console Examples (log, assert)Use the console to print out error messages to determine what a program is doing. Use log, assert and count.
This page was last reviewed on Dec 27, 2023.
Console. In Node.js we often need to determine what actions a program is taking. We can use console.log, along with some other methods, to print out messages.
Shows a console
The console.log method can receive multiple arguments, and it can even perform substitutions for arguments. With assert meanwhile we can tell if a necessary condition is true.
Example. This program uses the console.log method in a variety of ways. It prints out the values of 2 constants in the program: animal and number.
Part 1 With substitution codes like "%s" and "%d" we can format arguments within a string.
Part 2 Sometimes it is best to just pass strings and numbers directly to console.log—the values are all printed in a single line.
Part 3 Here we print 2 string literals, a string and a number to the console. A newline is automatically added on the end.
Part 4 String interpolation syntax, with the backtick char, can be used to create a string that is then passed to console.log.
Shows a console
const animal = "bird"; const number = "10"; // Part 1: use substitution format. console.log("animal: %s number: %d", animal, number); // Part 2: write 2 values to the console. console.log("animal:", animal); // Part 3: write 4 values to the console. console.log("animal:", animal, "number:", number); // Part 4: use string interpolation to write a string to the console. console.log(`animal: ${animal} number: ${number}`);
animal: bird number: 10 animal: bird animal: bird number: 10 animal: bird number: 10
Assert. Suppose we have a function that needs to always have a certain condition be true—an argument must not be null, for example. With assert, we can print a message if an expression is false.
Step 1 We enforce that the number equals 10. Because the number is 10, nothing is printed.
Step 2 When we assert that the number is 10, but the number is 5, we get the error message with "assertion failed" printed at its start.
// Step 1: assert passes if the condition is true, and nothing is written. var number = 10; console.assert(number === 10, "Number must be 10!"); // Step 2: assert fails. number = 5; console.assert(number === 10, "(ASSERT 2) Number must be 10!")
Assertion failed: (ASSERT 2) Number must be 10!
Count. How often is a certain value encountered at a point in a program? With console.count we can determine this with minimal additional code.
// Use count to keep track of a count at a key. console.count("a"); console.count("a"); // This count is 1. console.count("b"); // This will increase count for this key to 3. console.count("a");
a: 1 a: 2 b: 1 a: 3
Summary. Though it is not essential to a program's functionality, many Node programs use console.log output. With log, assert and count, we can understand the actions a program takes.
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Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Dec 27, 2023 (new).
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