A contrived example showing the difference between pure and impure functions.

contrived-pure-function-example.js

class ImpureCalc {
    constructor() {
        this.currentTotal = 0;
    }
    addToTotal(valueToAdd) {
        this.currentTotal += valueToAdd;
        return this.currentTotal;
    }  
}

class PureCalc {
    addToTotal(currentTotal, valueToAdd) {
        return currentTotal + valueToAdd;
    }
}

const pureCalc = new PureCalc();
const impureCalc = new ImpureCalc();

console.log(`Impure call 1: ${impureCalc.addToTotal(5)}`);
console.log(`Impure call 2: ${impureCalc.addToTotal(5)}`);

console.log(`Pure call 1: ${pureCalc.addToTotal(0, 5)}`);
console.log(`Pure call 2: ${pureCalc.addToTotal(0, 5)}`);

// Prints:
// Impure call 1: 5
// Impure call 2: 10
// Pure call 1: 5
// Pure call 2: 5