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You can play around with JavaScript in one of multiple shells, in interactive batch mode. This means that you can enter JavaScript one line at a time and have it immediately executed; and if you enter a statement that returns a value without assigning the value anywhere, the value is displayed.

For a list of shells, see the list referenced from Enternal links.

Keywords: REPL.

Standalone[edit | edit source]

Mozilla Firefox uses SpiderMonkey JavaScript engine, which is available as a standalone interactive shell for multiple platforms. You can download it from:

Unzip the file, and run "js" from the command line. A prompt appears:


You can enter statements, one at a time:

js> function incr(i) { return i+1; }
js> incr(1)
js> function plus2(i) {
return i+2;
js> plus2(1)
js> incr
function incr(i) { return i+1; }
js> print ("1+1:"+incr(1))
js> console.log("Yep.") // Console is available

Multi-line function definitions can be entered one line at a time, pressing enter after each line.

To run JavaScript snippets that use alert function--since they are intended for web browsers, you can define your own alert function:

js> function alert(message) { print ("Alert: "+message); }

From browser[edit | edit source]

You can have an interactive mode, entering JavaScript one line at a time and have it immediately executed, directly from your web browser.

In many versions of Firefox, press Control + Shift + K to get a web console window. At the bottom of the console window, there is a separate one-line field into which you can enter JavaScript lines and have them run by pressing Enter. Even multi-line function definitions can be entered, but not by pressing Enter but rather Shift + Enter and pressing Enter only after the whole definition was entered.

External links[edit | edit source]