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Debug

A helper for evalutating and logging an expression at the same time. Idea taken from Rust dbg! macro.

npm npm type definitions npm bundle size

Instead of separating "working" logic in order to log out individual pieces, use this function to log the expression and value in place!

Features

  • display called location (file and line number)
  • show unevaluated expression (with variable names intact!)
  • value of expression result
  • returns value so it can be used in place! (very useful - see examples)

Installation

# yarn
$ yarn add -D dbg-expr

# npm
$ npm install --save-dev dbg-expr

Please note that the $ character above is to denote use in a terminal. The command will fail if you enter that character when installing.

Usage

Just import the function and call it around an expression, either on its own line or an existing line.

For example:

import dbg from 'dbg-expr';

// on its own line (like console.log())
dbg(() => 4 - 1); // [/src/index.js:16] 4 - 1 = 3

// on an existing line
const value = 8 * dbg(() => 1 + 1); // [/src/index.js:44] 1 + 1 = 2

// using named variables
const age = api.getUser().age;
dbg(() => age); // [/models/user.js:25] age = 30

That last one is easier to type than:

console.log('age', age);

Log Format

The output format depends on whether you are logging to a terminal in Node vs the browser console via client-side JavaScript.

Node: [location] expression = value

Client-side: browser console example

FAQ

But why??

Well, I believe this has multiple benefits. An incomplete list might be:

  1. It's less to type 😊.
  2. If you want to log the output of a function, you would normally have to run the function twice: once to log the value and another to return the value. This could have adverse affects (ie: a non-immutable action like dispatching);
  3. Logging part of an expression. What if you want to see the value of the whole object, but then return only a property? This would mean you have to separate it into a variable, log the variable, and then continue on - like returning it; logging in-place solves this! Here's an example that illustrates how easy this package makes it!
// data coming from somewhere else
const people = [
  { first: 'Andy', last: 'Taylor' },
  { first: 'Opie', last: 'Taylor' },
  { first: 'Barney', last: 'Fife' },
  { first: 'Otis', last: 'Campbell' },
];

// original code
const uppercaseFirstNames = people.map(person => person.first.toUpperCase());

// verbose console.log() code
const uppercaseFirstNames = people.map(person => {
  const firstName = person.first;
  console.log('firstName', firstName);
  return firstName.toUpperCase();
});

// concise code by just wrapping in dbg()
const uppercaseFirstNames = people.map(person => dbg(() => person.first).toUpperCase());

Why do I need to pass in a function?

You'll notice that the expression is wrapped in a function. This is the only way to keep the expression unevaluated and logged out for you! If you pass only the expression, you'll still see the location and value, but not the stringified expression.

Why does the logged expression look so much different than my code? / Why am I not seeing a file location and line number?

You may have uglify/minify enabled in your bundler or framework which alters the output significantly. See the section below for help on this.

Do I have to use ES6 arrow functions?

You may have noticed that all of the examples above are using ES6 arrow functions. You may also use regular anonymous functions with the function keyword, like so:

var variable = 98;

// [/src/index.js:8] variable + 1 = 99
dbg(function() {
  return variable + 1;
});

Can I globally define this?

Currently you could attach this function to whatever long-lived object you wish, such as the client-side window object. Perhaps this is done in a Nuxt client-side plugin.

Disable Uglify / Minify in Development

This is the key to making this work well in some cases.. Below is a list of popular frameworks and setups and how you can disable these features in development. If you figure it out for one that is not listed, please submit a PR so others can benefit!