JS: Test Array Equality as JSON String 🐞

By Xah Lee. Date: . Last updated: .

Test Array Equality by Comparison as JSON String

One workaround of comparing array is to convert them into JSON string then compare the string.

🛑 WARNING: This got several problems, such as array involving undefined or sparse array. But even so, it is extremely inefficient. Do not do this for array with lots items or in a loop.

Problem: undefined vs null

JSON string converts undefined to null. 

// json convert undefined to null
console.assert(JSON.stringify([undefined, 5]) === JSON.stringify([null, 5]));

Problem: sparse array

// these 2 arrays are not same

const aa = [1, , 3];
const bb = [1, undefined, 3];

console.assert((Reflect.ownKeys(aa) === Reflect.ownKeys(bb)) === false);

// but in json they are the same
console.assert((JSON.stringify(aa) === JSON.stringify(bb)) === true);

Test Equality of Flat Arrays by Converting to JSON

If arrays are flat (no nesting), then one convenient way to test equality is by converting them to JSON.

// test equality of flat arrays by JSON.stringify
console.assert(JSON.stringify([4, 5, 6]) === JSON.stringify([4, 5, 6]));

Test Equality of Nested Arrays by Converting to JSON

Equality of content of nested true arrays can also be tested by first converting them to JSON.

// test equality of nested arrays
const aa = [4, [4, 5, 6], 6];
const bb = [4, [4, 5, 6], 6];
console.assert(JSON.stringify(aa) === JSON.stringify(bb));

// example of diff order. should be false
const cc = [4, [4, 6, 5], 6];
const dd = [4, [4, 5, 6], 6];
console.assert((JSON.stringify(cc) === JSON.stringify(dd)) === false);

JSON cannot be used to test equality of objects

🛑 WARNING: comparison array with nested object as json string doesn't work, because array element may be object, and their keys may have different order.

// array comparison by JSON.stringify is not reliable
const xx = [4, { "a": 1, "b": 2 }];
const yy = [4, { "b": 2, "a": 1 }];
console.log((JSON.stringify(xx) === JSON.stringify(yy)) === false);

Test Equality of Arrays-Like Objects

Best thing to do on Arrays-Like Objects, is to turn them into true array first. Use Array.from

but for curiosity, here's analysis.

Equality of content of flat Array-Like Objects can also be tested by first converting them to JSON.

// test equality of non-nested array-like objects by JSON.stringify

const aa = { 0: "a", 1: "b", length: 2 };
const bb = { 0: "a", 1: "b", length: 2 };
console.log(JSON.stringify(aa) === JSON.stringify(bb));
// it's ok if order are not the same, since they are still considered array-like objects
const xx = { 1: "b", 0: "a", length: 2 };
const yy = { 0: "a", 1: "b", length: 2 };
console.log(JSON.stringify(xx) === JSON.stringify(yy));

JavaScript. Test array equality by comparison as JSON string