Assert

An assertion library with user-friendly error messages

Features

• When an assertion succeeds, the value is returned. Useful for assertion + assignment.

• Throws appropriate error types for each assertion (e.g. TypeError, RangeError, etc.)

• Error messages include the invalid value, humanized and sanitized

• You can customize the field name used in error messages

Installation

You can install this library via npm.

npm install @jsdevtools/assert

Usage

The exported assert function supports fluent-like chaining with various assertion functions. Each of the assertion functions returns the value if valid, or throws an error if invalid.

assert.value(value, [fieldName], [defaultValue])

Asserts that a value that is not undefined. Any other value will pass, even null and NaN.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.value(0);                    // ✔
assert.value(false);                // ✔
assert.value(null);                 // ✔
assert.value(NaN);                  // ✔

assert.value(undefined);            // ❌ Invalid value: undefined. A value is required.

assert.value.oneOf(value, values, [fieldName], [defaultValue])

Assets that a value that is one of the specified values.

• value - The value to check
• values - The allowed values
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.value.oneOf("a", ["a", "b", "c"]);     // ✔
assert.value.oneOf(4, [1, 2, 3, 4, 5]);       // ✔
assert.value.oneOf(true, [1, true, "yes"]);   // ✔

assert.value.oneOf("b", ["x", "y", "z"]);     // ❌ Invalid value: "b". Expected "x", "y", or "z".

assert.type(value, type, [fieldName], [defaultValue])

Asserts that a value is the specified type.

• value - The value to check
• type - The expected type. This can be a class, a primitive wrapper (e.g. String), null, or undefined
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type("John", String);          // ✔
assert.type(42, Number);              // ✔
assert.type(false, Boolean);          // ✔
assert.type(null, null);              // ✔
assert.type({ x: 1 }, Object);        // ✔
assert.type(/^regex$/, RegExp);       // ✔
assert.type(new Date(), Date);        // ✔

assert.type("Fred", Object);          // ❌ Invalid value: "Fred". Expected an Object.
assert.type(100, BigInt);             // ❌ Invalid value: 100. Expected a bigint.
assert.type(undefined, null);         // ❌ Invalid value: undefined. Expected null.
assert.type(null, Object);            // ❌ Invalid value: null. Expected an Object.
assert.type(new Date(), RangeError);  // ❌ Invalid value: Date. Expected a RangeError.

assert.type.oneOf(value, types, [fieldName], [defaultValue])

Asserts that a value is one of the specified types.

• value - The value to check
• types - An array of the allowed types.
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.oneOf("John", [Number, String, Boolean]);           // ✔
assert.type.oneOf(42, [Number, BigInt, Date]);                  // ✔
assert.type.oneOf(null, [null, undefined]);                     // ✔
assert.type.oneOf(new RangeError(), [TypeError, RangeError]);   // ✔

assert.type.oneOf("Fred", [Number, Boolean, Object]);           // ❌ Invalid value: "Fred". Expected a number, boolean, or Object.
assert.type.oneOf(undefined, [Boolean, Number, null]);          // ❌ Invalid value: undefined. Expected a boolean, number, or null.
assert.type.oneOf(new SyntaxError(), [TypeError, RangeError]);  // ❌ Invalid value: SyntaxError. Expected a TypeError or RangeError.

assert.type.string(value, [fieldName], [defaultValue])

Asserts that a value is a primitive string (including empty strings).

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.string("John");                      // ✔
assert.type.string("");                          // ✔
assert.type.string("           ");               // ✔
assert.type.string("\n");                        // ✔
assert.type.string("\t");                        // ✔

assert.type.string(123);                         // ❌ Invalid value: 123. Expected a string.
assert.type.string(null);                        // ❌ Invalid value: null. Expected a string.
assert.type.string(new String());                // ❌ Invalid value: String. Expected a string.

assert.type.number(value, [fieldName], [defaultValue])

Asserts that a value is a primitive number. NaN is not considered a number.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.number(0)                     // ✔
assert.type.number(123)                   // ✔
assert.type.number(-42.1245)              // ✔
assert.type.number(Math.PI)               // ✔
assert.type.number(Infinity)              // ✔

assert.type.number("123");                // ❌ Invalid value: "123". Expected a number.
assert.type.number(NaN);                  // ❌ Invalid value: NaN. Expected a number.
assert.type.number(new Number());         // ❌ Invalid value: Number. Expected a number.

assert.type.boolean(value, [fieldName], [defaultValue])

