The ABC’s of Javascript: Apply, Bind, and Call

Let me start by saying that although grouping the three methods in the order above makes for a nice play on words, the functions should be better categorized as call and apply, then bind separately. And although the title is about these three functions, we’ll also learn how to manipulate the arguments object into an array, what happens when we set the context to null, and how to avoid scope problems with functions like setTimeout.

 The Call Function

The reason why we categorize the functions call and apply together is because they perform essentially the same thing: to invoke a given function with a target object as the context and optionally pass in arguments to the given function. Setting the function’s context is the same thing as setting the this variable inside of the function. Here is an example of call

var ann = {
  name: "Ann",
  greet: function{
    console.log("Hi, my name is " + this.name);
  }
};

var bob = {
  name: "Bob"
};

//prints out "Hi, my name is Bob"
ann.greet.call(bob); 

It’s nice to have when you need to control the context of the function, but another common usage is invoking Array’s slice method on the arguments keyword to make it a full fledged array. The arguments keyword stores all the parameters passed into the function. It stores the indices as incrementing, zero-based numeric properties and has a property length. However, that’s pretty much it. arguments is classified as an array-like object. Here’s how to use slice to transform arguments to an array:

var borat = function {
  var argumentsArray = Array.prototype.slice.call(arguments);
  argumentsArray.push("...NOT!");
  console.log(argumentsArray.join(" "));
};

//prints out "Pamela, I am no longer attracted to you ...NOT!"
borat("Pamela,", "I", "am", "no", "longer", "attracted", "to", "you");

What happened here? We asked the Array pseudo-class for its slice method, Array.prototype.slice; similar to static methods in other languages. Then, we asked slice to be invoked in the context of arguments. This works because the arguments variable has all of the basic requirements of an array-like structure for slice to be executed correctly. We can imagine slice looping from 0 to arguments.length and calling arguments[i] without error, then returning an array with the same contents as arguments. Now we can manipulate the array with nice methods like push and join!

 The Apply Function

Let’s take the previous example and replace call with apply.

var borat = function {
  var argumentsArray = Array.prototype.slice.apply(arguments);
  argumentsArray.push("...NOT!");
  console.log(argumentsArray.join(" "));
};

//prints out "Pamela, I am no longer attracted to you ...NOT!"
borat("Pamela,", "I", "am", "no", "longer", "attracted", "to", "you");

One might be tempted to assume that call and apply are interchangeable. And in this situation, they kinda* are (*Though call performs better than apply when no arguments are passed in!). The main difference between call and apply is the way they accept arguments to be passed into the invoked function, after the context. Apply requires arguments to be passed in as a single array, and call requires zero or more arguments as individual, comma separated parameters. Let’s see this in action.

var memorize = function(letter, word){
  console.log(letter + " for " + word);
};

//prints "A for apply()"
memorize.apply(null, ["A", "array"]);

//prints "C for call()"
memorize.call(null, "C", "comma");

Notice how the parameters after the context are passed directly into the invoked function. Apply is useful when you do not know the exact amount of arguments. Also notice how we passed null in as the context. This defaults to setting the context as the global Javascript object. We can override this by adding "use strict"; anywhere before our code, enabling “strict mode”. The context will then be null or undefined instead of the global object.

 The Bind Function

Like call, bind also sets the given function’s context to the target object and optionally accepts comma separated arguments, but it doesn’t invoke it. It’ll return a copy of the function. This function’s context will be readily set to the target object, any additional arguments will also be readily passed into the bound function. This can prove to be useful when we want to pass in a callback function with the correct scope. A common scenario is when we use setTimeOut:

var foo = {
  message: "I'm bound!",
  bar: function(){
    console.log(this.message);
  }
};

foo.bar(); //prints "I'm bound!"
setTimeout(foo.bar, 1); //prints "undefined"
setTimeout(foo.bar.bind(foo), 1); //prints "I'm bound!"

The first foo.bar() runs fine; the keyword this is correctly set to foo. The second foo.bar() within setTimeout will cause us problems. Why does this.message return undefined? The method setTimeout itself is being called by the another object, which means that the keyword this will also be set to that object. Chances are, this object will not have a property message. We can easily fix this by calling bind and passing the correct context in.

 
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Kudos
 
10
Kudos

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