Another essential concept in coding is 函数 , which allow you to store a piece of code that does a single task inside a defined block, and then call that code whenever you need it using a single short command — rather than having to type out the same code multiple times. In this article we'll explore fundamental concepts behind functions such as basic syntax, how to invoke and define them, scope, and parameters.
| Prerequisites: | Basic computer literacy, a basic understanding of HTML and CSS, JavaScript 第一步 . |
|---|---|
| Objective: | To understand the fundamental concepts behind JavaScript functions. |
In JavaScript, you'll find functions everywhere. In fact, we've been using functions all the way through the course so far; we've just not been talking about them very much. Now is the time, however, for us to start talking about functions explicitly, and really exploring their syntax.
Pretty much anytime you make use of a JavaScript structure that features a pair of parentheses —
()
— and you're
not
using a common built-in language structure like a
for 循环
,
while or do...while loop
，或
if...else statement
, you are making use of a function.
We've made use of functions built in to the browser a lot in this course. Every time we manipulated a text string, for example:
const myText = 'I am a string';
const newString = myText.replace('string', 'sausage');
console.log(newString);
// the replace() string function takes a source string,
// and a target string and replaces the source string,
// with the target string, and returns the newly formed string
Or every time we manipulated an array:
const myArray = ['I', 'love', 'chocolate', 'frogs'];
const madeAString = myArray.join(' ');
console.log(madeAString);
// the join() function takes an array, joins
// all the array items together into a single
// string, and returns this new string
Or every time we generated a random number:
const myNumber = Math.random();
// the random() function generates a random number between
// 0 and up to but not including 1, and returns that number
...we were using a function!
注意： Feel free to enter these lines into your browser's JavaScript console to re-familiarize yourself with their functionality, if needed.
The JavaScript language has many built-in functions to allow you to do useful things without having to write all that code yourself. In fact, some of the code you are calling when you invoke (a fancy word for run, or execute) a built in browser function couldn't be written in JavaScript — many of these functions are calling parts of the background browser code, which is written largely in low-level system languages like C++, not web languages like JavaScript.
Bear in mind that some built-in browser functions are not part of the core JavaScript language — some are defined as part of browser APIs, which build on top of the default language to provide even more functionality (refer to this early section of our course for more descriptions). We'll look at using browser APIs in more detail in a later module.
Programmers call 函数 that are part of objects 方法 . You don't need to learn about the inner workings of structured JavaScript objects yet — you can wait until our later module that will teach you all about the inner workings of objects, and how to create your own. For now, we just wanted to clear up any possible confusion of method versus function — you are likely to meet both terms as you look at the available related resources across the Web.
The built-in code we've made use of so far come in both forms: 函数 and 方法。 You can check the full list of the built-in functions, as well as the built-in objects and their corresponding methods here .
You've also seen a lot of
custom functions
in the course so far — functions defined in your code, not inside the browser. Anytime you saw a custom name with parentheses straight after it, you were using a custom function. In our
random-canvas-circles.html
example (see also the full
源代码
) from our
loops article
, we included a custom
draw()
function that looked like this:
function draw() {
ctx.clearRect(0,0,WIDTH,HEIGHT);
for (let i = 0; i < 100; i++) {
ctx.beginPath();
ctx.fillStyle = 'rgba(255,0,0,0.5)';
ctx.arc(random(WIDTH), random(HEIGHT), random(50), 0, 2 * Math.PI);
ctx.fill();
}
}
This function draws 100 random circles inside a
<canvas>
element. Every time we want to do that, we can just invoke the function with this:
draw();
rather than having to write all that code out again every time we want to repeat it. And functions can contain whatever code you like — you can even call other functions from inside functions. The above function for example calls the
random()
function three times, which is defined by the following code:
function random(number) {
return Math.floor(Math.random()*number);
}
We needed this function because the browser's built-in Math.random() function only generates a random decimal number between 0 and 1. We wanted a random whole number between 0 and a specified number.
You are probably clear on this by now, but just in case ... to actually use a function after it has been defined, you've got to run — or invoke — it. This is done by including the name of the function in the code somewhere, followed by parentheses.
function myFunction() {
alert('hello');
}
myFunction();
// calls the function once
注意： This form of creating a function is also known as 函数声明 . It is always hoisted, so you can call function above function definition and it will work fine.
Some functions require 参数 to be specified when you are invoking them — these are values that need to be included inside the function parentheses, which it needs to do its job properly.
注意： Parameters are sometimes called arguments, properties, or even attributes.
