`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.

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.

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

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.