Asserts that a value is a primitive boolean value.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.boolean(false)                 // ✔
assert.type.boolean(true)                  // ✔

assert.type.boolean("true");               // ❌ Invalid value: "true". Expected a boolean.
assert.type.boolean(0);                    // ❌ Invalid value: 0. Expected a boolean.
assert.type.boolean(1);                    // ❌ Invalid value: 1. Expected a boolean.
assert.type.boolean(new Boolean());        // ❌ Invalid value: Boolean. Expected a boolean.

assert.type.object(value, [fieldName], [defaultValue])

Asserts that a value is an object. null is not considered an object.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.object({})                 // ✔
assert.type.object(/^regex$/)          // ✔
assert.type.object(new Date())         // ✔
assert.type.object(new Object())       // ✔
assert.type.object(Object.prototype)   // ✔

assert.type.object(null);              // ❌ Invalid value: null. Expected an object.
assert.type.object(undefined);         // ❌ Invalid value: undefined. A value is required.
assert.type.object(Object);            // ❌ Invalid value: function. Expected an object.

assert.type.array(value, [fieldName], [defaultValue])

Asserts that a value is an array.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.array([])                  // ✔
assert.type.array([1, 2, 3])           // ✔
assert.type.array(new Array())         // ✔
assert.type.array(Array.prototype)     // ✔

assert.type.array("hello");            // ❌ Invalid value: "hello". Expected an array.
assert.type.array(Array);              // ❌ Invalid value: function. Expected an array.

assert.type.function(value, [fieldName], [defaultValue])

Asserts any type of function, including async, generators, arrow functions, classes, etc.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.type.function(Object)                   // ✔
assert.type.function(Object.toString)          // ✔
assert.type.function(function foo() {})        // ✔
assert.type.function(() => null)               // ✔
assert.type.function(class Foo {})             // ✔

assert.type.function(null);                    // ❌ Invalid value: null. Expected a function.
assert.type.function(new Object());            // ❌ Invalid value: {}. Expected a function.
assert.type.function("function");              // ❌ Invalid value: "function". Expected a function.

assert.string(value, [fieldName], [defaultValue])

This is an alias for assert.type.string()

assert.string.nonEmpty(value, [fieldName], [defaultValue])

Asserts that a value is a string with at least one character (including whitespace).

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.nonEmpty("John")                   // ✔
assert.string.nonEmpty("    ")                   // ✔
assert.string.nonEmpty("\n")                     // ✔
assert.string.nonEmpty("\t")                     // ✔

assert.string.nonEmpty("");                      // ❌ Invalid value: "". It cannot be empty.
assert.string.nonEmpty(null);                    // ❌ Invalid value: null. Expected a string.
assert.string.nonEmpty(new String());            // ❌ Invalid value: String. Expected a string.

assert.string.nonWhitespace(value, [fieldName], [defaultValue])

Asserts that a value is a string with at least one non-whitespace character.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.nonWhitespace("John")                   // ✔
assert.string.nonWhitespace("  a  ")                  // ✔

assert.string.nonWhitespace("");                      // ❌ Invalid value: "". It cannot be empty.
assert.string.nonWhitespace("    ");                  // ❌ Invalid value: "    ". It cannot be all whitespace.
assert.string.nonWhitespace("\n")                     // ❌ Invalid value: "\n". It cannot be all whitespace.
assert.string.nonWhitespace("\t")                     // ❌ Invalid value: "\t". It cannot be all whitespace.
assert.string.nonWhitespace(new String());            // ❌ Invalid value: String. Expected a string.

assert.string.minLength(value, minLength, [fieldName], [defaultValue])

Asserts that a value is a string with at least the specified number of characters (including whitespace).

• value - The value to check
• minLength - The minimum allowed length
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.minLength("John", 1)                // ✔
assert.string.minLength("  a  ", 5)               // ✔
assert.string.minLength("", 0)                    // ✔

assert.string.minLength("", 1);                   // ❌ Invalid value: "". It cannot be empty.
assert.string.minLength("John", 10);              // ❌ Invalid value: "John". It should be at least 10 characters.

assert.string.maxLength(value, maxLength, [fieldName], [defaultValue])

Asserts that a value is a string with no more than the specified number of characters (including whitespace).

