Assert

Ajouté en: v0.1.21

Code source: lib/assert.js

The node:assert module provides a set of assertion functions for verifying invariants.

Strict assertion mode

In strict assertion mode, non-strict methods behave like their corresponding strict methods. For example, assert.deepEqual() will behave like assert.deepStrictEqual().

In strict assertion mode, error messages for objects display a diff. In legacy assertion mode, error messages for objects display the objects, often truncated.

To use strict assertion mode:

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Example error diff:

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To deactivate the colors, use the NO_COLOR or NODE_DISABLE_COLORS environment variables. This will also deactivate the colors in the REPL. For more on color support in terminal environments, read the tty getColorDepth() documentation.

Legacy assertion mode

Legacy assertion mode uses the == operator in:

• assert.deepEqual()
• assert.equal()
• assert.notDeepEqual()
• assert.notEqual()

To use legacy assertion mode:

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Legacy assertion mode may have surprising results, especially when using assert.deepEqual():

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C assert.AssertionError

• Extends: errors.Error

Indicates the failure of an assertion. All errors thrown by the node:assert module will be instances of the AssertionError class.

M new assert.AssertionError(options)

Ajouté en: v0.1.21

• options Object
  • message string If provided, the error message is set to this value.
  • actual any The actual property on the error instance.
  • expected any The expected property on the error instance.
  • operator string The operator property on the error instance.
  • stackStartFn Function If provided, the generated stack trace omits frames before this function.

A subclass of Error that indicates the failure of an assertion.

All instances contain the built-in Error properties (message and name) and:

• actual any Set to the actual argument for methods such as assert.strictEqual().
• expected any Set to the expected value for methods such as assert.strictEqual().
• generatedMessage boolean Indicates if the message was auto-generated (true) or not.
• code string Value is always ERR_ASSERTION to show that the error is an assertion error.
• operator string Set to the passed in operator value.

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C assert.CallTracker

Ajouté en: v14.2.0, v12.19.0

This feature is currently experimental and behavior might still change.

M new assert.CallTracker()

Ajouté en: v14.2.0, v12.19.0

Creates a new CallTracker object which can be used to track if functions were called a specific number of times. The tracker.verify() must be called for the verification to take place. The usual pattern would be to call it in a process.on('exit') handler.

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M tracker.calls([fn][, exact])

Ajouté en: v14.2.0, v12.19.0

• fn Function Default: A no-op function.
• exact number Default: 1.
• Returns: Function that wraps fn.

The wrapper function is expected to be called exactly exact times. If the function has not been called exactly exact times when tracker.verify() is called, then tracker.verify() will throw an error.

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M tracker.getCalls(fn)

Ajouté en: v16.18.0

• fn Function.

• Returns: Array with all the calls to a tracked function.

• Object Object

  • thisArg Object
  • arguments Array the arguments passed to the tracked function

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M tracker.report()

Ajouté en: v14.2.0, v12.19.0

• Returns: Array of objects containing information about the wrapper functions returned by tracker.calls().
• Object Object
  • message string
  • actual number The actual number of times the function was called.
  • expected number The number of times the function was expected to be called.
  • operator string The name of the function that is wrapped.
  • stack Object A stack trace of the function.

The arrays contains information about the expected and actual number of calls of the functions that have not been called the expected number of times.

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M tracker.reset([fn])

Ajouté en: v16.18.0

• fn Function a tracked function to reset.

reset calls of the call tracker. if a tracked function is passed as an argument, the calls will be reset for it. if no arguments are passed, all tracked functions will be reset

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M tracker.verify()

Ajouté en: v14.2.0, v12.19.0

Iterates through the list of functions passed to tracker.calls() and will throw an error for functions that have not been called the expected number of times.

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M assert(value[, message])

Ajouté en: v0.5.9

• value any The input that is checked for being truthy.
• message string | Error

An alias of assert.ok().

M assert.deepEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

Strict assertion mode

An alias of assert.deepStrictEqual().

Legacy assertion mode

Tests for deep equality between the actual and expected parameters. Consider using assert.deepStrictEqual() instead. assert.deepEqual() can have surprising results.

Deep equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules.

Comparison details

• Primitive values are compared with the == operator, with the exception of NaN. It is treated as being identical in case both sides are NaN.
• Type tags of objects should be the same.
• Only enumerable "own" properties are considered.
• Error names and messages are always compared, even if these are not enumerable properties.
• 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 both sides encounter a circular reference.
• Implementation does not test the [[Prototype]] of objects.
• Symbol properties are not compared.
• WeakMap and WeakSet comparison does not rely on their values.

The following example does not throw an AssertionError because the primitives are compared using the == operator.

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"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

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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 an Error then it will be thrown instead of the AssertionError.

M assert.deepStrictEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

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.
• Error names and messages are always compared, even if these are not enumerable properties.
• 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 both sides encounter a circular reference.
• WeakMap and WeakSet comparison does not rely on their values. See below for further details.

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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 an Error then it will be thrown instead of the AssertionError.

M assert.doesNotMatch(string, regexp[, message])

• string string
• regexp RegExp
• message string | Error

Expects the string input not to match the regular expression.

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If the values do match, or if the string argument is of another type than string, 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 an Error then it will be thrown instead of the AssertionError.

M assert.doesNotReject(asyncFn[, error][, message])

Ajouté en: v10.0.0

• asyncFn Function | Promise
• error RegExp | Function
• message string

Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is not rejected.

If asyncFn is a function and it throws an error synchronously, assert.doesNotReject() will return a rejected Promise with that error. If the function does not return a promise, assert.doesNotReject() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

Using assert.doesNotReject() is actually not useful because there is little benefit in catching a rejection and then rejecting it again. Instead, consider adding a comment next to the specific code path that should not reject and keep error messages as expressive as possible.

