Excellent! We found an error. As Descartes said: “If the bug is reproduced, then it exists”.
Where is the error? Now the program has two separate checks. And these checks can work simultaneously. When this happens, two messages appear.
To correct the error, let’s first introduce new checks with negation:
-
no developers are on vacation;
-
no developers are out sick.
In these conditions, the keyword is “no”, which means that they must be executed in cases where the value of the variable is false and not executed if the value is true.
To create checks with negation, unary (single) logical operator ! is used. Example:
var condition = false;
if (!condition) {
// code will be executed
}
Practice using the negation in the program “Can I start the project?”.
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Simple fork</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/if-else">2. Let’s use an alternative</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/simple-compare">3. Simple comparisons</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/or-equal">4. Comparisons permitting equality</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/equality">5. Equality, inequality</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/string-compare">6. Strict string comparison</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/numeric-compare">7. Strict number comparison</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/then-action">8. Condition-based actions</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/else-action">9. If the condition is not fulfilled</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/value-condition">10. Values as a condition</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/nested-if">11. Nested conditions</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/and-or">12. Logical operators: &&, ||</a></li><li class="main-nav__course-contents-item"><a href="/courses/javascript-programming/conditions/negation">13. Logic traps</a></li><li class="main-nav__course-contents-item main-nav__course-contents-item--current"><a href="/courses/javascript-programming/conditions/nesting-deeper">14. 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2.334.725-.13.837-1.46.93-2.13h.112c.937.492 1.647 1.33 1.98 2.334-.388.176-.793.31-1.21.4zm18.365-2.864c-1.104-.145-2.215-.232-3.33-.26-.546-.01-1.093.05-1.626.176-1.166.483-2.08 1.425-2.53 2.604l-.158.79 1.795.623c.558.153 1.132.24 1.71.26h.28c3.245-.177 4.137-3.72 4.137-3.72l.084-.353zm-1.264 1.86c-.54 1.394-2.882 1.646-2.966 1.646-.517-.02-1.03-.11-1.525-.26-.186-.085-.633-.26-1.08-.43.374-.966 1.117-1.746 2.065-2.165 0 .68.158 1.953.81 2.083.65.13.613-1.535.566-2.26.934.005 1.865.064 2.79.177-.11.316-.473.65-.66 1.17z" fill="#151841"/><path d="M19.26 39.27c.535 1.362 1.56 2.473 2.874 3.115.534.312 1.204-.07 1.207-.69.002-.285-.15-.55-.397-.695-.918-.46-1.65-1.222-2.073-2.158-.344-.957-1.972-.52-1.61.428zm29.45 3.125c1.083-.59 2.01-1.428 2.706-2.446.3-.556-.112-1.228-.743-1.21-.293.007-.56.17-.698.428-.568.776-1.306 1.412-2.158 1.86-.394.23-.517.742-.27 1.125.263.376.773.482 1.163.242z" fill="#EFA75B" stroke-width="13"/><path d="M16.025 32.436c1.525-1.386 2.427-4.408 2.79-6.835-.084.354 2.613.43 4.138-1.208 1.078-1.125.446-2-.186-3.375-1.71-3.552-4.585-6.323-7.384-8.918-.716-.69-1.44-2.428-2.706-2-.995.353-.93 3.2-.716 4.073 1.09 7.448 4.065 18.263 4.065 18.263zm37.372-.78c-1.704-1.826-2.822-4.12-3.208-6.585 0 .354-2.614.354-4.056-1.29-.995-1.127-.362-1.86.363-3.117 1.86-3.198 4.863-5.718 7.746-8.136.81-.604 1.627-1.47 2.79-.93.994.428.715 2.334.548 3.115-1.083 5.72-2.48 11.376-4.183 16.943z" fill="#FFF"/><path d="M53.564 20.422c-1.98.51-4.138 1.636-5.105 3.487-.43-2.196 1.645-4.65 3.393-5.832.333-.21.312-.703-.037-.886-.162-.085-.357-.076-.512.022-2.157 1.562-4.417 4.5-3.784 7.188.076.262.354.41.614.325l-.075.28c-.085.603.93.864.994.26.446-2.158 2.706-3.376 4.77-3.897.372-.076.522-.526.27-.81-.133-.15-.342-.204-.53-.138zM17.457 18.06c-.55-.354-.995.428-.55.865 1.433 1.004 3.182 3.05 3.395 4.956-.93-1.86-3.05-2.975-5.105-3.486-.386-.005-.633.41-.445.747.042.076.103.14.176.184 2.073.52 4.324 1.73 4.77 3.896.106.328.5.46.78.26.273.192.653.06.745-.26.735-2.66-1.6-5.728-3.766-7.16z" fill="#F1662A"/></svg></span><p class="course-loading-overlay__message"><b class="course-loading-overlay__message-title">Loading…</b><br><span class="course-loading-overlay__message-text">Everything will be ready in few seconds</span></p></div></div><div class="course-layout__sidebar course-theory"><div class="course-theory__inner course-theory__inner--left-stub"><button class="course-theory__close icon-close icon-close--light" type="button">Close</button><ul class="course-tabs course-tabs--non-interactive