• value - The value to check
• maxLength - The maximum allowed length
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.maxLength("John", 10)               // ✔
assert.string.maxLength("  a  ", 5)               // ✔
assert.string.maxLength("", 50)                   // ✔

assert.string.maxLength("John Doe", 5);           // ❌ Invalid value: "John Doe". It cannot be more than 5 characters.

assert.string.length(value, minLength, [maxLength], [fieldName], [defaultValue])

Asserts that a value is a string with the specified number of characters (including whitespace)

• value - The value to check
• minLength - The minimum allowed length
• maxLength - (optional) The maximum allowed length. If not specified, it defaults to the same as minLength.
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.length("John", 1, 10)            // ✔
assert.string.length("  a  ", 5, 25)           // ✔
assert.string.length("", 0, 100)               // ✔

assert.string.length("John Doe", 1, 5);        // ❌ Invalid value: "John Doe". It cannot be more than 5 characters.
assert.string.length("John Doe", 20, 50);      // ❌ Invalid value: "John Doe". It should be at least 20 characters.
assert.string.length("John Doe", 5);           // ❌ Invalid value: "John Doe". It must be exactly 5 characters.

assert.string.pattern(value, regex, [fieldName], [defaultValue])

Asserts that a value is a string that matches the specified Regular Expression pattern.

• value - The value to check
• regex - The regular expression to test
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.pattern("Foo", /^fo+$/i);                 // ✔
assert.string.pattern("image.jpg", /\.(jpg|jpeg)/);     // ✔
assert.string.pattern("", /^\w*$/);                     // ✔

assert.string.pattern("foobar", /^fo+$/i);              // ❌ Invalid value: "foobar". It must match /^fo+$/i.
assert.string.pattern("jpeg.gif", /\.(jpg|jpeg)/);      // ❌ Invalid value: "jpeg.gif". It must match /\\.(jpg|jpeg)/.

You can customize the assertion error message by adding an example or examples property to the RegExp object.

import assert from "@jsdevtools/assert";

let pattern = /\.(jpg|jpeg)/;
pattern.example = "image.jpg";
assert.string.pattern("jpeg.gif", pattern);       // ❌ Invalid value: "jpeg.gif". It should look like "image.jpg".

let pattern = /\.(jpg|jpeg)/;
pattern.examples = ["image.jpg", "image.jpeg"];
assert.string.pattern("jpeg.gif", pattern);       // ❌ Invalid value: "jpeg.gif". It should look like "image.jpg" or "image.jpeg".

assert.string.enum(value, enum, [fieldName], [defaultValue])

Asserts that a value is a string that is a member of the specified enumeration object.

• value - The value to check
• enum - The object whose values are the enumveration members
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.string.enum("foo", { Foo: "foo", Bar: "bar" });            // ✔
assert.string.enum("3", { One: "1", Two: "2", Three: "3" });      // ✔
assert.string.enum("", { None: "", Some: "some", All: "all" });   // ✔

assert.string.enum("Bar", { Foo: "foo", Bar: "bar" });            // ❌ Invalid value: "Bar". Expected foo or bar.
assert.string.enum("5", { One: "1", Two: "2", Three: "3" });      // ❌ Invalid value: "5". Expected 1, 2, or 3.
assert.string.enum("", { None: "none", Some: "some" });           // ❌ Invalid value: "". Expected none or some.

assert.number(value, [fieldName], [defaultValue])

This is an alias for assert.type.number()

assert.number.positive(value, [fieldName], [defaultValue])

Asserts that a value is a positive number (greater than zero).

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.number.positive(42);              // ✔
assert.number.positive(12345.678);       // ✔
assert.number.positive(Infinity);        // ✔
assert.number.positive(Math.PI);         // ✔

assert.number.positive(0);               // ❌ Invalid value: 0. Expected a positive number.
assert.number.positive(-42);             // ❌ Invalid value: -42. Expected a positive number.
assert.number.positive(NaN);             // ❌ Invalid value: NaN. Expected a number.

assert.number.nonNegative(value, [fieldName], [defaultValue])

Asserts that a value is a number that is zero or greater.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.number.nonNegative(0);               // ✔
assert.number.nonNegative(42);              // ✔
assert.number.nonNegative(12345.6789);      // ✔
assert.number.nonNegative(Infinity);        // ✔
assert.number.nonNegative(Math.PI);         // ✔

assert.number.nonNegative(-42);             // ❌ Invalid value: -42. Expected zero or greater.
assert.number.nonNegative(-Infinity);       // ❌ Invalid value: -Infinity. Expected zero or greater.
assert.number.nonNegative(NaN);             // ❌ Invalid value: NaN. Expected a number.

assert.number.integer(value, [fieldName], [defaultValue])

Asserts that a value is an integer value (positive or negative).

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.number.integer(0);               // ✔
assert.number.integer(42);              // ✔
assert.number.integer(-42);             // ✔
assert.number.integer(12345.0);         // ✔

assert.number.integer(Math.PI);         // ❌ Invalid value: 3.141592653589793. Expected an integer.
assert.number.integer(Infinity);        // ❌ Invalid value: Infinity. Expected an integer.
assert.number.integer(NaN);             // ❌ Invalid value: NaN. Expected a number.

assert.number.integer.positive(value, [fieldName], [defaultValue])

Asserts that a value is a positive integer value (one or more).

