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Functions

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Functions are the core building blocks of program architecture. TypeScript enhances standard functions with strict parameter and return type validation rules.

Video Explanation​

1. Explicit Type Annotations​

Functions require explicit annotations for parameters and final return profiles to ensure total type safety:

function calculateRatio(numerator: number, denominator: number): number {
    return numerator / denominator;
}

2. Optional & Default Parameter Behaviors​

Optional parameters are marked with a trailing question mark (?) and must follow mandatory arguments.

Default parameters fall back to preset values if arguments are omitted or passed as undefined.

function catalogItem(id: number, taxonomy?: string, label: string = "Generic"): void {
    console.log(`ID: ${id}, Class: ${taxonomy ?? "Unclassified"}, Item Label: ${label}`);
}

catalogItem(101); // Valid
catalogItem(102, "Hardware", "Smart Sensor"); // Valid

3. Rest Parameter Collections​

To accept an indefinite count of runtime arguments, wrap parameters with spread array notation structures:

function sumAllMeasurements(...measurements: number[]): number {
    return measurements.reduce((accumulator, current) => accumulator + current, 0);
}

4. Overloading Function Signatures​

TypeScript allows you to specify multiple function type configurations (overload signatures) before writing a single unified, type-safe implementation block:

// Overload signatures mapping structural interfaces
function formatInput(data: string): string;
function formatInput(data: number): string;

// Single unified implementation block mapping underlying variants
function formatInput(data: string | number): string {
    if (typeof data === "string") {
        return data.trim().toUpperCase();
    }
    return data.toFixed(2);
}
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