As an example, the browser's built-in Math.random() function doesn't require any parameters. When called, it always returns a random number between 0 and 1:
const myNumber = Math.random();
The browser's built-in string replace() function however needs two parameters — the substring to find in the main string, and the substring to replace that string with:
const myText = 'I am a string';
const newString = myText.replace('string', 'sausage');
注意： When you need to specify multiple parameters, they are separated by commas.
Sometimes parameters are optional — you don't have to specify them. If you don't, the function will generally adopt some kind of default behavior. As an example, the array join() function's parameter is optional:
const myArray = ['I', 'love', 'chocolate', 'frogs'];
const madeAString = myArray.join(' ');
console.log(madeAString);
// returns 'I love chocolate frogs'
const madeAnotherString = myArray.join();
console.log(madeAnotherString);
// returns 'I,love,chocolate,frogs'
If no parameter is included to specify a joining/delimiting character, a comma is used by default.
If you're writing a function and want to support optional parameters, you can specify default values by adding
=
after the name of the parameter, followed by the default value:
function hello(name='Chris') {
console.log(`Hello ${name}!`);
}
hello('Ari'); // Hello Ari!
hello(); // Hello Chris!
So far we have just created a function like so:
function myFunction() {
alert('hello');
}
But you can also create a function that doesn't have a name:
function() {
alert('hello');
}
This is called an anonymous function , because it has no name. You'll often see anonymous functions when a function expects to receive another function as a parameter. In this case the function parameter is often passed as an anonymous function.
注意： This form of creating a function is also known as 函数表达式 . Unlike function declaration, function expressions are not hoisted.
For example, let's say you want to run some code when the user types into a text box. To do this you can call the
addEventListener()
function of the text box. This function expects you to pass it (at least) two parameters:
"keydown"
When the user presses a key, the browser will call the function you provided, and will pass it a parameter containing information about this event, including the particular key that the user pressed:
function logKey(event) {
console.log(`You pressed "${event.key}".`);
}
textBox.addEventListener('keydown', logKey);
Instead of defining a separate
logKey()
function, you can pass an anonymous function into
addEventListener()
:
textBox.addEventListener('keydown', function(event) {
console.log(`You pressed "${event.key}".`);
});
If you pass an anonymous function like this, there's an alternative form you can use, called a
arrow function
. Instead of
function(event)
, you write
(event) =>
:
textBox.addEventListener('keydown', (event) => {
console.log(`You pressed "${event.key}".`);
});
If the function only has one line in the curly brackets, you omit the curly brackets:
textBox.addEventListener('keydown', (event) => console.log(`You pressed "${event.key}".`));
If the function only takes one parameter, you can also omit the brackets around the parameter:
textBox.addEventListener('keydown', event => console.log(`You pressed "${event.key}".`));
Finally, if your function needs to return a value, and contains only one line, you can also omit the
return
statement. In the following example we're using the
map()
方法为
数组
to double every value in the original array:
const originals = [1, 2, 3];
const doubled = originals.map(item => item * 2);
console.log(doubled); // [2, 4, 6]
map()
method takes each item in the array in turn, passing it into the given function. It then takes the value returned by that function and adds it to a new array.
So in the example above,
item => item * 2
is the arrow function equivalent of:
function doubleItem(item) {
return item * 2;
}
We recommend that you use arrow functions, as they can make your code shorter and more readable. To learn more, see the section on arrow functions in the JavaScript guide , and our reference page on arrow functions .
注意： There are some subtle differences between arrow functions and normal functions. They're outside the scope of this introductory guide, and are unlikely to make a difference in the cases we've discussed here. To learn more, see the arrow function reference documentation .
Here's a complete working example of the "keydown" example we discussed above:
The HTML:
<input id="textBox" type="text"></input>
<div id="output"></div>
The JavaScript:
const textBox = document.querySelector("#textBox");
const output = document.querySelector("#output");
textBox.addEventListener('keydown', event => output.textContent = `You pressed "${event.key}".`);
The result - try typing into the text box and see the output:
Let's talk a bit about scope — a very important concept when dealing with functions. When you create a function, the variables and other things defined inside the function are inside their own separate scope , meaning that they are locked away in their own separate compartments, unreachable from code outside the functions.
The top level outside all your functions is called the global scope . Values defined in the global scope are accessible from everywhere in the code.
JavaScript is set up like this for various reasons — but mainly because of security and organization. Sometimes you don't want variables to be accessible from everywhere in the code — external scripts that you call in from elsewhere could start to mess with your code and cause problems because they happen to be using the same variable names as other parts of the code, causing conflicts. This might be done maliciously, or just by accident.
