A small package for consistently asserting if a value exists

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401.0.47 years ago7 years agoMinified + gzip package size for assert-exists in KB


Assert Exists
Travis Build XO code style
A package for consistently asserting if a variable exists.


  1. Install.
npm install assert-exists --save
  1. Use it.
var a = require("assert-exists");

var errorMsg = a.msg("MyPackage");

function MyPackage(options) {
  a.exists(options, errorMsg('options'));
  a.exists(options.someString, errorMsg('someString'), 'string');


  • .exists(value, errorMessage, *type)
Checks to see if value is not null, AND typeof type (if it's provided), otherwise throws AssertionError: "errorMessage".
  • .msg(packageString)
Returns a function that takes one argument and returns a string like:
"ERROR: Expected {1} to be passed into {packageString}."


When passing configs into constructors, I ran into a problem where the same code was copied and pasted lots of times. That was annoying, verbose and error prone. It looked like this:
function MyPackage(configs) {
  assert(configs, "Expected configs to be passed into MyPackage");
    typeof configs.enabled === 'bool',
    "Expected configs to be passed into MyPackage" // not correct
  this.enabled = configs.enabled;
    configs.db && typeof === 'string',
    'Expected host to be passed into MyPackage' // not really correct
    configs.db && typeof configs.db.port === 'string',
    'Expected host to be passed into MyPackage' // wrong string
This sucks. So, I made the package to constrict my ability to mess this stuff up, and take away some of the copy-pasta incentive.
Now we have one function to check if it exists, and one to get an error message. Simple.
a.exists(finished, a.msg('README')('finished'), 'bool');