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Decision Tables in TypeScript: An Underrated Pattern for Cleaner Code

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Decision Tables in TypeScript: An Underrated Pattern for Cleaner Code

Decision tables are a powerful tool for managing complex business logic in software development. While they've been around for decades, they often go overlooked in favor of more traditional coding techniques. In TypeScript, specifically, decision tables can be a game-changer, leading to cleaner, more maintainable code.

Why Use Decision Tables?

Traditional conditional logic, often expressed using nested if-else statements, can become convoluted and difficult to understand as the number of conditions increases. Decision tables offer a more structured and intuitive approach, making it easier to:

Visualize complex logic:
Decision tables present a clear and visual representation of the decision-making process, making it easier to grasp the logic at a glance.
Improve maintainability:
Changes to the logic can be easily implemented by updating the decision table without having to hunt through nested conditionals.
Reduce errors:
The structured format of decision tables helps prevent logic errors by ensuring that all possible combinations of conditions are considered.
Enhance collaboration:
Decision tables can facilitate collaboration between developers and domain experts by providing a common, easy-to-understand language for expressing business rules.

Building Decision Tables in TypeScript

Let's explore how to implement decision tables in TypeScript using a practical example:

Define the Table Structure

First, we define a data structure to represent our decision table. This structure typically involves:

Input conditions: These are the factors that influence the decision.
Output actions: These are the actions taken based on the input conditions.
Rules: Each rule defines a specific combination of input conditions and the corresponding output action.

Here's an example of a basic decision table structure in TypeScript:


interface DecisionTable {
inputConditions: string[];
outputActions: string[];
rules: Rule[];
}


interface Rule {

  conditions: boolean[];

  action: string;

}

Implement the Decision Logic

Now, we create a function that uses the decision table to determine the appropriate output action based on the input conditions:


function evaluateDecisionTable(table: DecisionTable, inputs: boolean[]): string {
for (const rule of table.rules) {
if (rule.conditions.every((condition, index) => condition === inputs[index])) {
  return rule.action;
}
}
// Handle cases where no rule matches the input conditions
return "No matching rule found";
}

Create and Use the Table

Finally, we create an instance of our decision table and use the evaluateDecisionTable function to execute the logic:


const decisionTable: DecisionTable = {
inputConditions: ["IsMember", "IsPremium", "HasDiscount"],
outputActions: ["ApplyRegularPrice", "ApplyMemberDiscount", "ApplyPremiumDiscount", "ApplySpecialDiscount"],
rules: [
{ conditions: [false, false, false], action: "ApplyRegularPrice" },
{ conditions: [true, false, false], action: "ApplyMemberDiscount" },
{ conditions: [false, true, false], action: "ApplyPremiumDiscount" },
{ conditions: [true, true, false], action: "ApplyPremiumDiscount" }, // Premium overrides Member
{ conditions: [false, false, true], action: "ApplySpecialDiscount" },
{ conditions: [true, false, true], action: "ApplySpecialDiscount" }, // Special overrides Member
{ conditions: [false, true, true], action: "ApplySpecialDiscount" }, // Special overrides Premium
{ conditions: [true, true, true], action: "ApplySpecialDiscount" },
],
};

const inputs = [true, true, false]; // IsMember: true, IsPremium: true, HasDiscount: false
const action = evaluateDecisionTable(decisionTable, inputs);
console.log(Action: ${action}); // Output: Action: ApplyPremiumDiscount

In this example, the decision table evaluates the customer's membership status, premium membership status, and any existing discount to determine the correct pricing action.

Improving Code Readability

While the above example demonstrates the basic principles, we can further improve readability and maintainability by using more descriptive data structures and functions:



interface Customer {

  isMember: boolean;

  isPremium: boolean;

  hasDiscount: boolean;

}

enum PricingAction {

  RegularPrice = "ApplyRegularPrice",

  MemberDiscount = "ApplyMemberDiscount",

  PremiumDiscount = "ApplyPremiumDiscount",

  SpecialDiscount = "ApplySpecialDiscount",

}

const pricingRules: PricingRule[] = [

  { conditions: { isMember: false, isPremium: false, hasDiscount: false }, action: PricingAction.RegularPrice },

  { conditions: { isMember: true, isPremium: false, hasDiscount: false }, action: PricingAction.MemberDiscount },

  { conditions: { isMember: false, isPremium: true, hasDiscount: false }, action: PricingAction.PremiumDiscount },

  { conditions: { isMember: true, isPremium: true, hasDiscount: false }, action: PricingAction.PremiumDiscount }, 

  { conditions: { isMember: false, isPremium: false, hasDiscount: true }, action: PricingAction.SpecialDiscount },

  { conditions: { isMember: true, isPremium: false, hasDiscount: true }, action: PricingAction.SpecialDiscount }, 

  { conditions: { isMember: false, isPremium: true, hasDiscount: true }, action: PricingAction.SpecialDiscount },

  { conditions: { isMember: true, isPremium: true, hasDiscount: true }, action: PricingAction.SpecialDiscount },

];

interface PricingRule {

  conditions: Customer;

  action: PricingAction;

}

function getPricingAction(customer: Customer): PricingAction {

  const matchingRule = pricingRules.find(rule => {

    return rule.conditions.isMember === customer.isMember &&

      rule.conditions.isPremium === customer.isPremium &&

      rule.conditions.hasDiscount === customer.hasDiscount;

  });

return matchingRule ? matchingRule.action : PricingAction.RegularPrice; // Default to regular price if no rule matches

}

const customer: Customer = {

  isMember: true,

  isPremium: true,

  hasDiscount: false,

};


const action = getPricingAction(customer);


console.log(Action: ${action}); // Output: Action: ApplyPremiumDiscount

This approach utilizes enums for improved readability, enhances data structures to represent customer information, and refactors the logic into a more readable function.

Best Practices for Decision Tables in TypeScript

Keep tables concise:

Avoid overly large and complex decision tables. Break down complex logic into smaller, more manageable tables.
Use descriptive names:

Choose meaningful names for conditions, actions, and rules to improve readability and maintainability.
Validate input conditions:

Ensure that the input conditions are properly validated to prevent unexpected behavior.
Document the tables:

Add comments to explain the purpose and logic behind each table and rule.
Consider tools:

Explore tools like decision table editors or libraries to streamline the creation and maintenance of decision tables.

Conclusion

Decision tables are a powerful and often overlooked pattern in TypeScript. By leveraging their structure and visual clarity, you can significantly enhance the readability, maintainability, and correctness of your code. Remember to apply best practices, document your tables thoroughly, and consider utilizing tools to maximize their benefits. As your codebase grows in complexity, decision tables can prove to be an invaluable asset for managing and evolving business logic.