assert.deepStrictEqual

History

assert.deepStrictEqual(actual, expected, message?): void

Attributes

actual:any

expected:any

message:string | Error | Function

Tests for deep equality between the actual and expected parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules.

Comparison details

Primitive values are compared using Object.is().
Type tags of objects should be the same.
[[Prototype]] of objects are compared using the === operator.
Only enumerable "own" properties are considered.
Object constructors are compared when available.
Error names, messages, causes, and errors are always compared, even if these are not enumerable properties. errors is also compared.
Enumerable own Symbol properties are compared as well.
Object wrappers are compared both as objects and unwrapped values.
Object properties are compared unordered.
Map keys and Set items are compared unordered.
Recursion stops when both sides differ or either side encounters a circular reference.
WeakMap, WeakSet and Promise instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different WeakMap, WeakSet, or Promise instances will result in inequality, even if they contain the same content.
RegExp lastIndex, flags, and source are always compared, even if these are not enumerable properties.

import assert from 'node:assert/strict';

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   {
// +   a: 1
// -   a: '1'
//   }

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}

assert.deepStrictEqual(NaN, NaN);
// OK because Object.is(NaN, NaN) is true.

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.

assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
//   Symbol(): 1
// }

const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap();
const obj = {};

weakMap1.set(obj, 'value');
weakMap2.set(obj, 'value');

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakMap1, weakMap2);
// AssertionError: Values have same structure but are not reference-equal:
//
// WeakMap {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakMap1, weakMap1);
// OK

const weakSet1 = new WeakSet();
const weakSet2 = new WeakSet();
weakSet1.add(obj);
weakSet2.add(obj);

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakSet1, weakSet2);
// AssertionError: Values have same structure but are not reference-equal:
// + actual - expected
//
// WeakSet {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakSet1, weakSet1);
// OK

JavaScript

const assert = require('node:assert/strict');

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   {
// +   a: 1
// -   a: '1'
//   }

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}

assert.deepStrictEqual(NaN, NaN);
// OK because Object.is(NaN, NaN) is true.

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.

assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
//   Symbol(): 1
// }

const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap();
const obj = {};

weakMap1.set(obj, 'value');
weakMap2.set(obj, 'value');

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakMap1, weakMap2);
// AssertionError: Values have same structure but are not reference-equal:
//
// WeakMap {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakMap1, weakMap1);
// OK

const weakSet1 = new WeakSet();
const weakSet2 = new WeakSet();
weakSet1.add(obj);
weakSet2.add(obj);

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakSet1, weakSet2);
// AssertionError: Values have same structure but are not reference-equal:
// + actual - expected
//
// WeakSet {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakSet1, weakSet1);
// OK

JavaScript

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of Error then it will be thrown instead of the AssertionError.