Echo

Fun with proxies. Do any combination of calls, constructs, and dot-walks starting with Echo, and an approximation of what you typed is logged to the console (with syntax highlighting!) Play with it in your browser on RunKit!

/* > */ Echo
/* < */ Echo

/* > */ new Echo.foo
/* < */ new Echo.foo

/* > */ new ((new Echo).bar.baz(1, Echo.qux, new Echo(7)))
/* < */ new ((new Echo).bar.baz(1, Echo.qux, new Echo(7)))

/* > */ Echo.gfdgfd[4][' fff '](undefined, true, -Infinity)
/* < */ Echo.gfdgfd[4][' fff '](undefined, true, -Infinity)

/* > */ Echo`foo${[1, 2, 3, 4]}bar`.baz
/* < */ Echo`foo${[1, 2, 3, 4]}bar`.baz

Usage

Requirements: Browsers released after 2018. Node 11 or newer.

RunKit

The easiest way to play with Echo is in your browser on RunKit!

Browser

Another way to use Echo is to paste dist/echo.min.js into your console (note: Echo is harmless, but you should always be careful about pasting code into your console!).

Node

npm install echo-proxy
node --require echo-proxy

You can also import it into your code. Note that Echo attaches itself to the global, so you don't actually want the import to be named Echo.

// in ES module (package.json type=module)
import {} from 'echo-proxy';

// or in script (non-module)
require('echo-proxy');

API

The public API gets special treatment, and is not echoed. This also goes for JS-internal things like __proto__ that lead to weird behavior when messed with. The following builtin functions are overridden to return the stringified Echo:

• Echo.toString
• Echo[Symbol.toPrimitive]
• Echo[Symbol.for('nodejs.util.inspect.custom')]

Echo.render()

Example: const { tokens } = Echo.foo().render()

Returns an object with the following shape:

{
  tokens: {value: string, type: string}[] -- The output broken up into typed "tokens," which is what Echo uses to do syntax highlighting.
  plaintext: string -- The tokens strung together without any syntax highlighting.
  formatted: string[] -- The tokens with syntax highlighting, formatted as arguments to console.log.
  theme: { [tokenType]: [colorString] } -- The "theme" object that maps "token types" to either ANSI colors or hsl colors (or null if options.colorMode='off').
}

Echo.print()

Example: const value = Echo.foo(); value.print();

Logs the value to the console.

Echo.then(...)

Example: const { tokens } = await Echo.foo()

Resolves to the value of Echo.render().

Echo.options

Example: Echo.options.theme = 'firefox'

Can be used to configure Echo. This includes options to make it reflect how you would write code (for example, to always use parentheses with a constructor), and options that affect its behavior in other ways.

Property	Type/Options	Default	Explanation
bracketNotationOptional	Boolean	true	When true, use heuristics to decide when to use dot notation. Otherwise, always use square-bracket notation.
parensOptional	Boolean	true	When true, use heuristics to decide whether to wrap sub-expressions in parentheses. When false, always wrap them.
constructorParensOptional	Boolean	true	When true, drop parentheses for constructors with no arguments.
stringDelimiter	String	"'"	What character is used to delimit (bookend) strings, and is escaped from strings. If the string contains a newline, this is ignored and the string is rendered as a template string.
colorMode	"browser", "ansi", "off"	[auto detected]	Enable or disable syntax highlighting and the mode used to set colors.
theme	"chrome", "firefox", Object	[auto detected]	Which DevTools' syntax highlighting theme to use. Defaults to Chrome unless you're using Firefox. Can also be a custom theme object whose keys are TokenTypes and whose values are ANSI color codes or CSS styles for the given token type (depending on colorMode).
autoLog	Boolean	true	When true, automatically console.log the output. See below.
autoLogMinLength	Number	1	The minimum number of tokens required to log to console. You can use this to avoid printing in some autocomplete situations. See below.

The Auto-Logging Problem

When Echo.options.autoLog=true, the output is automatically logged to the console. If you're in an environment with autocomplete (like Chrome console), this mode may cause Echo to be logged several times as you type.

A little explanation of the way Echo's auto-logging works: it never knows if the code has finished evaluating and if it's time to print the output. Here are a couple examples to illustrate the problem:

• Consider the expression Echo.foo.bar. The sub-expression Echo.foo is completely evaluated before it looks for .bar. While this sub-expression is evaluating, all Echo knows is that the user tried to access Echo.foo, it can never tell if that getter is the last one in the chain or if there are more to come.
• Consider the expression Echo(Echo.foo). We don't want to print the inner Echo on its own line, we want them both to appear in one line.

Echo circumvents this by using setTimeout to wait 1 tick. All the various Echoes in your code calculate their outputs individually, and then the longest output is printed.

This timeout process is skipped when you manually call Echo.render() or Echo.print(). Instead of waiting a tick and logging, everything is handled immediately.

So that's how auto-logging works. So why does your browser sometimes cause it to print while you're typing? This happens when your browser tries to inspect the Echo object to populate the console's autocomplete/intellisense system. From Echo's perspective, this snooping looks the same as accessing values on it, so it tries to render the output.

To mitigate the extra rendering, we have Echo.options.autoLogMinLength. This says that any Echo output less than minLength items long shouldn't be printed.