course-layout__theory-heading js--theory-tabs"><li class="course-tabs__item course-tabs__item--external course-tab course-tab--centered"><span class="course-tab__icon"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-theory"></use></svg></span><span class="course-tab__title">Theory</span></li><li class="course-tabs__item course-tabs__item--internal course-tab course-tab--centered course-tab--active js--theory-tab__theory"><span class="course-tab__icon"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-theory"></use></svg></span><span class="course-tab__title">Theory</span></li><li class="course-tabs__item course-tabs__item--internal course-tab course-tab--centered js--theory-tab__comments"><span class="course-tab__icon"><svg aria-hidden="true" style="width: 16px; height: 16px; fill: #2e2c9c;"><use xlink:href="/img/sprites/up.svg#comment"></use></svg></span><span class="course-tab__title">Comments</span></li></ul><section class="course-theory__content"><div class="js--theory-pane__theory course-theory__content-inner"><h1 class="course-theory__content-heading">Logical negation</h1><div class="course-theory__content-text"><p>Excellent! We found an error. As Descartes said: “If the bug is reproduced, then it exists”.</p><p>Where is the error? Now the program has two separate checks. And these checks can work simultaneously. When this happens, two messages appear.</p><p>To correct the error, let’s first introduce new checks with negation:</p><ul><li><em>no</em> developers are on vacation;</li><li><em>no</em> developers are out sick.</li></ul><p>In these conditions, the keyword is “no”, which means that they must be executed in cases where the value of the variable is <code> false</code> and not executed if the value is <code>true</code>.</p><p>To create checks with negation, unary (single) logical operator <code>!</code> is used. Example:</p><pre><code>var condition = false;
if (!condition) {
// code will be executed
}</code></pre><p>Practice using the negation in the program “Can I start the project?”.</p></div></div><div class="js--theory-pane__comments hidden course-theory__content-inner"><div class="course-theory__content-text"><h2 class="course-theory__content-heading">Comments</h2><div id="disqus_thread"></div><script>
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</script><noscript>Please enable JavaScript to view the <a href="https://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript></div></div></section><div class="course-theory__controls"><button class="button button--close" type="button">To the task</button></div></div></div><div class="course-layout__column course-layout__column--left"><ul class="js--course-tabs course-tabs course-layout__course-tabs"><li class="course-tabs__item course-tab js--editor-tab course-tabs__item--limited course-tabs__item--limited-large course-tab--active" data-editor="js"><span class="course-tab__title course-tab__title--indent"><span class="course-tab__title-part course-tab__title-part--active">script.js</span></span></li></ul><div class="course-layout__editors"><div class="js--editor course-editor course-layout__editor" id="" data-tab="js" data-split=""><span class="course-editor__label"><span class="course-editor__label-item">JavaScript</span></span><div class="course-editor__item" id="js-editor">var enoughDevelopers = false;
var techAvailable = false;
var onVacation = false;
var onSickLeave = false;
if (enoughDevelopers && techAvailable) {
console.log('You can start the project');
}
if (onVacation) {
console.log('No one is on vacation');
} else {
console.log('Someone is on vacation');
}
if (onSickLeave) {
console.log('No one is on sick leave');
} else {
console.log('Someone is on sick leave');
}
</div></div></div><div class="course-editor-controls"><button class="course-editor-controls__item course-editor-controls__item--answer" type="button">Show the answer</button></div><div class="js--feedback course-unhappy"><button class="js--feedback-btn course-unhappy__button" type="button"><span class="sr-only">Complain</span><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#unhappy"></use></svg></button><div class="js--feedback-modal course-unhappy__modal course-unhappy__modal--step-one"><form method="POST" action="/courses/javascript-programming/conditions/nesting-deeper/signal" class="course-unhappy__form form"><input type="hidden" name="csrf_name" value="csrf69aea6fe8695e"><input type="hidden" name="csrf_value" value="f9617c704299805330453efbabecc847"><p class="course-unhappy__text">What didn’t you like in this task?