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# assert
[](https://travis-ci.org/browserify/commonjs-assert)
This module is used for writing unit tests for your applications, you can access it with `require('assert')`.
It aims to be fully compatibe with the [node.js assert module](http://nodejs.org/api/assert.html), same API and same behavior, just adding support for web browsers.
The API and code may contain traces of the [CommonJS Unit Testing 1.0 spec](http://wiki.commonjs.org/wiki/Unit_Testing/1.0) which they were based on, but both have evolved significantly since then.
A `strict` and a `legacy` mode exist, while it is recommended to only use `strict mode`.
## Strict mode
When using the `strict mode`, any `assert` function will use the equality used in the strict function mode. So `assert.deepEqual()` will, for example, work the same as `assert.deepStrictEqual()`.
It can be accessed using:
```js
const assert = require('assert').strict;
```
## Legacy mode
> Deprecated: Use strict mode instead.
When accessing `assert` directly instead of using the `strict` property, the
[Abstract Equality Comparison](https://tc39.github.io/ecma262/#sec-abstract-equality-comparison) will be used for any function without a
"strict" in its name (e.g. `assert.deepEqual()`).
It can be accessed using:
```js
const assert = require('assert');
```
It is recommended to use the `strict mode` instead as the Abstract Equality Comparison can often have surprising results. Especially
in case of `assert.deepEqual()` as the used comparison rules there are very lax.
E.g.
```js
// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());
```
## assert.fail(actual, expected, message, operator)
Throws an exception that displays the values for actual and expected separated by the provided operator.
## assert(value, message), assert.ok(value, [message])
Tests if value is truthy, it is equivalent to assert.equal(true, !!value, message);
## assert.equal(actual, expected, [message])
Tests shallow, coercive equality with the equal comparison operator ( == ).
## assert.notEqual(actual, expected, [message])
Tests shallow, coercive non-equality with the not equal comparison operator ( != ).
## assert.deepEqual(actual, expected, [message])
Tests for deep equality.
## assert.deepStrictEqual(actual, expected, [message])
Tests for deep equality, as determined by the strict equality operator ( === )
## assert.notDeepEqual(actual, expected, [message])
Tests for any deep inequality.
## assert.strictEqual(actual, expected, [message])
Tests strict equality, as determined by the strict equality operator ( === )
## assert.notStrictEqual(actual, expected, [message])
Tests strict non-equality, as determined by the strict not equal operator ( !== )
## assert.throws(block, [error], [message])
Expects block to throw an error. error can be constructor, regexp or validation function.
Validate instanceof using constructor:
```javascript
assert.throws(function() { throw new Error("Wrong value"); }, Error);
```
Validate error message using RegExp:
```javascript
assert.throws(function() { throw new Error("Wrong value"); }, /value/);
```
Custom error validation:
```javascript
assert.throws(function() {
throw new Error("Wrong value");
}, function(err) {
if ( (err instanceof Error) && /value/.test(err) ) {
return true;
}
}, "unexpected error");
```
## assert.doesNotThrow(block, [message])
Expects block not to throw an error, see assert.throws for details.
## assert.ifError(value)
Tests if value is not a false value, throws if it is a true value. Useful when testing the first argument, error in callbacks.