June 17, 2017Last modified May 9, 2025

Javascript code snippets

Array flattener

Implement a function flatten that returns a newly-created array with all sub-array elements concatenated recursively into a single level.

export default function flatten(value) {
  if (!Array.isArray(value)) {
    return [value];
  }

  let flattenedArray = [];

  for (let i = 0; i < value.length; i++) {
    if (Array.isArray(value[i])) {
      flattenedArray = flattenedArray.concat(flatten(value[i]));
    } else {
      flattenedArray.push(value[i]);
    }
  }

  return flattenedArray;
}

export default function flatten(value) {
  if (!Array.isArray(value)) {
    return [value];
  }

  let flattenedArray = [];

  for (let i = 0; i < value.length; i++) {
    if (Array.isArray(value[i])) {
      flattenedArray = flattenedArray.concat(flatten(value[i]));
    } else {
      flattenedArray.push(value[i]);
    }
  }

  return flattenedArray;
}

Deep equal

Implement a function deepEqual that performs a deep comparison between two values. It returns true if two input values are deemed equal, and returns false if not.

export default function deepEqual(a, b) {

  // case - both values are same in type and value 
  if (a === b) {
    return true;
  }

  // case - both values have different type
  if (Array.isArray(a) !== Array.isArray(b)) {
    return false;
  }

  // case - no value is null
  if (
    a === null ||
    typeof a !== "object" ||
    b === null ||
    typeof b !== "object"
  ) {
    return false;
  }

  const keysA = Object.keys(a);
  const keysB = Object.keys(b);

  // case - both objects have diff no of keys
  if (keysA.length !== keysB.length) {
    return false;
  }

  for (const k of keysA) {
    // case - values of both keys are same
    if (!keysB.includes(k) || !deepEqual(a[k], b[k])) {
      return false;
    }
  }

  return true;
}

export default function deepEqual(a, b) {

  // case - both values are same in type and value 
  if (a === b) {
    return true;
  }

  // case - both values have different type
  if (Array.isArray(a) !== Array.isArray(b)) {
    return false;
  }

  // case - no value is null
  if (
    a === null ||
    typeof a !== "object" ||
    b === null ||
    typeof b !== "object"
  ) {
    return false;
  }

  const keysA = Object.keys(a);
  const keysB = Object.keys(b);

  // case - both objects have diff no of keys
  if (keysA.length !== keysB.length) {
    return false;
  }

  for (const k of keysA) {
    // case - values of both keys are same
    if (!keysB.includes(k) || !deepEqual(a[k], b[k])) {
      return false;
    }
  }

  return true;
}

getElementsByClassName() Implementation

getElementsByClassName() is a method which exists on HTML Documents and Elements to return an HTMLCollection of descendant elements within the Document/Element which has the specified class name(s).

export default function getElementsByClassName(element, classNames) {
  let elements = [];
  const classes = new Set(classNames.trim().split(/\s+/));

  function isSubset(a, b) {
    return Array.from(a).every((cls) => b.contains(cls));
  }

  function traverse(el) {
    // case - all nodes traversed
    if (el == null) {
      return;
    }

    // case - if element contains all classes
    if (isSubset(classes, el.classList)) {
      elements.push(el);
    }

    for (const child of el.children) {
      traverse(child);
    }
  }

  for (const ch of element.children) {
    traverse(ch);
  }

  return elements;
}

export default function getElementsByClassName(element, classNames) {
  let elements = [];
  const classes = new Set(classNames.trim().split(/\s+/));

  function isSubset(a, b) {
    return Array.from(a).every((cls) => b.contains(cls));
  }

  function traverse(el) {
    // case - all nodes traversed
    if (el == null) {
      return;
    }

    // case - if element contains all classes
    if (isSubset(classes, el.classList)) {
      elements.push(el);
    }

    for (const child of el.children) {
      traverse(child);
    }
  }

  for (const ch of element.children) {
    traverse(ch);
  }

  return elements;
}

JSON.stringify

Implement a function jsonStringify, similar to JSON.stringify that converts a JavaScript value into a JSON string.

export default function jsonStringify(value) {
  if (value === null) return "null";

  let valueType = typeof value;

  if (Array.isArray(value)) {
    valueType = "array";
  }

  switch (valueType) {
    case "array":
      const allVals = value.map((v) => jsonStringify(v)).join(",");
      return `[${allVals}]`;
    case "object":
      const objVals = Object.entries(value)
        .map(([key, value]) => `"${key}":${jsonStringify(value)}`)
        .join(",");
      return `{${objVals}}`;
    case "string":
      return `"${value}"`;
    default:
      return String(value);
  }
}

export default function jsonStringify(value) {
  if (value === null) return "null";

  let valueType = typeof value;

  if (Array.isArray(value)) {
    valueType = "array";
  }

  switch (valueType) {
    case "array":
      const allVals = value.map((v) => jsonStringify(v)).join(",");
      return `[${allVals}]`;
    case "object":
      const objVals = Object.entries(value)
        .map(([key, value]) => `"${key}":${jsonStringify(value)}`)
        .join(",");
      return `{${objVals}}`;
    case "string":
      return `"${value}"`;
    default:
      return String(value);
  }
}