When passing object methods as callbacks, for instance to setTimeout, there’s a known problem: "losing this".

In this chapter we’ll see the ways to fix it.

Losing “this”

We’ve already seen examples of losing 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, 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 a 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);

// ...the value of user changes 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 come a 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); // (*)

// can run it without an object
sayHi(); // Hello, John!

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

// even if the value of user changes within 1 second
// sayHi uses the pre-bound value
user = {
  sayHi() { alert("Another user in setTimeout!"); }
};

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)
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.

Partial functions

Until now we have only been talking about binding this. Let’s take it a step further.

We can bind not only this, but also arguments. That’s rarely done, but sometimes can be handy.

The full syntax of bind:

let bound = func.bind(context, [arg1], [arg2], ...);

It allows to bind context as this and starting arguments of the function.

For instance, we have a multiplication function mul(a, b):

function mul(a, b) {
  return a * b;
}

Let’s use bind to create a function double on its base:

function mul(a, b) {
  return a * b;
}

let double = mul.bind(null, 2);

alert( double(3) ); // = mul(2, 3) = 6
alert( double(4) ); // = mul(2, 4) = 8
alert( double(5) ); // = mul(2, 5) = 10

The call to mul.bind(null, 2) creates a new function double that passes calls to mul, fixing null as the context and 2 as the first argument. Further arguments are passed “as is”.

That’s called partial function application – we create a new function by fixing some parameters of the existing one.

Please note that here we actually don’t use this here. But bind requires it, so we must put in something like null.

The function triple in the code below triples the value:

function mul(a, b) {
  return a * b;
}

let triple = mul.bind(null, 3);

alert( triple(3) ); // = mul(3, 3) = 9
alert( triple(4) ); // = mul(3, 4) = 12
alert( triple(5) ); // = mul(3, 5) = 15

Why do we usually make a partial function?

The benefit is that we can create an independent function with a readable name (double, triple). We can use it and not provide the first argument every time as it’s fixed with bind.

In other cases, partial application is useful when we have a very generic function and want a less universal variant of it for convenience.

For instance, we have a function send(from, to, text). Then, inside a user object we may want to use a partial variant of it: sendTo(to, text) that sends from the current user.

Going partial without context

What if we’d like to fix some arguments, but not the context this? For example, for an object method.

The native bind does not allow that. We can’t just omit the context and jump to arguments.

Fortunately, a helper function partial for binding only arguments can be easily implemented.

Like this:

function partial(func, ...argsBound) {
  return function(...args) { // (*)
    return func.call(this, ...argsBound, ...args);
  }
}

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

// add a partial method with fixed time
user.sayNow = partial(user.say, new Date().getHours() + ':' + new Date().getMinutes());

user.sayNow("Hello");
// Something like:
// [10:00] John: Hello!

The result of partial(func[, arg1, arg2...]) call is a wrapper (*) that calls func with:

  • Same this as it gets (for user.sayNow call it’s user)
  • Then gives it ...argsBound – arguments from the partial call ("10:00")
  • Then gives it ...args – arguments given to the wrapper ("Hello")

So easy to do it with the spread operator, right?

Also there’s a ready _.partial implementation from lodash library.

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 for an object method, so that we can pass it somewhere. For example, to setTimeout.

When we fix some arguments of an existing function, the resulting (less universal) function is called partially applied or partial.

Partials are convenient when we don’t want to repeat the same argument over and over again. Like if we have a send(from, to) function, and from should always be the same for our task, we can get a partial and go on with it.

Tasks

importance: 5

What will be the output?

function f() {
  alert( this ); // ?
}

let user = {
  g: f.bind(null)
};

user.g();

The answer: null.

function f() {
  alert( this ); // null
}

let user = {
  g: f.bind(null)
};

user.g();

The context of a bound function is hard-fixed. There’s just no way to further change it.

So even while we run user.g(), the original function is called with this=null.

importance: 5

Can we change this by additional binding?

What will be the output?

function f() {
  alert(this.name);
}

f = f.bind( {name: "John"} ).bind( {name: "Ann" } );

f();

The answer: John.

function f() {
  alert(this.name);
}

f = f.bind( {name: "John"} ).bind( {name: "Pete"} );

f(); // John

The exotic bound function object returned by f.bind(...) remembers the context (and arguments if provided) only at creation time.

A function cannot be re-bound.

importance: 5

There’s a value in the property of a function. Will it change after bind? Why, or why not?

function sayHi() {
  alert( this.name );
}
sayHi.test = 5;

let bound = sayHi.bind({
  name: "John"
});

alert( bound.test ); // what will be the output? why?

The answer: undefined.

The result of bind is another object. It does not have the test property.

importance: 5

The call to askPassword() in the code below should check the password and then call user.loginOk/loginFail depending on the answer.

But it leads to an error. Why?

Fix the highlighted line for everything to start working right (other lines are not to be changed).

function askPassword(ok, fail) {
  let password = prompt("Password?", '');
  if (password == "rockstar") ok();
  else fail();
}

let user = {
  name: 'John',

  loginOk() {
    alert(`${this.name} logged in`);
  },

  loginFail() {
    alert(`${this.name} failed to log in`);
  },

};

askPassword(user.loginOk, user.loginFail);

The error occurs because ask gets functions loginOk/loginFail without the object.

When it calls them, they naturally assume this=undefined.

Let’s bind the context:

function askPassword(ok, fail) {
  let password = prompt("Password?", '');
  if (password == "rockstar") ok();
  else fail();
}

let user = {
  name: 'John',

  loginOk() {
    alert(`${this.name} logged in`);
  },

  loginFail() {
    alert(`${this.name} failed to log in`);
  },

};

askPassword(user.loginOk.bind(user), user.loginFail.bind(user));

Now it works.

An alternative solution could be:

//...
askPassword(() => user.loginOk(), () => user.loginFail());

Usually that also works and looks good.

It’s a bit less reliable though in more complex situations where user variable might change after askPassword is called, but before the visitor answers and calls () => user.loginOk().

importance: 5

The task is a little more complex variant of Fix a function that loses "this".

The user object was modified. Now instead of two functions loginOk/loginFail, it has a single function user.login(true/false).

What should we pass askPassword in the code below, so that it calls user.login(true) as ok and user.login(false) as fail?

function askPassword(ok, fail) {
  let password = prompt("Password?", '');
  if (password == "rockstar") ok();
  else fail();
}

let user = {
  name: 'John',

  login(result) {
    alert( this.name + (result ? ' logged in' : ' failed to log in') );
  }
};

askPassword(?, ?); // ?

Your changes should only modify the highlighted fragment.

  1. Either use a wrapper function, an arrow to be concise:

    askPassword(() => user.login(true), () => user.login(false));

    Now it gets user from outer variables and runs it the normal way.

  2. Or create a partial function from user.login that uses user as the context and has the correct first argument:

    askPassword(user.login.bind(user, true), user.login.bind(user, false));
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