src/language-js/printer-estree.js/printAssignment
function printAssignment(
  leftNode,
  printedLeft,
  operator,
  rightNode,
  printedRight,
  options
) {
  if (!rightNode) {
    return printedLeft;
  }

  const canBreak =
    (isBinaryish(rightNode) && !shouldInlineLogicalExpression(rightNode)) ||
    (rightNode.type === "ConditionalExpression" &&
      isBinaryish(rightNode.test) &&
      !shouldInlineLogicalExpression(rightNode.test)) ||
    ((leftNode.type === "Identifier" ||
      isStringLiteral(leftNode) ||
      leftNode.type === "MemberExpression") &&
      (isStringLiteral(rightNode) || isMemberExpressionChain(rightNode)));

  const printed = printAssignmentRight(
    rightNode,
    printedRight,
    canBreak,
    options
  );

  return group(concat([printedLeft, operator, printed]));
}