---
title: "Introduce Assertion"
type: "refactoring-technique"
slug: "introduce-assertion"
url: "http://localhost:3000/en/introduce-assertion.md"
category: "Simplifying Conditional Expressions"
description: "Problem: For a portion of code to work correctly, certain conditions or values must be true. Solution: Replace these assumptions with specific assertion checks."
languages: ["java", "csharp", "php", "python", "typescript"]
---
# Introduce Assertion

> Problem: For a portion of code to work correctly, certain conditions or values must be true. Solution: Replace these assumptions with specific assertion checks.

## Problem

For a portion of code to work correctly, certain conditions or values must be true.

## Solution

Replace these assumptions with specific assertion checks.

## Why Refactor

Say that a portion of code assumes something about, for example, the current condition of an object or value of a parameter or local variable. Usually this assumption will always hold true except in the event of an error.

Make these assumptions obvious by adding corresponding assertions. As with type hinting in method parameters, these assertions can act as live documentation for your code.

As a guideline to see where your code needs assertions, check for comments that describe the conditions under which a particular method will work.

## Benefits

* If an assumption isn’t true and the code therefore gives the wrong result, it’s better to stop execution before this causes fatal consequences and data corruption. This also means that you neglected to write a necessary test when devising ways to perform testing of the program.

## How to Refactor

When you see that a condition is assumed, add an assertion for this condition in order to make sure.

Adding the assertion shouldn’t change the program’s behavior.

Don’t overdo it with use of assertions for **everything** in your code. Check for only the conditions that are necessary for correct functioning of the code. If your code is working normally even when a particular assertion is false, you can safely remove the assertion.
## Relations

**Eliminates smells**

- [Comments](/en/smells/comments.md)

## Code Examples

### java

```java
// Before
double getExpenseLimit() {
  // Should have either expense limit or
  // a primary project.
  return (expenseLimit != NULL_EXPENSE) ?
    expenseLimit :
    primaryProject.getMemberExpenseLimit();
}

// After
double getExpenseLimit() {
  Assert.isTrue(expenseLimit != NULL_EXPENSE || primaryProject != null);

  return (expenseLimit != NULL_EXPENSE) ?
    expenseLimit:
    primaryProject.getMemberExpenseLimit();
}
```

### csharp

```csharp
// Before
double GetExpenseLimit() 
{
  // Should have either expense limit or
  // a primary project.
  return (expenseLimit != NULL_EXPENSE) ?
    expenseLimit :
    primaryProject.GetMemberExpenseLimit();
}

// After
double GetExpenseLimit() 
{
  Assert.IsTrue(expenseLimit != NULL_EXPENSE || primaryProject != null);

  return (expenseLimit != NULL_EXPENSE) ?
    expenseLimit:
    primaryProject.GetMemberExpenseLimit();
}
```

### php

```php
// Before
function getExpenseLimit() {
  // Should have either expense limit or
  // a primary project.
  return ($this->expenseLimit !== NULL_EXPENSE) ?
    $this->expenseLimit:
    $this->primaryProject->getMemberExpenseLimit();
}

// After
function getExpenseLimit() {
  assert($this->expenseLimit !== NULL_EXPENSE || isset($this->primaryProject));

  return ($this->expenseLimit !== NULL_EXPENSE) ?
    $this->expenseLimit:
    $this->primaryProject->getMemberExpenseLimit();
}
```

### python

```python
// Before
def getExpenseLimit(self):
    # Should have either expense limit or
    # a primary project.
    return self.expenseLimit if self.expenseLimit != NULL_EXPENSE else \
        self.primaryProject.getMemberExpenseLimit()

// After
def getExpenseLimit(self):
    assert (self.expenseLimit != NULL_EXPENSE) or (self.primaryProject != None)

    return self.expenseLimit if (self.expenseLimit != NULL_EXPENSE) else \
        self.primaryProject.getMemberExpenseLimit()
```

### typescript

```typescript
// Before
getExpenseLimit(): number {
  // Should have either expense limit or
  // a primary project.
  return (expenseLimit != NULL_EXPENSE) ?
    expenseLimit:
    primaryProject.getMemberExpenseLimit();
}

// After
getExpenseLimit(): number {
  // TypeScript and JS doesn't have built-in assertions, so we'll use
  // good-old console.error(). You can always extract this into a
  // designated assertion function.
  if (!(expenseLimit != NULL_EXPENSE ||
       (typeof primaryProject !== 'undefined' && primaryProject))) {
      console.error("Assertion failed: getExpenseLimit()");
  }

  return (expenseLimit != NULL_EXPENSE) ?
    expenseLimit:
    primaryProject.getMemberExpenseLimit();
}
```