Enforce: The Assertion Library for Vest

Enforce is a powerful assertion library that powers Vest's validations. It's designed to be:

• Fluent - Chain multiple assertions together naturally
• Composable - Build reusable validators from smaller pieces
• Extensible - Add your own custom rules

Basic Usage

Import enforce from Vest and use it inside your tests:

import { enforce, test } from 'vest';

test('username', 'Must be at least three characters long', () => {
  enforce(username).longerThan(2);
});

When an assertion fails, it throws an error that Vest catches and records as a failed test.

Fluent Chaining

Chain multiple assertions together to test various conditions:

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All assertions must pass for the test to pass. If any assertion fails, the test stops at that point.

Common Patterns

Validating Strings

// Required field
enforce(email).isNotBlank();

// Email format
enforce(email).isEmail();

// Length constraints
enforce(password).longerThanOrEquals(8).shorterThanOrEquals(128);

Validating Numbers

// Range check
enforce(age).isNumber().greaterThanOrEquals(18).lessThan(120);

// Positive number
enforce(price).isPositive();

Validating Objects

// Check shape/structure
enforce(user).shape({
  name: enforce.isString(),
  email: enforce.isEmail(),
  age: enforce.isNumber(),
});

Composing Rules

For rules you use together frequently, create reusable validators:

import { enforce, compose } from 'vest';

const isValidAge = compose(
  enforce.isNumber(),
  enforce.greaterThanOrEquals(18),
  enforce.lessThan(120),
);

// Use like any other rule
test('age', 'Must be a valid age', () => {
  enforce(data.age).condition(isValidAge);
});

Functional Programming

Enforce rules are just functions. The compose utility lets you build complex validators from simple, testable pieces - exactly like function composition in FP.

Learn more about composing rules →

Available Rules

Enforce comes with a rich set of built-in rules:

Category	Examples
Type Checks	isString(), isNumber(), isBoolean(), isArray()
String Rules	isNotBlank(), isEmail(), matches(), startsWith()
Number Rules	greaterThan(), lessThan(), isPositive(), isNegative()
Comparison	equals(), notEquals(), inside(), notInside()
Collection	lengthEquals(), longerThan(), shorterThan(), isEmpty()
Shape	shape(), loose(), isArrayOf()

View all rules →

Custom Rules

Need validation logic that isn't built-in? Create your own:

import { enforce } from 'vest';

enforce.extend({
  isValidUsername(value) {
    return /^[a-zA-Z0-9_]+$/.test(value);
  },
});

// Now use it anywhere
enforce(username).isValidUsername();

Learn more about custom rules →

Next Steps

• All Built-in Rules - Complete reference
• Composing Rules - Build reusable validators
• Custom Rules - Extend with your own logic