In JavaScript, console is the object you use to print messages while a program runs. console.log("Saved") writes Saved to the browser’s DevTools console, or to the terminal when you run the same file in Node.js.
Logging is a side effect. The call prints the value and then returns undefined. Use log() for ordinary values, warn() when something looks wrong, and error() when a step failed. count() tells you how many times a line ran.
This page teaches what the console is. The Console Methods index lists every method in this JavaScript tutorial, including console.log(), console.table(), console.group(), and console.time().
const result = console.log("Saved") prints Saved. result is undefined.
Common console calls you will meet in a JavaScript tutorial:
console.log("Saved");
console.info("Cache warm");
console.warn("Retrying");
console.error("Request failed");
console.assert(score >= 60, "below passing");
console.count("click");
console.table(rows);Ideas every JavaScript console tutorial builds on:
| Idea | Example | Remember |
|---|---|---|
console.log(name) | Shows the value, returns undefined | |
| Severity | console.warn(msg) | warn and error are easier to spot |
| Check | console.assert(ok, msg) | Prints only when ok is false |
| Count | console.count("click") | Same label shares one counter |
| Table | console.table(rows) | Best for arrays of objects |
| Timer | console.time("load") | End it with the same label |
Three short programs that show how the console works in a JS tutorial. The output below is what those calls print.
log() writes the argument. The variable that catches the call holds undefined.
const score = 80;
const written = console.log(score);
console.log(written);80.undefined, because log() does not return the message.undefined.Use a different method when the message means something different. The text is still just a string.
console.log("Saved");
console.info("Cache warm");
console.warn("Retrying");
console.error("Request failed");Retrying. DevTools marks it as a warning.Request failed. DevTools marks it as an error.Each label has its own counter. A new label starts at 1.
console.count("click");
console.count("click");
console.count("save");click: 1.click: 2 on the next call.1.After you know what the console prints, open the methods index for the rest of the calls in this JavaScript tutorial.
| Look up | Why |
|---|---|
console.log() | Print any value while you debug |
console.table() | Show an array of objects as columns |
console.group() | Nest related lines under one heading |
console.time() | Measure how long a block of code takes |
Full list: Console Methods index.
Open the index for log(), warn(), table(), group(), time(), and the rest.
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