What is assert in JavaScript

Question

What does assert mean in JavaScript   I   ve seen something like   assert function1    amp  amp  function2    amp  amp  function3     some text      And would like to know what the method assert   does

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It probably came with a testing library that some of your code is using. Here's an example of one (chances are it's not the same library as your code is using, but it shows the general idea):

http://chaijs.com/guide/styles/#assert

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There is no standard assert in JavaScript itself  Perhaps you re using some library that provides one  for instance  if you re using Node js  perhaps you re using the assertion module   Browsers and other environments that offer a console implementing the Console API provide console assert   The usual meaning of an assert function is to throw an error if the expression passed into the function is false  this is part of the general concept of assertion checking  Usually assertions  as they re called  are used only in  quot testing quot  or  quot debug quot  builds and stripped out of production code  Suppose you had a function that was supposed to always accept a string  You d want to know if someone called that function with something that wasn t a string  without having a type checking layer like TypeScript or Flow   So you might do  assert typeof argumentName      quot string quot        where assert would throw an error if the condition were false  A very simple version would look like this  function assert condition  message        if   condition            throw message     quot Assertion failed quot            Better yet  make use of the Error object  which has the advantage of collecting a stack trace and such  function assert condition  message        if   condition            throw new Error message     quot Assertion failed quot

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In addition to other options like console assert or rolling your own  you can use invariant  It has a couple of unique features    It supports formatted error messages  using a  s specifier   In production environments  as determined by the Node js or Webpack environment   the error message is optional  allowing for  slightly  smaller  js

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Assertion throws error message if first attribute is false  and the second attribute is the message to be thrown  console assert condition message    There are many comments saying assertion does not exist in JavaScript but console assert   is the assert function in JavaScript The idea of assertion is to find why where the bug occurs  console assert document getElementById  quot title quot     quot You have no element with ID  title  quot    console assert document getElementById  quot image quot     quot You have no element with ID  image  quot     Here depending on the message you can find what the bug is  These error messages will be displayed to console in red color as if we called console error    You can use assertions to test your functions eg  console assert myAddFunction 5 8     5 8   quot Failed on 5 and 8 quot     Note the condition can be anything like     lt   gt  etc This is commonly used to test if the newly created function works as expected by providing some test cases and is not meant for production  To see more functions in console execute console log console

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Previous answers can be improved in terms of performances and compatibility   Check once if the Error object exists  if not declare it    if  typeof Error      undefined         Error   function message            this message   message             Error prototype message           Then  each assertion will check the condition  and always throw an Error object  function assert condition  message        if   condition  throw new Error message     Assertion failed        Keep in mind that the console will not display the real error line number  but the line of the assert function  which is not useful for debugging

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The other answers are good  there isn t an assert function built into ECMAScript5  e g  JavaScript that works basically everywhere  but some browsers give it to you or have add-ons that provide that functionality  While it s probably best to use a well-established   popular   maintained library for this  for academic purposes a  poor man s assert  function might look something like this   const assert   function condition  message        if   condition          throw Error  Assert failed       message              assert 1     1      Executes without problem assert false   Expected true       Yields  Error  Assert failed  Expected true  in console

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If using a modern browser or nodejs  you can use console assert expression  object    For more information    Chrome API Reference Firefox Web Console Firebug Console API IE Console API Opera Dragonfly Nodejs Console API

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As mentioned by T J   There is no assert in JavaScript  However  there is a node module named assert  which is used mostly for testing  so  you might see code like   const assert   require  assert    assert 5  gt  7

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If you use webpack  you can just use the node js assertion library   Although they claim that it s  not intended to be a general purpose assertion library   it seems to be more than OK for ad hoc assertions  and it seems no competitor exists in the Node space anyway  Chai is designed for unit testing    const assert   require  assert        assert jqXHR status    201   create response should be 201      You need to use webpack or browserify to be able to use this  so obviously this is only useful if those are already in your workflow

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If the assertion is false  the message is displayed   Specifically  if the first argument is false  the second argument  the string message  will be be logged in the developer tools console   If the first argument is true  basically nothing happens   A simple example     I   m using Google Developer Tools     var isTrue   true  var isFalse   false  console assert isTrue   Equals true so will NOT log to the console     console assert isFalse   Equals false so WILL log to the console

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Here is a really simple implementation of an assert function  It takes a value and a description of what you are testing         function assert value  description            var result   value    pass     fail           console log result     -      description             If the value evaluates to true it passes   assert  1   1   testing if 1 1        If it returns false it fails    assert  1   2   testing if 1 1

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Word or function  assert  is mostly used in testing parts of application   Assert functions are a short way of instructing the program to check the condition  also called  assertion   and if the condition is not True  it will throw error   So let s see how it would look like in  normal code   if  typeof  string       array       throw Error  Error   string       array        With assert you can simply write   assert typeof  string       array    In Javascript  there s no native assert function  so you have to use one from some library   For simple introduction  you can check this article   http   fredkschott com post 2014 05 nodejs-testing-essentials   I hope it helps

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assert   is not a native javascript function   It is a custom function someone made   You will have to look for it on your page or in your files and post it for anybody to help determine what it s doing

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Java has a assert statement  the JVM disables assertion validation by default  They must be explicitly enabled using command line argument -enableassertions  or its shorthand -ea   while JavaScript supports console assert    it s just a logging method and won t interrupt current procedure if assertion failed  To bring things together and satisfy various needs  here is a tiny js assertion lib   x000D   x000D  globalThis assert        gt      class AssertionError extends Error       constructor message          super message         this name    AssertionError               let config         async  true      silent  false        function assert condition  message   undefined        if   condition          if  config silent              NOOP         else if  config async            console assert condition  message     assert            else           throw new AssertionError message     assertion failed                        assert config   config    return assert             global assert    Object assign assert config         silent  true     to disable assertion validation   async  false     to validate assertion synchronously  will interrupt if assertion failed  like Java s       let items        id  1      id  2      id  3     function deleteItem item      let index   items findIndex  e   gt  e id     item id     assert index  gt  -1   index should be  gt  0  the item id   item id   to be deleted doesn t exist  or was already deleted      items splice index  1      console log  begin    deleteItem  id  1    deleteItem  id  1    console log  end    x000D   x000D   x000D

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check this http   net tutsplus com tutorials javascript-ajax quick-tip-quick-and-easy-javascript-testing-with-assert   it is for testing  JavaScript  Amazingly  at barely five or six lines  this code provides a great level of power and control over your code  when testing   The assert function accepts two parameters   outcome  A boolean  which references whether your test passed or failed  description  A short description of your test   The assert function then simply creates a list item  applies a class of either    pass    or    fail     dependent upon whether your test returned true or false  and then appends the description to the list item  Finally  that block of coded is added to the page  It   s crazy simple  but works perfectly

[javascript] What is “assert” in JavaScript?

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