Comments are used to add hints, notes, suggestions, or warnings to JavaScript code. This can make it easier to read and understand. They can also be used to disable code to prevent it from being executed; this can be a valuable debugging tool.
JavaScript has two long-standing ways to add comments to code.
The first way is the // comment; this makes all text following it on the same line into a comment. For example:
function comment() {
console.log('Hello world!');
}
comment();
The second way is the /* */ style, which is much more flexible.
For example, you can use it on a single line:
function comment() {
console.log('Hello world!');
}
comment();
You can also make multiple-line comments, like this:
function comment() {
console.log('Hello world!');
}
comment();
You can also use it in the middle of a line, if you wish, although this can make your code harder to read so it should be used with caution:
function comment(x) {
console.log('Hello ' + x + ' !');
}
comment('world');
In addition, you can use it to disable code to prevent it from running, by wrapping code in a comment, like this:
function comment() {
}
comment();
In this case, the console.log() call is never issued, since it's inside a comment. Any number of lines of code can be disabled this way.
There's a special third comment syntax, the hashbang comment. A hashbang comment behaves exactly like a single line-only (//) comment, except that it begins with #! and is only valid at the absolute start of a script or module. Note also that no whitespace of any kind is permitted before the #!. The comment consists of all the characters after #! up to the end of the first line; only one such comment is permitted.
Hashbang comments in JavaScript resemble shebangs in Unix which provide the path to a specific JavaScript interpreter that you want to use to execute the script. Before the hashbang comment became standardized, it had already been de-facto implemented in non-browser hosts like Node.js, where it was stripped from the source text before being passed to the engine. An example is as follows:
console.log("Hello world");
The JavaScript interpreter will treat it as a normal comment — it only has semantic meaning to the shell if the script is directly run in a shell.
Warning: If you want scripts to be runnable directly in a shell environment, encode them in UTF-8 without a BOM. Although a BOM will not cause any problems for code running in a browser, it is not advised to use a BOM with a hashbang in a script — because the BOM will prevent the script from working when you try to run it in a Unix/Linux shell environment. So if you want scripts to be runnable directly in a shell environment, encode them in UTF-8 without a BOM.
You must only use the #! comment style to specify a JavaScript interpreter. In all other cases just use a // comment (or multiline comment).