Free JavaScript Specialist Practice Questions

Question 1

An order form uses price to reference a text input, and price.value is the string "18.75". A delivery charge of 4 must be added. The input has already been validated, and the total must remain a number for later calculations. Which expression produces the required total?

Accepted Answer

Number(price.value) + 4

Answer

Number(price.value + 4)

Answer

parseInt(price.value, 10) + 4

Answer

price.value + Number(4)

Question 2

A profile editor sends the request below. The same-origin server responds with HTTP 409 and the valid JSON body {"error":"Name already used"}. The response is readable, and showMessage returns normally.

fetch('/api/profile', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ displayName: 'Robin' })
})
  .then(response => response.json())
  .then(() => showMessage('Saved'))
  .catch(() => showMessage('Save failed'));

Why can 'Saved' appear after this response?

Accepted Answer

Fetch fulfills on HTTP 409; parsing its valid JSON also fulfills.

Answer

response.json() recovers a rejected Fetch promise when the error body is valid JSON.

Answer

The POST success callback runs before the HTTP status becomes available.

Answer

The catch handler applies only to the last then callback, not the preceding chain.

Question 3

A user-interface component temporarily overrides one instance's label method:

class Badge {
  label() { return 'Member'; }
}
const badge = new Badge();
badge.label = function () { return 'Editor'; };
delete badge.label;
const text = badge.label();

After the override is removed, what happens at the final assignment?

Accepted Answer

text becomes "Member"; lookup reaches the prototype after deletion.

Answer

text becomes "Editor"; the assignment changed the prototype method.

Answer

text becomes undefined; delete leaves a missing own-property value.

Answer

A TypeError is thrown; deleting the method prevents another call.

Question 4

A support dashboard displays customer-supplied ticket subjects inside a div. A subject containing an image tag with an event handler has executed script in a support agent's session. Subjects must appear literally, including angle brackets; embedded markup is not a supported feature. Which replacement for subjectElement.innerHTML = subject directly fixes this rendering defect?

Accepted Answer

subjectElement.textContent = subject;

Answer

subjectElement.innerHTML = JSON.stringify(subject);

Answer

subjectElement.innerHTML = encodeURIComponent(subject);

Answer

subjectElement.innerHTML = subject.replace(/<script>/gi, '');

Question 5

A translation helper is created before the application's language setting changes:

let language = 'en';
function makeLabel(language) {
  return key => `${language}:${key}`;
}
const label = makeLabel(language);
language = 'fr';

Without recreating label, a test calls label('save'). What should the assertion expect?

Accepted Answer

"en:save", because the closure retains makeLabel's separate parameter binding.

Answer

"fr:save", because the closure reads the updated outer language binding.

Answer

"undefined:save", because the parameter loses its value when makeLabel returns.

Answer

A ReferenceError, because the returned function outlives its parameter scope.

Question 6

A publishing tool checks page approvals with this function:

What does the approval check actually do with these three pages?

Accepted Answer

Returns true after visiting all three pages in the array.

Answer

Returns false and stops after the second page is inspected.

Answer

Returns false after visiting all three pages in the array.

Answer

Returns undefined because forEach does not return a collection.

Question 7

library.js defines window.makeChart. dashboard.js calls makeChart and reads an element declared later in the body. Both files are external classic scripts placed in the head. Parsing must continue while they download, and dashboard.js must execute after both its dependency and the required HTML are ready. Choose the appropriate pair of script tags, in source order.

Accepted Answer

Answer

Answer

Answer

Question 8

An editor wants to clear the needs-review class from every marked paragraph. The page has exactly these three matching elements, in this order, when the code runs:

Introduction

Details

Summary

const pending = document.getElementsByClassName('needs-review'); for (let i = 0; i < pending.length; i++) { pending[i].classList.remove('needs-review'); } Which paragraph remains marked when the loop finishes?

Accepted Answer

The details paragraph.

Answer

The introduction paragraph.

Answer

The summary paragraph.

Answer

All three paragraphs are cleared.

Question 9

After a component closes, its click handler still runs. Registration and cleanup use the same existing button and component objects:

const component = {
  count: 0,
  record() { this.count++; }
};
button.addEventListener('click', component.record.bind(component));
button.removeEventListener('click', component.record.bind(component));

The button has no other listeners. What makes this cleanup ineffective?

Accepted Answer

Each bind call produces a new function, so the callback references do not match.

Answer

The listener was registered for bubbling, but removal searches only capturing listeners.

Answer

Binding changes component.record itself, so cleanup refers to a replaced method.

Answer

The bound receiver must be button for removeEventListener to recognize the callback.

Question 10

A product-code validator must accept exactly two uppercase ASCII letters followed by four digits. It passes once for AB2048, fails for the same string on the next call, then passes again. No code alters the input.

const codePattern = /^[A-Z]{2}[0-9]{4}$/g;
function isCode(value) {
  return codePattern.test(value);
}

Which edit removes the alternating result without weakening the format rule?

Accepted Answer

Remove g while retaining the anchors and character classes.

Answer

Convert value with String(value) before passing it to test.

Answer

Set codePattern.lastIndex to 1 immediately before every test.

Answer

Wrap each result in Boolean(codePattern.test(value)) before returning it.

Free JavaScript Specialist Practice Questions

Domain 1: Essential JavaScript Principles and Practices

Question 1

Question 2

Question 3

Domain 2: Intermediate JavaScript Programming Techniques

Question 4

Question 5

Question 6

Domain 3: Applied JavaScript

Question 7

Question 8

Domain 4: Advanced JavaScript Development

Question 9

Question 10

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