let isAdditionOperator =
    (token) => (token === '+' || token === 'add')

let isSubtractionOperator =
    (token) => (token === '-' || token ==='sub')

let isMultiplicationOperator =
    (token) => (token === '*' || token === 'mul')

let isDivisionOperator =
    (token) => (token === '/' || token == 'div')

let isNumberLiteral =
    (token) => !isNaN(token)

let calculate = (tokens) => {
    let currentToken = tokens.shift();

    if(isNumberLiteral(currentToken)) {
        return parseFloat(currentToken);
    }
    if(isAdditionOperator(currentToken)) {
        return calculate(tokens) + calculate(tokens);
    }
    if(isSubtractionOperator(currentToken)) {
        return - calculate(tokens);
    }
    if(isMultiplicationOperator(currentToken)) {
        return calculate(tokens) * calculate(tokens);
    }
    if(isDivisionOperator(currentToken)) {
        return calculate(tokens) / calculate(tokens);
    }
    throw 'Invalid token ' + currentToken;
}
let expression = "* + 1 2 * 3 + 7 - 2"
let tokens = expression.split(' ');
console.log(calculate(tokens))