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:

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 <anonymous>: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

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