Read-eval-print-loop

deno repl starts a read-eval-print-loop, which lets you interactively build up program state in the global context, it is especially useful for quick prototyping and checking snippets of code.

⚠️ Deno REPL supports JavaScript as well as TypeScript, however TypeScript code is not type-checked, instead it is transpiled to JavaScript behind the scenes.

⚠️ To make it easier to copy-paste code samples, Deno REPL supports import and export declarations. It means that you can paste code containing import ... from ...;, export class ... or export function ... and it will work as if you were executing a regular ES module.

Special variables

The REPL provides a couple of special variables, that are always available:

Identifier Description
_ Yields the last evaluated expression
_error Yields the last thrown error
Deno 1.14.3
exit using ctrl+d or close()
> "hello world!"
"hello world!"
> _
"hello world!"
> > const foo = "bar";
undefined
> _
undefined

--eval flag

--eval flag allows you to run some code in the runtime before you are dropped into the REPL. This is useful for importing some code you commonly use in the REPL, or modifying the runtime in some way:

$ deno repl --eval 'import { assert } from "https://deno.land/std@$STD_VERSION/testing/asserts.ts"'
Deno 1.14.3
exit using ctrl+d or close()
> assert(true)
undefined
> assert(false)
Uncaught AssertionError
    at assert (https://deno.land/std@0.110.0/testing/asserts.ts:224:11)
    at :2:1

Tab completions

Tab completions are crucial feature for quick navigation in REPL. After hitting tab key, Deno will now show a list of all possible completions.

$ deno repl
Deno 1.14.3
exit using ctrl+d or close()
> Deno.read
readTextFile      readFile          readDirSync       readLinkSync      readAll           read
readTextFileSync  readFileSync      readDir           readLink          readAllSync       readSync

Keyboard shortcuts

Keystroke Action
Ctrl-A, Home Move cursor to the beginning of line
Ctrl-B, Left Move cursor one character left
Ctrl-C Interrupt and cancel the current edit
Ctrl-D If if line is empty, signal end of line
Ctrl-D, Del If line is not empty, delete character under cursor
Ctrl-E, End Move cursor to end of line
Ctrl-F, Right Move cursor one character right
Ctrl-H, Backspace Delete character before cursor
Ctrl-I, Tab Next completion
Ctrl-J, Ctrl-M, Enter Finish the line entry
Ctrl-K Delete from cursor to end of line
Ctrl-L Clear screen
Ctrl-N, Down Next match from history
Ctrl-P, Up Previous match from history
Ctrl-R Reverse Search history (Ctrl-S forward, Ctrl-G cancel)
Ctrl-T Transpose previous character with current character
Ctrl-U Delete from start of line to cursor
Ctrl-V Insert any special character without performing its associated action
Ctrl-W Delete word leading up to cursor (using white space as a word boundary)
Ctrl-X Ctrl-U Undo
Ctrl-Y Paste from Yank buffer
Ctrl-Y Paste from Yank buffer (Meta-Y to paste next yank instead)
Ctrl-Z Suspend (Unix only)
Ctrl-_ Undo
Meta-0, 1, ..., - Specify the digit to the argument. starts a negative argument.
Meta-< Move to first entry in history
Meta-> Move to last entry in history
Meta-B, Alt-Left Move cursor to previous word
Meta-Backspace Kill from the start of the current word, or, if between words, to the start of the previous word
Meta-C Capitalize the current word
Meta-D Delete forwards one word
Meta-F, Alt-Right Move cursor to next word
Meta-L Lower-case the next word
Meta-T Transpose words
Meta-U Upper-case the next word
Meta-Y See Ctrl-Y