quick-lint-js

let headingLinks = [];
for (let el of $("a")) {
  if (el.parentNode.tag = "H1") {
    headingLinks.push(el);
  }
}

Expected a conditional expression and instead saw an assignment;
Unexpected constant condition

let headingLinks = [];
for (let el of $("a")) {
  if (el.parentNode.tag = "H1") {
    headingLinks.push(el);
  }
}

'=' changes variables; to compare, use '===' instead [E0188]

function TodoEntry({title, children}) {
  return (
    <h3>{title}</h3>
    <div className="body">{children}</div>
  );
}

Adjacent JSX elements must be wrapped in an enclosing tag

function TodoEntry({title, children}) {
  return (
    <h3>{title}</h3>
    <div className="body">{children}</div>
  );
}

missing '<>' and '</>' to enclose multiple children;
children end here [E0189]

function Link({text, ...props}) {
  return <a {props}>{text}</a>;
} 

Parsing error: Unexpected token props

function Link({text, ...props}) {
  return <a {props}>{text}</a>;
} 

missing '...' in JSX attribute spread [E0186]