As we already know, a function in JavaScript is a value.
Every value in JavaScript has a type. What type is a function?
In JavaScript, functions are objects.
A good way to imagine functions is as callable “action objects”. We can not only call them, but also treat them as objects: add/remove properties, pass by reference etc.
Function objects contain some useable properties.
For instance, a function’s name is accessible as the “name” property:
function sayHi() {
alert("Hi");
}
alert(sayHi.name); // sayHiWhat’s kind of funny, the name-assigning logic is smart. It also assigns the correct name to a function even if it’s created without one, and then immediately assigned:
let sayHi = function() {
alert("Hi");
};
alert(sayHi.name); // sayHi (there's a name!)It also works if the assignment is done via a default value:
function f(sayHi = function() {}) {
alert(sayHi.name); // sayHi (works!)
}
f();In the specification, this feature is called a “contextual name”. If the function does not provide one, then in an assignment it is figured out from the context.
Object methods have names too:
let user = {
sayHi() {
// ...
},
sayBye: function() {
// ...
}
}
alert(user.sayHi.name); // sayHi
alert(user.sayBye.name); // sayByeThere’s no magic though. There are cases when there’s no way to figure out the right name. In that case, the name property is empty, like here:
// function created inside array
let arr = [function() {}];
alert( arr[0].name ); // <empty string>
// the engine has no way to set up the right name, so there is noneIn practice, however, most functions do have a name.
There is another built-in property “length” that returns the number of function parameters, for instance:
function f1(a) {}
function f2(a, b) {}
function many(a, b, ...more) {}
alert(f1.length); // 1
alert(f2.length); // 2
alert(many.length); // 2Here we can see that rest parameters are not counted.
The length property is sometimes used for introspection in functions that operate on other functions.
For instance, in the code below the ask function accepts a question to ask and an arbitrary number of handler functions to call.
Once a user provides their answer, the function calls the handlers. We can pass two kinds of handlers:
A zero-argument function, which is only called when the user gives a positive answer.
A function with arguments, which is called in either case and returns an answer.
To call handler the right way, we examine the handler.length property.
The idea is that we have a simple, no-arguments handler syntax for positive cases (most frequent variant), but are able to support universal handlers as well:
function ask(question, ...handlers) {
let isYes = confirm(question);
for(let handler of handlers) {
if (handler.length == 0) {
if (isYes) handler();
} else {
handler(isYes);
}
}
}
// for positive answer, both handlers are called
// for negative answer, only the second one
ask("Question?", () => alert('You said yes'), result => alert(result));This is a particular case of so-called polymorphism – treating arguments differently depending on their type or, in our case depending on the length. The idea does have a use in JavaScript libraries.
We can also add properties of our own.
Here we add the counter property to track the total calls count:
function sayHi() {
alert("Hi");
// let's count how many times we run
sayHi.counter++;
}
sayHi.counter = 0; // initial value
sayHi(); // Hi
sayHi(); // Hi
alert( `Called ${sayHi.counter} times` ); // Called 2 timesA property is not a variable
A property assigned to a function like sayHi.counter = 0 does not define a local variable counter inside it. In other words, a property counter and a variable let counter are two unrelated things.
We can treat a function as an object, store properties in it, but that has no effect on its execution. Variables are not function properties and vice versa. These are just parallel worlds.
Function properties can replace closures sometimes. For instance, we can rewrite the counter function example from the chapter Variable scope, closure to use a function property:
function makeCounter() {
// instead of:
// let count = 0
function counter() {
return counter.count++;
};
counter.count = 0;
return counter;
}
let counter = makeCounter();
alert( counter() ); // 0
alert( counter() ); // 1The count is now stored in the function directly, not in its outer Lexical Environment.
Is it better or worse than using a closure?
The main difference is that if the value of count lives in an outer variable, then external code is unable to access it. Only nested functions may modify it. And if it’s bound to a function, then such a thing is possible:
function makeCounter() {
function counter() {
return counter.count++;
};
counter.count = 0;
return counter;
}
let counter = makeCounter();
counter.count = 10;
alert( counter() ); // 10So the choice of implementation depends on our aims.
Named Function Expression, or NFE, is a term for Function Expressions that have a name.
For instance, let’s take an ordinary Function Expression:
let sayHi = function(who) {
alert(`Hello, ${who}`);
};And add a name to it:
let sayHi = function func(who) {
alert(`Hello, ${who}`);
};Did we achieve anything here? What’s the purpose of that additional "func" name?
First let’s note, that we still have a Function Expression. Adding the name "func" after function did not make it a Function Declaration, because it is still created as a part of an assignment expression.
Adding such a name also did not break anything.
The function is still available as sayHi():
let sayHi = function func(who) {
alert(`Hello, ${who}`);
};
sayHi("John"); // Hello, JohnThere are two special things about the name func, that are the reasons for it:
It allows the function to reference itself internally.
It is not visible outside of the function.
