Test a Value Against a Predicate Function in JavaScript

In JavaScript, we can test a value against a predicate function and apply another function to it when the condition is satisfied. If the predicate returns true, fn(x) is returned; otherwise, the original value x is returned.

• A predicate function takes a value as input and returns true or false.
• The specified function is executed only when the predicate returns true.
• If the predicate returns false, the original value is returned.
• This approach is useful for performing conditional transformations on values.

Approach 1: Using an Even Number Predicate

In this approach, the value is checked using an isEven() predicate function. If the value is even, its factorial is calculated; otherwise, the original value is returned.

function factorial(x) {
    let fact = 1;

    for (let i = 1; i <= x; i++) {
        fact *= i;
    }

    return fact;
}

function isEven(x) {
    return x % 2 === 0;
}

function test(x) {
    if (isEven(x)) {
        return factorial(x);
    }

    return x;
}

const result = test(6);

console.log(`This function returns ${result}`);

This function returns 720

Approach 2: Using a Comparison Predicate

In this approach, the predicate function checks whether the given value is less than a specified maximum value. If the condition is satisfied, the Fibonacci value is returned; otherwise, the original value is returned.

const maxValue = 20;

function fibonacci(x) {
    if (x <= 1) {
        return x;
    }

    let previous = 0;
    let current = 1;

    for (let i = 2; i <= x; i++) {
        const next = previous + current;
        previous = current;
        current = next;
    }

    return current;
}

function isLess(x) {
    return x < maxValue;
}

function test(x) {
    if (isLess(x)) {
        return fibonacci(x);
    }

    return x;
}

const result = test(15);

console.log(`This function returns ${result}`);

This function returns 610

Approach 3: Using a Multiple-of-Three Predicate

In this approach, the predicate function checks whether the given value is a multiple of 3. If the condition is satisfied, the value is raised to its own power; otherwise, the original value is returned.

function powerOfSelf(x) {
    return x ** x;
}

function isMultipleOfThree(x) {
    return x % 3 === 0;
}

function test(x) {
    if (isMultipleOfThree(x)) {
        return powerOfSelf(x);
    }

    return x;
}

const result = test(6);

console.log(`This function returns ${result}`);

This function returns 46656

Approach 4: Using a Higher-Order Function

A more reusable approach is to pass the predicate function and the function to be executed as arguments to a higher-order function.

• The predicate function checks the condition.
• The fn function performs the required operation.
• If the predicate returns true, fn(x) is returned.
• Otherwise, x is returned.

function test(x, predicate, fn) {
    if (predicate(x)) {
        return fn(x);
    }

    return x;
}

function isEven(x) {
    return x % 2 === 0;
}

function square(x) {
    return x * x;
}

console.log(test(6, isEven, square));
console.log(test(5, isEven, square));