For example, say you have an HTML file that is calling in two external JavaScript files, and both of them have a variable and a function defined that use the same name:
<!-- Excerpt from my HTML -->
<script src="first.js"></script>
<script src="second.js"></script>
<script>
greeting();
</script>
// first.js
const name = 'Chris';
function greeting() {
alert(`Hello ${name}: welcome to our company.`);
}
// second.js
const name = 'Zaptec';
function greeting() {
alert(`Our company is called ${name}.`);
}
Both functions you want to call are called
greeting()
, but you can only ever access the
first.js
文件的
greeting()
function (the second one is ignored). In addition, an error results when attempting (in the
second.js
file) to assign a new value to the
名称
variable — because it was already declared with
const
, and so can’t be reassigned.
注意： You can see this example running live on GitHub (see also the 源代码 ).
Keeping parts of your code locked away in functions avoids such problems, and is considered the best practice.
It is a bit like a zoo. The lions, zebras, tigers, and penguins are kept in their own enclosures, and only have access to the things inside their enclosures — in the same manner as the function scopes. If they were able to get into other enclosures, problems would occur. At best, different animals would feel really uncomfortable inside unfamiliar habitats — a lion or tiger would feel terrible inside the penguins' watery, icy domain. At worst, the lions and tigers might try to eat the penguins!
The zoo keeper is like the global scope — they have the keys to access every enclosure, to restock food, tend to sick animals, etc.
Let's look at a real example to demonstrate scoping.
a()
and
b()
, and three variables —
x
,
y
，和
z
— two of which are defined inside the functions, and one in the global scope. It also contains a third function called
output()
, which takes a single parameter and outputs it in a paragraph on the page.
output(x);
You should see the value of variable
x
printed to the browser viewport.
output(y);
output(z);
Both of these should throw an error into the console along the lines of "
ReferenceError: y is not defined
". Why is that? Because of function scope —
y
and
z
are locked inside the
a()
and
b()
functions, so
output()
can't access them when called from the global scope.
a()
and
b()
so they look like this:
function a() {
const y = 2;
output(y);
}
function b() {
const z = 3;
output(z);
}
Save the code and reload it in your browser, then try calling the
a()
and
b()
functions from the JavaScript console:
a();
b();
You should see the
y
and
z
values printed in the browser viewport. This works fine, as the
output()
function is being called inside the other functions — in the same scope as the variables it is printing are defined in, in each case.
output()
itself is available from anywhere, as it is defined in the global scope.
function a() {
const y = 2;
output(x);
}
function b() {
const z = 3;
output(x);
}
a();
b();
两者
a()
and
b()
call should print the value of x to the browser viewport. These work fine because even though the
output()
calls are not in the same scope as
x
is defined in,
x
is a global variable so is available inside all code, everywhere.
function a() {
const y = 2;
output(z);
}
function b() {
const z = 3;
output(y);
}
a();
b();
This time the
a()
and
b()
calls will throw that annoying
ReferenceError:
variable name
is not defined
error into the console — this is because the
output()
calls and the variables they are trying to print are not in the same function scopes — the variables are effectively invisible to those function calls.
注意：
The same scoping rules do not apply to loop (e.g.
for() { ... }
) and conditional blocks (e.g.
if() { ... }
) — they look very similar, but they are not the same thing! Take care not to get these confused.
注意： ReferenceError: "x" is not defined error is one of the most common you'll encounter. If you get this error and you are sure that you have defined the variable in question, check what scope it is in.
Keep in mind that you can call a function from anywhere, even inside another function. This is often used as a way to keep code tidy — if you have a big complex function, it is easier to understand if you break it down into several sub-functions:
function myBigFunction() {
const myValue;
subFunction1();
subFunction2();
subFunction3();
}
function subFunction1() {
console.log(myValue);
}
function subFunction2() {
console.log(myValue);
}
function subFunction3() {
console.log(myValue);
}
Just make sure that the values being used inside the function are properly in scope. The example above would throw an error
ReferenceError: myValue is not defined
, because although the
myValue
variable is defined in the same scope as the function calls, it is not defined inside the function definitions — the actual code that is run when the functions are called. To make this work, you'd have to pass the value into the function as a parameter, like this:
function myBigFunction() {
const myValue = 1;
subFunction1(myValue);
subFunction2(myValue);
subFunction3(myValue);
}
function subFunction1(value) {
console.log(value);
}
function subFunction2(value) {
console.log(value);
}
function subFunction3(value) {
console.log(value);
}
You've reached the end of this article, but can you remember the most important information? You can find some further tests to verify that you've retained this information before you move on — see Test your skills: Functions . These tests require skills that are covered in the next two articles, so you might want to read those first before trying it.
This article has explored the fundamental concepts behind functions, paving the way for the next one in which we get practical and take you through the steps to building up your own custom function.
最后修改： , 由 MDN 贡献者