Some US adverts say "React developer". Others say "JavaScript React developer", or list "strong HTML and JavaScript" on a line of its own, above React. That is not padding. It is a signal about the codebase and about the interview, and it is worth reading properly.
Two things usually sit behind it. The team has a mixed codebase — older JavaScript, perhaps jQuery, being moved to React a piece at a time — so framework knowledge alone will not carry you. And the technical screen will test the language separately from the library.
This page is about that fundamentals round: where its questions actually come from, and the distinction that will make you sound like someone who has read the specifications rather than a tutorial. For the market picture and pay, see our React developer jobs guide.
A "JavaScript Fundamentals" Round Draws On Two Different Standards
This is the useful part, and very few candidates can state it.
The JavaScript language is standardised as ECMA-262, published by Ecma International and revised every year; the current text is the ECMAScript 2027 Language Specification. That document defines the language itself: types and coercion, scope and closures, prototypes, classes, iterators and generators, promises, async functions, modules, and the array and object methods.
It does not define the environment the language runs in. The event loop, the task and microtask queues, setTimeout and setInterval, the DOM, fetch and the rest of the browser surface are specified elsewhere — principally in the WHATWG HTML Standard and its companion standards. In the HTML Standard each agent is given its own event loop, and timers are defined in its own Timers section, which states plainly that "the setTimeout() and setInterval() methods allow authors to schedule timer-based callbacks".
So when an interviewer says "explain the event loop", they are asking about a browser specification, not about the JavaScript language. When they ask "what does async actually do", that is ECMA-262. Both are fair questions. Knowing which document each comes from is what separates an answer that recites a diagram from one that explains a mechanism.
A detail worth having ready
Here is a specific, verifiable fact from the HTML Standard's timers section that almost no candidate knows, and which answers the classic "why doesn't setTimeout(fn, 0) run immediately" question properly: timers nest, and "after five such nested timers, however, the interval is forced to be at least four milliseconds". That is a clamp written into the standard, not a browser quirk and not a consequence of the event loop being busy.
"ES6+" Names a Baseline From 2015
Adverts almost universally ask for "ES6+" or "modern JavaScript (ES6)". It is worth knowing what that phrase actually refers to, because it dates the advert and sometimes the codebase.
ES6 is the sixth edition of ECMA-262, adopted by the Ecma General Assembly on 17 June 2015 as ECMAScript 2015. It was the largest revision since 1999, and it brought in the things now taken for granted: modules, class declarations, block scoping with let and const, iterators and generators, promises and destructuring.
What changed after it matters more for how you read the advert. TC39 moved to an annual release cycle, publishing a new edition each June: the 7th in 2016 through to the 16th edition in June 2025. There is no "ES7 developer" or "ES12 developer" because nobody counts that way any more; the editions are named by year.
Two practical readings follow:
- "ES6+" means "not the language as it was before 2015". It is a floor, not a target, and it has been a floor for a decade. Do not treat it as the ceiling of what to learn.
- An advert that stops at ES6 may be describing its codebase. In a modernisation role that is useful information rather than a red flag: it tells you what you will be reading.
What the Modernisation Work Actually Is
A large share of adverts with this phrasing are about moving an existing application to React piece by piece rather than building a new one. It is unfashionable work and it is a genuinely good place to learn, because you cannot get through it on framework knowledge alone.
What it tends to require:
- Reading code you did not write, in styles that predate the conventions you learned. This is the actual skill, and it is the one that transfers everywhere.
- Making two paradigms coexist. React owning part of a page while older code owns the rest means being deliberate about who controls the DOM node, and what happens on teardown.
- Understanding the language rather than the framework's idioms, because half the code you meet will not be using them.
- Not breaking what works. Incremental migration is judged on the absence of regressions, which is a different discipline from shipping features.
If you are weighing this against a greenfield role: the greenfield role teaches you one stack, and this one teaches you how software actually ages. Both are defensible. Only one of them is usually available to someone without a long CV.
How To Prepare For the Two-Part Interview
Prepare them as two subjects, because that is how they will be asked.
Language, from ECMA-262 territory: scope and closures; this and how it is determined; prototypes and the class syntax over them; value and reference; equality and coercion; the array and object methods you would otherwise reach for a library for; promises, and what async and await desugar to; modules and what a circular import does; iterators and generators.
