js
function isValid(str) {
 const stack = [];
 const tokens = ["()", "{}", "[]"];
 const map = tokens.reduce((acc, [open, close]) => {
  acc.opening[open] = close;
  acc.closing[close] = open;
  return acc;
 }, { opening: {}, closing: {} });
 for (let i = 0; i < str.length; i++) {
  if (map.opening[str[i]]) {
   stack.push(str[i]);
  }
  if (map.closing[str[i]]) {
   const lastToken = stack.pop();
   if (lastToken !== map.closing[str[i]]) {
    return false;
   }
  }
 }
 return stack.length === 0;
}

js
function isValid(str) {
 const stack = [];
 const tokens = ["()", "{}", "[]"];
 const map = tokens.reduce(
  (acc, [opening, closing]) => Object.assign(acc, { opening, closing }),
  { opening: {}, closing: {} },
 );
 for (let i = 0; i < str.length; i++) {
  if (map.opening[str[i]]) {
   stack.push(str[i]);
  }
  if (map.closing[str[i]]) {
   const lastToken = stack.pop();
   if (lastToken !== map.closing[str[i]]) {
    return false;
   }
  }
 }
 return stack.length === 0;
}

js
function isValid(str) {
 const stack = [];
 const tokens = ["()", "{}", "[]"];
 const map = tokens.reduce(
  (acc, [opening, closing]) => Object.assign(acc, { opening, closing }),
  { opening: {}, closing: {} },
 );
 for (let i = 0; i < str.length; i++) {
  if (map.opening[str[i]]) {
   stack.push(str[i]);
  }
  if (map.closing[str[i]]) {
   const lastToken = stack.pop();
   if (lastToken !== map.closing[str[i]]) {
    return false;
   }
  }
 }
 return stack.length === 0;
}