</p><div class="course-unhappy__slide"><div class="course-unhappy__wrap"><div class="course-unhappy__list"><label class="checkbox checkbox--small"><input class="checkbox__input" name="problems" value="1" type="checkbox"><span class="checkbox__text">The code check doesn’t work</span></label><label class="checkbox checkbox--small"><input class="checkbox__input" name="problems" value="2" type="checkbox"><span class="checkbox__text">The task is too difficult</span></label><label class="checkbox checkbox--small"><input class="checkbox__input" name="problems" value="3" type="checkbox"><span class="checkbox__text">The theory is hard to understand</span></label><label class="checkbox checkbox--small"><input class="js--feedback-other-checkbox checkbox__input" name="problems" value="0" type="checkbox"><span class="checkbox__text">Other (will write below)</span></label></div><div class="course-unhappy__message"><label class="sr-only" for="id-message">Другое (сейчас напишу)</label><textarea class="field field--area" id="id-message" name="message" placeholder="For example, a typo in the word «heading»"></textarea></div></div></div><button class="course-unhappy__control course-unhappy__control--prev" type="button"><span class="sr-only">Prev</span><svg aria-hidden="true"><use xlink:href="/img/sprites/landings.svg#arrow"></use></svg></button><button class="course-unhappy__control course-unhappy__control--next" type="button" disabled><span class="sr-only">Next</span><svg aria-hidden="true"><use xlink:href="/img/sprites/landings.svg#arrow"></use></svg></button><button class="js--submit-btn button" disabled>Submit</button></form><p class="course-unhappy__result hidden">Thanks! We’ll fix everything at once!</p></div></div></div><div class="course-layout__column course-layout__column--right"><span class="course-separator-control"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-arrows-separator"></use></svg></span><div class="course-browser"><div class="course-browser__menu"><span class="course-browser__menu-item course-browser__address"><span class="course-browser__menu-item-inner">Console</span></span><button class="js--autorun course-browser__menu-item course-browser__menu-button course-browser__menu-button--text-with-icon course-browser__menu-button--autorun course-browser__menu-button--autorun-on" title="Autorun" type="button"><span class="course-browser__menu-item-inner"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-autorun"></use></svg>
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</span></button><button class="course-browser__menu-item course-browser__menu-button course-browser__menu-button--expand" type="button"><svg xmlns="http://www.w3.org/2000/svg" width="11" height="11" viewBox="0 0 11 11"><path d="M1.375 5.125v4.5h4.5m3.75-3.75v-4.5h-4.5" fill="none" fill-rule="evenodd" stroke="currentColor"></path></svg></button></div><div class="course-browser__notification js--browser-change-heading hidden"><p class="course-browser__notification-text">The code has changed, click “Run” or turn autorun on.</p><button class="course-browser__notification-button course-browser__notification-button--play js--close" type="button"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-play"></use></svg>
Run</button></div><div class="course-browser__content"><div class="console js--console"><h2 class="sr-only">Result</h2><div class="console__content js--console__content"></div></div></div><div class="course-goals js--goals"><div class="course-goals__heading"><span class="course-goals__heading-title"><span class="course-goals__count-label">Goals</span><span class="course-goals__count">completed</span></span><button class="course-goals__button course-goals__button--expand js--goals__expand" type="button"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-collapse"></use></svg></button><a class="course-goals__button course-goals__button--next button button--green button--without-border hidden" href="/courses/javascript-programming/conditions/logic-operators" title="Let’s combine logical operators"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#arrow-white--mini"></use></svg></a></div><div class="course-goals__content"><p>Let’s experiment with negations.</p><ol class="course-goals__list"><li class="course-goals__list-item">Change the vacation check to the check with negation.</li><li class="course-goals__list-item">Do the same with the check about sick leave.</li><li class="course-goals__list-item">Indicate that one of the developers is on vacation.</li><li class="course-goals__list-item">Indicate that one of the developers is out sick.</li></ol></div></div></div></div></article></div><div class="course-interface__overlay course-interface__overlay--active"></div></main><script
src="https://assets.htmlacademy.org/scripts/courses-spa/htmlacademy-task.v43.js"
data-task-type="js"
data-assets-path="https://assets.htmlacademy.org/scripts/courses-spa/"
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