How to test a value x against predicate function and returns fn(x) or x in JavaScript ?

In this article, we are given a value x, and the task is to check the value against the predicate function. If the value satisfies the condition or predicate return fn(x) else return x.

Predicate functions are the function that takes one item as input and returns true or false based on the whether item satisfies that condition or not.

Example 1: In this example, we have given a number and a condition, if condition is true then it returns the fn(number) otherwise it returns a simple numeric value.

Javascript

<script>
    // Function that we have to return
    function fn(x) {
        let fact = 1;
        for (i = 1; i <= x; i++) {
            fact *= i;
        }

        return fact;
    }

    // Predicate function checking even value
    function isEven(x) {
        return (x % 2 === 0);
    }


    // Function returning fn(x) if predicate
    // function return true else return x
    function test(x) {
        let e = isEven(x);

        if (e) {
            return fn(x);

        } else
            return x;
    }

    // Printing return value
    let ans = test(6);
    console.log(`This function returns ${ans}`);
</script>

Output: Here the condition is a number that should be even because our input data is even so it returns the fn(6) value.

This function returns 720

Example 2: In this example, we have given a number and a test condition, if condition is true then it returns the fn(number) otherwise it returns number value only. 

Javascript

<script>
    // Initializing the max_Integer
    let max_Integer = 20;

    // Function we have to return
    function fn(x) {
        var b = 0;
        var a = 1;
        var sum;
        for (i = 0; i < x - 1; i++) {
            sum = a + b;
            b = a;
            a = sum;
        }
        return a;
    }

    // Predicate function checking value
    // is less than max_Integer
    function isless(x) {
        return (x < max_Integer);
    }

    // Function return fn(x) if predicate
    // function return true else return x
    function test(x) {
        let e = isless(x);

        if (e) {
            return fn(x);

        } else
            return x;
    }

    // Printing return value
    let ans = test(15);
    console.log(`This function returns ${ans}`);
</script>

Output:

The function returns 610

Example 3: In this example, we have given a number and a test condition, if condition is true then it returns the fn(number) otherwise it returns number value only. 

Javascript

<script>
    // Function we have to return
    function fn(x) {

        let j = 1;
        for (i = 0; i < x - 1; i++) {
            j = j * x;
        }
        return j;
    }

    // Predicate function checking
    // multiple of 3
    function ismultiple(x) {
        return (x % 3 == 0);
    }

    // Function that returns fn(x) if
    // predicate function return true
    // else return false
    function test(x) {
        let e = ismultiple(x);

        if (e) {
            return fn(x);

        } else
            return x;
    }

    // Printing value
    let ans = test(6);
    console.log(`This function returns ${ans}`);
</script>

Output:

This function returns 7776