If specified, error can be a Class, RegExp, or a validation function. See assert.throws() for more details.

Besides the async nature to await the completion behaves identically to assert.doesNotThrow().

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M assert.doesNotThrow(fn[, error][, message])

• fn Function
• error RegExp | Function
• message string

Asserts that the function fn does not throw an error.

Using assert.doesNotThrow() is actually not useful because there is no benefit in catching an error and then rethrowing it. Instead, consider adding a comment next to the specific code path that should not throw and keep error messages as expressive as possible.

When assert.doesNotThrow() is called, it will immediately call the fn function.

If an error is thrown and it is the same type as that specified by the error parameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller.

If specified, error can be a Class, RegExp, or a validation function. See assert.throws() for more details.

The following, for instance, will throw the TypeError because there is no matching error type in the assertion:

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However, the following will result in an AssertionError with the message 'Got unwanted exception...':

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If an AssertionError is thrown and a value is provided for the message parameter, the value of message will be appended to the AssertionError message:

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M assert.equal(actual, expected[, message])

• actual any
• expected any
• message string | Error

Strict assertion mode

An alias of assert.strictEqual().

Legacy assertion mode

Tests shallow, coercive equality between the actual and expected parameters using the == operator. NaN is specially handled and treated as being identical if both sides are NaN.

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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 an Error then it will be thrown instead of the AssertionError.

M assert.fail([message])

Ajouté en: v0.1.21

• message string | Error Default: 'Failed'

Throws an AssertionError with the provided error message or a default error message. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

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Using assert.fail() with more than two arguments is possible but deprecated. See below for further details.

M assert.fail(actual, expected[, message[, operator[, stackStartFn]]])

• actual any
• expected any
• message string | Error
• operator string Default: '!='
• stackStartFn Function Default: assert.fail

If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. If just the two actual and expected arguments are provided, operator will default to '!='. If message is provided as third argument it will be used as the error message and the other arguments will be stored as properties on the thrown object. If stackStartFn is provided, all stack frames above that function will be removed from stacktrace (see Error.captureStackTrace). If no arguments are given, the default message Failed will be used.

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In the last three cases actual, expected, and operator have no influence on the error message.

Example use of stackStartFn for truncating the exception's stacktrace:

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M assert.ifError(value)

• value any

Throws value if value is not undefined or null. This is useful when testing the error argument in callbacks. The stack trace contains all frames from the error passed to ifError() including the potential new frames for ifError() itself.

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M assert.match(string, regexp[, message])

• string string
• regexp RegExp
• message string | Error

Expects the string input to match the regular expression.

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If the values do not match, or if the string argument is of another type than string, 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 an Error then it will be thrown instead of the AssertionError.

M assert.notDeepEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

Strict assertion mode

An alias of assert.notDeepStrictEqual().

Legacy assertion mode

Tests for any deep inequality. Opposite of assert.deepEqual().

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If the values are deeply 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 an Error then it will be thrown instead of the AssertionError.

M assert.notDeepStrictEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

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If the values are deeply and strictly 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 an Error then it will be thrown instead of the AssertionError.

M assert.notEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

Strict assertion mode

An alias of assert.notStrictEqual().

Legacy assertion mode

Tests shallow, coercive inequality with the != operator. NaN is specially handled and treated as being identical if both sides are NaN.

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If the values are 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 an Error then it will be thrown instead of the AssertionError.

M assert.notStrictEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

Tests strict inequality between the actual and expected parameters as determined by Object.is().

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If the values are strictly 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 an Error then it will be thrown instead of the AssertionError.

M assert.ok(value[, message])

• value any
• message string | Error

Tests if value is truthy. It is equivalent to assert.equal(!!value, true, message).

If value is not truthy, 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 an Error then it will be thrown instead of the AssertionError. If no arguments are passed in at all message will be set to the string: 'No value argument passed to `assert.ok()`'.

Be aware that in the repl the error message will be different to the one thrown in a file! See below for further details.

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M assert.rejects(asyncFn[, error][, message])

Ajouté en: v10.0.0

• asyncFn Function | Promise
• error RegExp | Function | Object | Error
• message string

Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is rejected.

If asyncFn is a function and it throws an error synchronously, assert.rejects() will return a rejected Promise with that error. If the function does not return a promise, assert.rejects() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

Besides the async nature to await the completion behaves identically to assert.throws().

If specified, error can be a Class, RegExp, a validation function, an object where each property will be tested for, or an instance of error where each property will be tested for including the non-enumerable message and name properties.

If specified, message will be the message provided by the AssertionError if the asyncFn fails to reject.

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error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Please read the example in assert.throws() carefully if using a string as the second argument gets considered.

M assert.strictEqual(actual, expected[, message])

• actual any
• expected any
• message string | Error

Tests strict equality between the actual and expected parameters as determined by Object.is().

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If the values are not strictly 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 an Error then it will be thrown instead of the AssertionError.

M assert.throws(fn[, error][, message])

• fn Function
• error RegExp | Function | Object | Error
• message string

Expects the function fn to throw an error.

If specified, error can be a Class, RegExp, a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable message and name properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples.

If specified, message will be appended to the message provided by the AssertionError if the fn call fails to throw or in case the error validation fails.

Custom validation object/error instance:

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Validate instanceof using constructor:

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Validate error message using RegExp:

Using a regular expression runs .toString on the error object, and will therefore also include the error name.

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Custom error validation:

The function must return true to indicate all internal validations passed. It will otherwise fail with an AssertionError.

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error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an ERR_AMBIGUOUS_ARGUMENT error. Please read the example below carefully if using a string as the second argument gets considered:

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Due to the confusing error-prone notation, avoid a string as the second argument.