ReactJS component names must begin with capital letters?

Question

In JSX, lower-case tag names are considered to be HTML tags. However, lower-case tag names with a dot (property accessor) aren't.

See HTML tags vs React Components.

• <component /> compiles to React.createElement('component') (html tag)
• <Component /> compiles to React.createElement(Component)
• <obj.component /> compiles to React.createElement(obj.component)

@Alexandre Kirszenberg gave a very good answer, just wanted to add another detail.

React used to contain a whitelist of well-known element names like div etc, which it used to differentiate between DOM elements and React components.

But because maintaining that list isn't all that fun, and because web components makes it possible to create custom elements, they made it a rule that all React components must start with a upper case letter, or contain a dot.

From the official React reference:

When an element type starts with a lowercase letter, it refers to a built-in component like or and results in a string 'div' or 'span' passed to React.createElement. Types that start with a capital letter like compile to React.createElement(Foo) and correspond to a component defined or imported in your JavaScript file.

Also note that:

We recommend naming components with a capital letter. If you do have a component that starts with a lowercase letter, assign it to a capitalized variable before using it in JSX.

Which means one has to use:

const Foo = foo; before using foo as a Component element in JSX.

The first part of a JSX tag determines the type of the React element, basically there is some convention Capitalized, lowercase, dot-notation.

Capitalized and dot-notation types indicate that the JSX tag is referring to a React component, so if you use the JSX <Foo /> compile to React.createElement(Foo)
OR
<foo.bar /> compile to React.createElement(foo.bar) and correspond to a component defined or imported in your JavaScript file.

While the lowercase type indicate to a built-in component like <div> or <span> and results in a string 'div' or 'span' passed to React.createElement('div').

React recommend naming components with a capital letter. If you do have a component that starts with a lowercase letter, assign it to a capitalized variable before using it in JSX.

In JSX, React Classes are capitalized to make XML compatible, so that it is not mistaken for an HTML tag. If the react classes are not capitalized, it is an HTML tag as pre-defined JSX syntax.

User define components must be Capitalized

When an element type starts with a lowercase letter, it refers to a built-in component like <div> or <span> and results in a string 'div' or 'span' passed to React.createElement. Types that start with a capital letter like <Foo /> compile to React.createElement(Foo) and correspond to a component defined or imported in your JavaScript file.

React recommend naming components with a capital letter. If you do have a component that starts with a lowercase letter, assign it to a capitalized variable before using it in JSX.

For example, this code will not run as expected:

import React from 'react';

// Wrong! This is a component and should have been capitalized:
function hello(props) {
  // Correct! This use of <div> is legitimate because div is a valid HTML tag:
  return <div>Hello {props.toWhat}</div>;
}

function HelloWorld() {
  // Wrong! React thinks <hello /> is an HTML tag because it's not capitalized:
  return <hello toWhat="World" />;
}

To fix this, we will rename hello to Hello and use <Hello /> when referring to it:

import React from 'react';

// Correct! This is a component and should be capitalized:
function Hello(props) {
  // Correct! This use of <div> is legitimate because div is a valid HTML tag:
  return <div>Hello {props.toWhat}</div>;
}

function HelloWorld() {
  // Correct! React knows <Hello /> is a component because it's capitalized.
  return <Hello toWhat="World" />;
}

Here is the reference