Example Functions in JavaScript

examples.js

// Example functions with comparison and logical operators
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators#equality_operators
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#comparison_operators

// returns true or false if the input variable is greater than 2.
// 3 > 2 is true
// 2.01 > 2 is true
// 2 > 2 is false because 2 is not greater than 2
// 1 > 2 is false 
function isGreaterThanTwo(x) {
    return x > 2;
}

// returns true or false if the input variable is less than 2.
function isLessThanTwo(x) {
    return x < 2;
}

function isTwo(x) {
    return x == 2;
}



function isNotTwo(n) {
    return n != 2;
}


// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_NOT
function not(x) {
    return !x;
}



function notNot(x) {
    return !!x;
}



function isEmptyString(str) {
    return str == "";
}



function aintEmptyString(str) {
    return str != "";
}



function isTheLetterA(str) {
    str = str.toLowerCase();
    return str == "a";
}


// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR
function isPorQ(x) {
    x = x.toLowerCase();
    return x == "p" || x == "q";
}



function isLessThanFive(x) {
    return x < 5;
}



function isGreaterThanFive(x) {
    return x > 5;
}



function isGreaterThanOrEqualToFive(x) {
    return x >= 5;
}



// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND
function betweenTenAndTwenty(x) {
    return 10 < x && x < 20;
}


// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR
function is6or12(x) {
    return x === 6 || x == 12;
}


// Notice each condition, each predicate, will live on either the left or the right side of an AND or an OR operator. 
// What would happen if we had 3 different conditions?
// The following code will check for a number above 1, below 10, and that the number is even.
function isAboveOneLessThan10AndEven(x) {
    return x > 1 && x < 10 && x % 2 == 0;
}

// Another approach to building up multiple conditions is to build tiny functions and use them together. 
// Consider the above example written with 3 helper functions

function isGreaterThan1(x) {
    return x > 1;
}

function isLessThan10(x) {
    return x < 10;
}

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

// Notice how this approach utilizes previously built functions in order to produce code that reads more like English.
function isAboveOneLessThan10AndEven(x) {
    return isGreaterThan1(x) && isLessThan10(x) && isEven(x);
}