For instance, the function sayHi below calls itself again with "Guest" if no who is provided:
let sayHi = function func(who) {
if (who) {
alert(`Hello, ${who}`);
} else {
func("Guest"); // use func to re-call itself
}
};
sayHi(); // Hello, Guest
// But this won't work:
func(); // Error, func is not defined (not visible outside of the function)Why do we use func? Maybe just use sayHi for the nested call?
Actually, in most cases we can:
let sayHi = function(who) {
if (who) {
alert(`Hello, ${who}`);
} else {
sayHi("Guest");
}
};The problem with that code is that sayHi may change in the outer code. If the function gets assigned to another variable instead, the code will start to give errors:
let sayHi = function(who) {
if (who) {
alert(`Hello, ${who}`);
} else {
sayHi("Guest"); // Error: sayHi is not a function
}
};
let welcome = sayHi;
sayHi = null;
welcome(); // Error, the nested sayHi call doesn't work any more!That happens because the function takes sayHi from its outer lexical environment. There’s no local sayHi, so the outer variable is used. And at the moment of the call that outer sayHi is null.
The optional name which we can put into the Function Expression is meant to solve exactly these kinds of problems.
Let’s use it to fix our code:
let sayHi = function func(who) {
if (who) {
alert(`Hello, ${who}`);
} else {
func("Guest"); // Now all fine
}
};
let welcome = sayHi;
sayHi = null;
welcome(); // Hello, Guest (nested call works)Now it works, because the name "func" is function-local. It is not taken from outside (and not visible there). The specification guarantees that it will always reference the current function.
The outer code still has its variable sayHi or welcome. And func is an “internal function name”, the way for the function to can call itself reliably.
There’s no such thing for Function Declaration
The “internal name” feature described here is only available for Function Expressions, not for Function Declarations. For Function Declarations, there is no syntax for adding an “internal” name.
Sometimes, when we need a reliable internal name, it’s the reason to rewrite a Function Declaration to Named Function Expression form.
Functions are objects.
Here we covered their properties:
name – the function name. Usually taken from the function definition, but if there’s none, JavaScript tries to guess it from the context (e.g. an assignment).
length – the number of arguments in the function definition. Rest parameters are not counted.
If the function is declared as a Function Expression (not in the main code flow), and it carries the name, then it is called a Named Function Expression. The name can be used inside to reference itself, for recursive calls or such.
Also, functions may carry additional properties. Many well-known JavaScript libraries make great use of this feature.
They create a “main” function and attach many other “helper” functions to it. For instance, the jQuery library creates a function named $. The lodash library creates a function _, and then adds _.clone, _.keyBy and other properties to it (see the docs when you want to learn more about them). Actually, they do it to lessen their pollution of the global space, so that a single library gives only one global variable. That reduces the possibility of naming conflicts.
So, a function can do a useful job by itself and also carry a bunch of other functionality in properties.
importance: 5
Modify the code of makeCounter() so that the counter can also decrease and set the number:
counter() should return the next number (as before).
counter.set(value) should set the counter to value.
counter.decrease() should decrease the counter by 1.
See the sandbox code for the complete usage example.
P.S. You can use either a closure or the function property to keep the current count. Or write both variants.
Open a sandbox with tests.
The solution uses count in the local variable, but addition methods are written right into the counter. They share the same outer lexical environment and also can access the current count.
function makeCounter() {
let count = 0;
function counter() {
return count++;
}
counter.set = value => count = value;
counter.decrease = () => count--;
return counter;
}Open the solution with tests in a sandbox.
importance: 2
Write function sum that would work like this:
sum(1)(2) == 3; // 1 + 2 sum(1)(2)(3) == 6; // 1 + 2 + 3 sum(5)(-1)(2) == 6 sum(6)(-1)(-2)(-3) == 0 sum(0)(1)(2)(3)(4)(5) == 15
P.S. Hint: you may need to setup custom object to primitive conversion for your function.
Open a sandbox with tests.
For the whole thing to work anyhow, the result of sum must be function.
That function must keep in memory the current value between calls.
According to the task, the function must become the number when used in ==. Functions are objects, so the conversion happens as described in the chapter Object to primitive conversion, and we can provide our own method that returns the number.
Now the code:
function sum(a) {
let currentSum = a;
function f(b) {
currentSum += b;
return f;
}
f.toString = function() {
return currentSum;
};
return f;
}
alert( sum(1)(2) ); // 3
alert( sum(5)(-1)(2) ); // 6
alert( sum(6)(-1)(-2)(-3) ); // 0
alert( sum(0)(1)(2)(3)(4)(5) ); // 15Please note that the sum function actually works only once. It returns function f.
Then, on each subsequent call, f adds its parameter to the sum currentSum, and returns itself.
There is no recursion in the last line of f.
Here is what recursion looks like:
function f(b) {
currentSum += b;
return f(); // <-- recursive call
}And in our case, we just return the function, without calling it:
function f(b) {
currentSum += b;
return f; // <-- does not call itself, returns itself
}This f will be used in the next call, again return itself, as many times as needed. Then, when used as a number or a string – the toString returns the currentSum. We could also use Symbol.toPrimitive or valueOf here for the conversion.
Open the solution with tests in a sandbox.