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

Special functions

The REPL provides several functions in the global scope:

--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@0.137.0/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

--eval-file flag

--eval-file flag allows you to run code from specified files before you are dropped into the REPL. Like the --eval flag, this is useful for importing code you commonly use in the REPL, or modifying the runtime in some way.

Files can be specified as paths or URLs. URL files are cached and can be reloaded via the --reload flag.

If --eval is also specified, then --eval-file files are run before the --eval code.

$ deno repl --eval-file=https://examples.deno.land/hello-world.ts,https://deno.land/std@0.137.0/encoding/ascii85.ts
Download https://examples.deno.land/hello-world.ts
Hello, World!
Download https://deno.land/std@0.137.0/encoding/ascii85.ts
Deno 1.20.5
exit using ctrl+d or close()
> rfc1924 // local (not exported) variable defined in ascii85.ts
"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!#$%&()*+-;<=>?@^_`{|}~"

Relative Import Path Resolution

If --eval-file specifies a code file that contains relative imports, then the runtime will try to resolve the imports relative to the current working directory. It will not try to resolve them relative to the code file's location. This can cause "Module not found" errors when --eval-file is used with module files:

$ deno repl --eval-file=https://deno.land/std@0.137.0/hash/md5.ts
error in --eval-file file https://deno.land/std@0.137.0/hash/md5.ts. Uncaught TypeError: Module not found "file:///home/encoding/hex.ts".
    at async <anonymous>:2:13
Deno 1.20.5
exit using ctrl+d or close()
> close()
$ deno repl --eval-file=https://deno.land/std@0.137.0/encoding/hex.ts
Download https://deno.land/std@0.137.0/encoding/hex.ts
Deno 1.20.5
exit using ctrl+d or close()
>

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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