In other words, calling boundFunc is like func with fixed this.

For instance, here funcUser passes a call to func with this=user:

let user = {
  firstName: "John"
};

function func() {
  alert(this.firstName);
}

*!*
let funcUser = func.bind(user);
funcUser(); // John  
*/!*

Here func.bind(user) as a "bound variant" of func, with fixed this=user.

All arguments are passed to the original func "as is", for instance:

let user = {
  firstName: "John"
};

function func(phrase) {
  alert(phrase + ', ' + this.firstName);
}

// bind this to user
let funcUser = func.bind(user);

*!*
funcUser("Hello"); // Hello, John (argument "Hello" is passed, and this=user)
*/!*

Now let's try with an object method:

let user = {
  firstName: "John",
  sayHi() {
    alert(`Hello, ${this.firstName}!`);
  }
};

*!*
let sayHi = user.sayHi.bind(user); // (*)
*/!*

sayHi(); // Hello, John!

setTimeout(sayHi, 1000); // Hello, John!

In the line (*) we take the method user.sayHi and bind it to user. The sayHi is a "bound" function, that can be called alone or passed to setTimeout -- doesn't matter, the context will be right.

Here we can see that arguments are passed "as is", only this is fixed by bind:

let user = {
  firstName: "John",
  say(phrase) {
    alert(`${phrase}, ${this.firstName}!`);
  }
};

let say = user.say.bind(user);

say("Hello"); // Hello, John ("Hello" argument is passed to say)
say("Bye"); // Bye, John ("Bye" is passed to say)

````smart header="Convenience method: bindAll" If an object has many methods and we plan to actively pass it around, then we could bind them all in a loop:

for (let key in user) {
  if (typeof user[key] == 'function') {
    user[key] = user[key].bind(user);
  }
}

JavaScript libraries also provide functions for convenient mass binding , e.g. _.bindAll(obj) in lodash.


## Summary

Method `func.bind(context, ...args)` returns a "bound variant" of function `func` that fixes the context `this` and first arguments if given.

Usually we apply `bind` to fix `this` in an object method, so that we can pass it somewhere. For example, to `setTimeout`. There are more reasons to `bind` in the modern development, we'll meet them later.