assert

Asserts that a given condition is true. If the condition is false, it throws an error.

assert(condition, message);

Relationship with invariant

assert has exactly the same functionality as the invariant function. The only difference is the name. For more details, see the invariant documentation.

Reference

assert(condition, message)

Use assert when a specific condition must be satisfied in your code. If the condition is false, it immediately throws an error and stops program execution.

import { assert } from 'es-toolkit/util';

// If the condition is true, nothing happens
assert(true, 'This message will not appear');

// If the condition is false, it throws an error
assert(false, 'This condition is false'); // Error: This condition is false

// When checking that a value is not null or undefined
const value = getValue();
assert(value !== null && value !== undefined, 'Value must not be null or undefined');
// Now you can be sure that value is neither null nor undefined

// When checking if a number is positive
const number = getNumber();
assert(number > 0, 'Number must be positive');

You can also pass an error object directly.

import { assert } from 'es-toolkit/util';

// Passing an Error object
assert(false, new Error('Custom error message'));

// Using a custom error class
class ValidationError extends Error {
  constructor(message: string) {
    super(message);
    this.name = 'ValidationError';
  }
}

assert(false, new ValidationError('Validation failed'));

It's especially useful for verifying code assumptions during development or checking that function inputs are within expected ranges.

Parameters

• condition (unknown): The condition to evaluate. If it evaluates to a falsy value, an error is thrown.
• message (string | Error): The error message or error object to throw when the condition is false.

Returns

(void): Returns nothing if the condition is true.

Throws

Throws the provided message or error object if the condition evaluates to false.