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.number.integer.positive(42);              // ✔
assert.number.integer.positive(12345.0);         // ✔

assert.number.integer.positive(0);               // ❌ Invalid value: 0. Expected a positive integer.
assert.number.integer.positive(-42);             // ❌ Invalid value: -42. Expected a positive integer.
assert.number.integer.positive(Infinity);        // ❌ Invalid value: Infinity. Expected an integer.
assert.number.integer.positive(NaN);             // ❌ Invalid value: NaN. Expected a number.

assert.number.integer.nonNegative(value, [fieldName], [defaultValue])

Asserts that a value is an integer value that is zero or greater.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.number.integer.nonNegative(0);               // ✔
assert.number.integer.nonNegative(42);              // ✔
assert.number.integer.nonNegative(12345.0);         // ✔

assert.number.integer.nonNegative(-42);             // ❌ Invalid value: -42. Expected zero or greater.
assert.number.integer.nonNegative(-Infinity);       // ❌ Invalid value: -Infinity. Expected an integer.
assert.number.integer.nonNegative(NaN);             // ❌ Invalid value: NaN. Expected a number.

assert.array(value, [fieldName], [defaultValue])

This is an alias for assert.type.array()

assert.array.nonEmpty(value, [fieldName], [defaultValue])

Asserts that a value is an array with at least one item.

• value - The value to check
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.array.nonEmpty(["a", "b", "c"]);         // ✔
assert.array.nonEmpty(new Array(1, 2, 3));      // ✔

assert.array.nonEmpty([]);                      // ❌ Invalid value: Array. It cannot be empty.
assert.array.nonEmpty(new Array());             // ❌ Invalid value: Array. It cannot be empty.

assert.array.minLength(value, minLength, [fieldName], [defaultValue])

Asserts that a value is an array with at least the specified number of items.

• value - The value to check
• minLength - The minimum allowed length
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.array.minLength([], 0);                   // ✔
assert.array.minLength(["a", "b", "c"], 1);      // ✔

assert.array.minLength([], 1);                   // ❌ Invalid value: Array. It cannot be empty.
assert.array.minLength(["a", "b", "c"], 10);     // ❌ Invalid value: [a,b,c]. It should have at least 10 items.

assert.array.maxLength(value, maxLength, [fieldName], [defaultValue])

Asserts that a value is an array with no more than the specified number of items.

• value - The value to check
• maxLength - The maximum allowed length
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.array.maxLength([], 5);                   // ✔
assert.array.maxLength(["a", "b", "c"], 3);      // ✔

assert.array.maxLength([1, 2, 3], 1);            // ❌ Invalid value: [1,2,3]. It cannot have more than 1 item.
assert.array.maxLength(["a", "b", "c"], 2);      // ❌ Invalid value: [a,b,c]. It cannot have more than 2 items.

assert.array.length(value, minLength, [maxLength], [fieldName], [defaultValue])

Asserts that a value is an array with the specified number of items.

• value - The value to check
• minLength - The minimum allowed length
• maxLength - (optional) The maximum allowed length. If not specified, it defaults to the same as minLength.
• fieldName - (optional) The name of the field being assertd. This is used in the error message if the assertion fails.
• defaultValue - (optional) The default value to use if value is undefined.

import assert from "@jsdevtools/assert";

assert.array.length([], 0);                   // ✔
assert.array.length([], 0, 5);                // ✔
assert.array.length(["a", "b", "c"], 3);      // ✔
assert.array.length(["a", "b", "c"], 1, 3);   // ✔

assert.array.length([], 1);                   // ❌ Invalid value: Array. It cannot be empty.
assert.array.length([1, 2, 3], 5, 10);        // ❌ Invalid value: [1,2,3]. It should have at least 5 items.
assert.array.length(["a", "b", "c"], 2);      // ❌ Invalid value: [a,b,c]. It must have exactly 2 items.

Contributing

Contributions, enhancements, and bug-fixes are welcome! Open an issue on GitHub and submit a pull request.

Building

To build the project locally on your computer:

1. Clone this repo
   git clone https://github.com/JS-DevTools/assert.git

2. Install dependencies
   npm install

3. Build the code
   npm run build

4. Run the tests
   npm test

License

@jsdevtools/assert is 100% free and open-source, under the MIT license. Use it however you want.

This package is Treeware. If you use it in production, then we ask that you buy the world a tree to thank us for our work. By contributing to the Treeware forest you’ll be creating employment for local families and restoring wildlife habitats.

Big Thanks To

Thanks to these awesome companies for their support of Open Source developers ❤