Flash cards
Review the key moves
What is the main idea behind JavaScript Comments?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
// ___ heading: document.getElementById("myH").innerHTML = "My First Page"; // Change paragraph: document.getElementById("myP").innerHTML = "My first paragraph.";Put the learning moves in the order that makes the concept easiest to apply.
JavaScript comments can be used to explain JavaScript code, and to make it more readable.
JavaScript comments can also be used to prevent execution, when testing alternative code.
Single Line Comments
Single line comments start with // .
Any text between // and the end of the line will be ignored by JavaScript (will not be executed).
This example uses a single-line comment before each code line:
Example
// Change heading: document.getElementById("myH").innerHTML = "My First Page"; // Change paragraph: document.getElementById("myP").innerHTML = "My first paragraph.";This example uses a single line comment at the end of each line to explain the code:
Example
let x = 5; // Declare x, give it the value of 5
let y = x + 2; // Declare y, give it the value of x + 2Multi-line Comments
Multi-line comments start with /* and end with */ .
Any text between /* and */ will be ignored by JavaScript.
This example uses a multi-line comment (a comment block) to explain the code:
Example
/* The code below will change
the heading with id = "myH"
and the paragraph with id = "myP"
in my web page: */
document.getElementById("myH").innerHTML = "My First Page";
document.getElementById("myP").innerHTML = "My first paragraph.";It is most common to use single line comments. Block comments are often used for formal documentation.
Using Comments to Prevent Execution
Using comments to prevent execution of code is suitable for code testing.
Adding // in front of a code line changes the code lines from an executable line to a comment.
This example uses // to prevent execution of one of the code lines:
Example
//document.getElementById("myH").innerHTML = "My First Page"; document.getElementById("myP").innerHTML = "My first paragraph.";This example uses a comment block to prevent execution of multiple lines:
Example
/* document.getElementById("myH").innerHTML = "My First Page";
document.getElementById("myP").innerHTML = "My first paragraph.";
*/