This is a simple javascript test exercise.

fluid_calculator.js

class FluidCalc {
    constructor() {
        // Array of operation objects
        this.operations = [];

        // Stores the first number in the chain
        this.first_number = null;

        // The latest operation in the chain
        this.current_operation = null;
    }

    one() {
        this.process_number(1);

        return this;
    }

    two() {
        this.process_number(2);

        return this;
    }

    three() {
        this.process_number(3);

        return this;
    }

    plus() {
        this.current_operation = '+';

        return this;
    }

    minus() {
        this.current_operation = '-';

        return this;
    }

    times() {
        this.current_operation = '*';

        return this;
    }

    /**
     *  Called for each number method of the class;
     *  Determine if this is the first number the chain, otherwise bundle number into an operator object using
     *  the latest operator used.
     */
    process_number(n) {
        if (this.first_number === null) {
            this.first_number = n;
            return;
        }

        // Operator Object
        let operation = {
            op: this.current_operation, // operation (+, -, *, etc...)
            value: n // value to apply operation to.
        };

        this.operations.push(operation);

        // Clear the last number used so we can deliniate the start of the next operation.
        this.current_number = null;
    }

    /**
     *  Given a starting value, this method will apply the operation specified in the object passed in using the value in the object.
     */
    perform_operation(value, operation) {
        if (operation.op === '+') {
            return value + operation.value;
        }

        if (operation.op === '-') {
            return value - operation.value;
        }

        if (operation.op === '*') {
            return value * operation.value;
        }
    }

    /**
     * Calculate a final value by iterating over the array of operations and appying each to the final result.
     */
    equals() {
        // Our operation starts with the first number passed in.
        let final_value = this.first_number;

        this.operations.forEach(item => {
            final_value = this.perform_operation(final_value, item);
        });

        return final_value;
    }
}

/** ----------------------------------- */

let calculator_obj = new FluidCalc();

let test = calculator_obj
    .one()
    .plus()
    .two()
    .minus()
    .three();

console.log(test.equals());