javascript - What do curly braces mean in JSX (React)?

Question

For an example, to set a style in react you could do

var css = {color: red}

and

<h1 style={css}>Hello world</h1>

Why do you need the curly braces around css in the second code snippet?

Answer 1

The curly braces are a special syntax to let the JSX parser know that it needs to interpret the contents in between them as JavaScript instead of a string.

You need them when you want to use a JavaScript expression like a variable or a reference inside JSX. Because if you use the standard double quote syntax like so:

var css = { color: red }

<h1 style="css">Hello world</h1>

JSX doesn't know you meant to use the variable css in the style attribute instead of the string. And by placing the curly braces around the variable css, you are telling the parser "take the contents of the variable css and put them here". (Technically its evaluating the content)

This process is generally referred to as "interpolation".