console.log() / error() / warn() / table() Since: ES5（ECMAScript 2009）

Outputs messages and data to the console in the browser's developer tools. This is the most commonly used method for debugging.

Syntax

console.log(value);      // General message
console.error(value);    // Error message
console.warn(value);     // Warning message
console.table(array);    // Display in table format

Method List

Method	Description
console.log()	Outputs a general message. You can output any value, including strings, numbers, objects, and arrays. Multiple values can be output at once by separating them with commas.
console.error()	Outputs an error message in red. A stack trace is also displayed, making it easier to identify where the error occurred.
console.warn()	Outputs a warning message in yellow. Use this when something is not an error but still requires attention.
console.table()	Displays arrays or objects in a readable table format.
console.time("label")	Starts a timer. Use this to measure how long a process takes.
console.timeEnd("label")	Stops the timer started by `console.time()` and outputs the elapsed time.
console.group("label")	Groups console output together. Useful for organizing nested log entries.
console.groupEnd()	Ends the group started by `console.group()`.

Sample Code

// Basic output
console.log("Hello"); // Outputs 'Hello'.
console.log(42); // Outputs '42'.
console.log({ name: "Taro", age: 25 }); // Outputs the object.

// Output multiple values separated by commas.
var x = 10;
console.log("Value of x:", x); // Outputs 'Value of x: 10'.

// Errors and warnings
console.error("Failed to fetch data");
console.warn("This function is deprecated");

// Display in table format.
var users = [
	{ name: "Taro", age: 25 },
	{ name: "Hanako", age: 30 }
];
console.table(users);

// Measure processing time.
console.time("loop");
for (var i = 0; i < 100000; i++) {
	// Some processing
}
console.timeEnd("loop"); // Outputs elapsed time like 'loop: 2.5ms'.

// Grouping
console.group("User Info");
console.log("Name: Taro");
console.log("Age: 25");
console.groupEnd();

Overview

console.log() is the most frequently used debugging method in JavaScript development. Whenever you want to check a variable's value or trace the flow of execution, you can quickly output a message to the console.

console.error() and console.warn() are color-coded in the console, making it easy to spot errors and warnings among a large number of log entries. console.table() displays the contents of arrays and objects in a neatly organized table, which is especially handy when you need to understand a data structure at a glance.

console.log() is intended for debugging during development. It is recommended to remove any unnecessary console.log() calls before releasing to production. Excessive log output can affect performance, and internal information may become visible to users.