Array.isArray()

ES5+

Determines whether the passed value is an Array.

Syntax

Array.isArray(value)

Parameters

value any

The value to be checked

Return Value

boolean

true if the value is an Array; otherwise, false

Examples

JavaScript

console.log(Array.isArray([1, 2, 3]));
console.log(Array.isArray('hello'));
console.log(Array.isArray({length: 3}));

Output:

// true false false

📌 When to Use

Use Array.isArray() whenever you need to check if a value is an array. It's the most reliable method for array detection, working correctly across different JavaScript contexts (iframes, different realms) where instanceof Array fails.

⚠️ Common Mistakes

Using typeof arr === "object" - arrays return "object", not "array"

Using instanceof Array - fails across different iframe/window contexts

Checking for .length property - many objects have length (strings, NodeList, arguments)

✅ Best Practices

Always use Array.isArray() over instanceof Array for reliable detection

Combine with type guards in TypeScript: if (Array.isArray(x)) { /* x is array */ }

Use for input validation in functions that accept arrays or single values

⚡ Performance Notes

Array.isArray() is O(1) and very fast. It's implemented natively and has negligible overhead. It's faster than checking constructor.name or using Object.prototype.toString.call(). Safe to use in hot paths.

🌍 Real World Example

Normalizing Function Input to Always Be an Array

Create a function that accepts either a single value or an array and handles both uniformly

function processItems(input) {
  // Normalize input to always be an array
  const items = Array.isArray(input) ? input : [input];

  return items.map(item => transform(item));
}

// Works with both:
processItems('single');           // processes ['single']
processItems(['a', 'b', 'c']);    // processes ['a', 'b', 'c']

// Type checking in validation
function validateData(data) {
  if (!Array.isArray(data.tags)) {
    throw new Error('tags must be an array');
  }
  // data.tags is guaranteed to be an array here
}

Related Methods

Array.from()