Check if a string is balanced

is-balanced.js

/**
 * Check if given string is balanced, i.e., if it has the same number of open and close delimiters.
 * The following delimiters are considered: '(', ')', '[', ']', '{', '}'.
 * A string with no delimiters is considered balanced.
 * 
 * @link https://twitter.com/coproduto/status/1782352142050775113
 * @param str {String} to check if it is balanced
 * @returns {Boolean}
 */
export function isBalanced(str) {
  const stack = []
  const openDelimiters = ['(', '[', '{']
  const closeDelimiters = [')', ']', '}']

  for (let i = 0; i < str.length; i++) {
    const char = str[i]

    if (openDelimiters.includes(char)) {
      stack.push(char)
      continue
    }
    
    if (closeDelimiters.includes(char)) {
      if (stack.length === 0) {
        return false
      }

      const lastOpenDelimiter = stack.pop()

      if (!lastOpenDelimiter) {
        return false
      }

      const correspondingCloseDelimiter = closeDelimiters[openDelimiters.indexOf(lastOpenDelimiter)]

      if (char !== correspondingCloseDelimiter) {
        return false
      }
    }
  }

  return stack.length === 0;
}

is-balanced.test.js

import assert from 'node:assert'
import { describe, it } from 'node:test'

import { isBalanced } from './challenge.js'

describe('isBalanced', () => {
  it('should return true for a balanced string', () => {
    assert.strictEqual(isBalanced('abcd'), true)
    assert.strictEqual(isBalanced('(abcd)'), true)
    assert.strictEqual(isBalanced('(abc[%def])'), true)
    assert.strictEqual(isBalanced('$(abc[de]fg{hi}jk)%//'), true)
  })

  it('should return false for an unbalanced string', () => {
    assert.strictEqual(isBalanced('(abcd'), false)
    assert.strictEqual(isBalanced('abcd]'), false)
    assert.strictEqual(isBalanced('(abc]d'), false)
    assert.strictEqual(isBalanced('(({abc})'), false)
    assert.strictEqual(isBalanced('(ab[cd)ef]'), false)
    assert.strictEqual(isBalanced('{ab(cd}ef)'), false)
  })
})