Environment, from the HTML Standard and its siblings: the event loop, and the difference between a task and a microtask; where a promise callback runs relative to a timer; the timer clamp above; event propagation, delegation and cancellation; fetch, abort signals and error handling; and what "blocking the main thread" physically means.
The single best answer you can give in this round is one that names the boundary out loud — that promises are the language, that the queue they resolve on is the host environment, and that this is why the same code behaves differently in a browser and on a server. Very few candidates say it, and it is not a trick; it is just reading the right documents once.
The assessment stage itself is regulated, and there are things you are entitled to ask for. Our guide to coding tests in the USA sets out what applies, including on a timed exercise. If the advert also names TypeScript, our React TypeScript guide covers what types do and, importantly, do not do at runtime.
Frequently Asked Questions
Why do adverts say "JavaScript React developer" rather than "React developer"?
Usually because the codebase is mixed and framework knowledge alone will not carry the work, and because the interview will test the language separately from the library. Both are worth knowing before you apply.
Is the event loop part of JavaScript?
Not of the language specification. ECMA-262 defines the language; the event loop, task queues and timers are defined in the WHATWG HTML Standard, which gives each agent its own event loop. Interviewers ask about both under one heading.
What does "ES6+" actually mean?
The sixth edition of ECMA-262, adopted on 17 June 2015 as ECMAScript 2015, and everything after it. The standard has been annual since, reaching its 16th edition in June 2025, so ES6 is a floor rather than a target.
Why doesn't setTimeout with zero delay run immediately?
Partly because the callback is queued rather than called, and partly because the HTML Standard clamps nested timers: after five nested timers the interval is forced to at least four milliseconds.
What is the difference between a task and a microtask?
Both are concepts from the HTML Standard rather than from the language. Promise callbacks are processed as microtasks and run before the next task, which is why a resolved promise runs ahead of a zero-delay timer.
Do I need jQuery for a modernisation role?
You need to be able to read it, which is not the same as writing it. The skill these roles actually test is reading unfamiliar code and making two paradigms coexist safely.
Is modernisation work bad for my career?
No. It teaches you to read code you did not write and to migrate without regressions, which transfers to every codebase. It is also more often open to candidates without a long CV.
Does this title pay differently?
There is no separate federal wage for it. It sits in the same occupations as the rest of React work, which the main React guide explains, and the scope of the role rather than the title decides the band.
People Also Search For
JavaScript React developer jobs
Usually mixed-codebase or modernisation work, with the language tested separately from the library.
JavaScript interview fundamentals
Prepare two subjects: the language from ECMA-262, and the environment from the WHATWG HTML Standard.
JavaScript event loop interview
A browser specification question, not a language one. Tasks, microtasks and the nested timer clamp are the substance of it.
ECMAScript editions
Annual since ES2015, the sixth edition adopted on 17 June 2015, reaching the 16th edition in June 2025.
Legacy modernisation React
Incremental migration judged on the absence of regressions, which is a different discipline from shipping features.
React JS developer jobs USA
Often advertised by staffing firms, where a separate set of pay rules applies. Our contract jobs guide covers those.
Is jQuery still used
Enough that being able to read it is an asset on migration work, which is where a lot of these adverts sit.
React developer jobs USA
The market picture, the two occupations this work is benchmarked against, and the pay bands are on the main React guide.
Related career guides
- React Developer Jobs in USA — the main guide: pay bands, the toolchain that dates a portfolio, and the eligibility filter.
- React TypeScript Developer Jobs in USA — what types do at compile time and, crucially, what they do not do at runtime.
- Frontend Developer Coding Tests in USA — the assessment stage, and the adjustment you are entitled to ask for.
- React JS Developer Contract Jobs in USA — the staffing layer many of these adverts come from.
- Entry Level React Developer Jobs in USA — what to build, and how to read a junior advert.
Official sources
- Ecma International — ECMA-262, the ECMAScript Language Specification, and the June 2015 adoption of the 6th edition as ECMAScript 2015.
- TC39 — the current ECMAScript Language Specification draft.
- WHATWG — the HTML Standard, web application APIs and the Timers section.
- Bureau of Labor Statistics — Occupational Outlook Handbook for software developers and for web developers and digital designers.
This article is for general informational purposes. Specifications are revised and employer practice varies — confirm the current position with Ecma International, the WHATWG and the employer's own advertisement before relying on any of it.
