The **`ifError`** method of the `assert` module in Node.js throws an error if the provided value is not `null` or `undefined`. The `ifError` method is commonly used to test `error` arguments in callbacks.

The process is illustrated below:

To use the `ifError` method, you will need to install the `assert` module using the command prompt, as shown below:

```Javascript
npm install assert
```

After the installation is complete, you will need to import the `assert` module into the program, as shown below:

```Java
const assert = require('assert');
```

The prototype of the `ifError` method is shown below:

```Java
ifError(value)
```

<h2>Paramaters</h2>

The `ifError` method takes a single mandatory parameter that represents the value to be checked for an error.

<h2>Return value</h2>

If `value` is not `null` or `undefined`, then the `ifError` method throws an [AssertionError](https://nodejs.org/api/assert.html#assert_class_assert_assertionerror), and the program terminates; otherwise, execution continues as normal.

<h2>Example</h2>

The code below shows how the `ifError` method works in Node.js:

const assert = require('assert');

// evaluating first expression
try{
  assert.ifError('23')
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}

// evaluating second expression
try{
  assert.ifError('error')
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}

// evaluating third expression
try{
  assert.ifError(null)
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}

// evaluating fourth expression
try{
  assert.ifError(undefined)
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}

<h2>Explanation</h2>

The code above uses $4$ different expressions to show the behavior of the `ifError` method.

In the first two expressions in lines $5$ and $14$, `value` is *"23"* and *"error"*, respectively. Since `value` is neither `null` nor `undefined`, the `ifError` method throws an error, which triggers the `catch` branch of the `try-catch` block. The code outputs the message associated with the error, i.e., the `value` parameter itself.

In the expressions in line $23$ and $32$, `value` is  `null` and `undefined`, respectively. Therefore, the `ifError` method does not throw any errors, and only the `try` branch of the `try-catch` block executes.

What is the ifError method of the assert module in Node.js?

Throws an error if the provided value is not null or undefined; used for testing error arguments in callbacks.