isEmpty

Write isEmpty(value). It returns true when value holds nothing and false otherwise, and it works for every kind of value.

null and undefined are empty. A string or an array is empty when its length is 0, and a Map or a Set when its size is 0.

Any other object, plain or a class instance, is empty when it has no own enumerable string keys: the ones Object.keys lists. Inherited and non-enumerable properties don't count, so Object.create({ a: 1 }) is empty.

Numbers (including 0 and NaN), booleans, symbols, bigints, and functions are never empty: they're values, not containers.

isEmpty(null);             // true
isEmpty("");               // true
isEmpty(new Map());        // true
isEmpty({});               // true
isEmpty({ a: undefined }); // false
isEmpty(new Set([0]));     // false
isEmpty(0);                // false
isEmpty(() => {});         // false

What the tests check

1. 1null and undefined are empty

   expect(isEmpty(null)).toBe(true);
   expect(isEmpty(undefined)).toBe(true);
   

2. 2strings and arrays are empty only when their length is 0

   expect(isEmpty("")).toBe(true);
   expect(isEmpty([])).toBe(true);
   expect(isEmpty(" ")).toBe(false);
   expect(isEmpty([0])).toBe(false);
   expect(isEmpty([undefined])).toBe(false);
   

3. 3Maps and Sets are empty only when their size is 0

   expect(isEmpty(new Map())).toBe(true);
   expect(isEmpty(new Set())).toBe(true);
   expect(isEmpty(new Map([["a", 1]]))).toBe(false);
   expect(isEmpty(new Set([0]))).toBe(false);
   

4. 4objects are empty when they have no own enumerable keys

   expect(isEmpty({})).toBe(true);
   expect(isEmpty(Object.create(null))).toBe(true);
   expect(isEmpty({ a: 1 })).toBe(false);
   expect(isEmpty({ a: undefined })).toBe(false);
   

5. 5inherited and non-enumerable properties don't count

   expect(isEmpty(Object.create({ a: 1 }))).toBe(true);
   expect(isEmpty(Object.defineProperty({}, "hidden", { value: 1, enumerable: false }))).toBe(true);
   

6. 6class instances follow the object rule

   class Empty {
     method() {}
   }
   class Point {
     constructor() {
       this.x = 1;
     }
   }
   expect(isEmpty(new Empty())).toBe(true);
   expect(isEmpty(new Point())).toBe(false);
   

7. 7numbers, booleans, symbols, and bigints are never empty

   for (const value of [0, NaN, 42, false, true, Symbol("s"), 0n]) {
     expect(isEmpty(value)).toBe(false);
   }
   

8. 8functions are never empty

   expect(isEmpty(() => {})).toBe(false);
   expect(isEmpty(function named() {})).toBe(false);
   

Learn more

• Checking types reliablyLesson
• Map & SetLesson
• Enumerability & ownershipLesson