Understanding the Key Differences Between | and || in TypeScript

This blog post will show the difference between these two operators.

In TypeScript, operators are essential tools that allow us to manipulate data and control the flow of applications. Among the various operators, the pipe symbol (|) and the double pipe symbol (||) are often used, but they serve very different purposes.

Understanding the difference between these two operators is crucial for writing efficient and error-free code.

What is this?

The single pipe (|) in TypeScript is known as the union type operator. It allows a variable to hold one of several types, essentially broadening the range of acceptable values for that variable.

On the other hand, the double pipe (||) is the logical OR operator, used to evaluate expressions and return the first truthy value encountered, or the last value if all are falsy.

Why is this?

The union type (|) is vital for situations where a variable can legitimately represent multiple types, offering flexibility while maintaining type safety.

The logical OR operator (||), however, is often used in control flow to set default values or short-circuit evaluations, making it a powerful tool in conditional statements.

Use Case

• Union Type (|): Suppose we're dealing with a function that can return a string or a number based on certain conditions. Using the union type allows us to define a return type that accommodates both possibilities.

  function getId(id: string | number): string {
    return `ID: ${id}`;
  }

• Logical OR (||): We might want to assign a default value to a variable if it’s undefined or null. The logical OR operator helps us to do that efficiently.

  const username = inputUsername || 'Guest';

Pros and Cons

• Union Type (|):

  • Pros:
  • Enhances flexibility and type safety.
  • Reduces the need for extensive type checks.

• Logical OR (||):

  • Pros:
  • Simplifies conditional logic.
  • Provides a quick way to set default values.
  • Cons:
  • May lead to unintended consequences if falsy values are legitimate (e.g., 0, '').
  • Overuse can reduce code clarity.

Conclusion

Both the union type (|) and the logical OR (||) operators are powerful tools in TypeScript, each with its specific use cases and benefits.

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