calculator.js

/* calculator in javascript using functional programming principles */

const operators = ["+", "*", "-", "/"];
//isOperator checks if a given character is an operator
const isOperator = char => operators.includes(char);

/* all operations to be used */
const operations = {
	"/": (a, b) => a / b,
	"*": (a, b) => a * b,
	"+": (a, b) => a + b,
	"-": (a, b) => a - b,
}

/* mergeOperation merge an operation around a given operator index
i.e : mergeOperation(["13", "+", "6", "*", "2"], 3)
--> ["13", "+", "12"]
*/
const mergeOperation = (calculation, i) => {
	if (i === -1) return calculation;
	const result = operations[calculation[i]](+calculation[i-1], +calculation[i+1]);
	return calculation.map((item, j) => i - 1 === j ? result : item)
	.filter((item, j) => j < i || j > i + 1);
}

/* getNextOperatorIndex retrieves the next operator index of a calculation array
prioritizing * and / operator
*/
const getNextOperatorIndex = calculation => {
	const nextIndex = calculation.findIndex(operator => ["*", "/"].includes(operator));
	return nextIndex !== -1 ? 
	nextIndex : 
	calculation.findIndex(operator => ["+", "-"].includes(operator));
}

/* operate reduces recursively a calculation untils all its operations have been done
i.e : ["13", "+", "6", "*", "2"]
--> ["13", "+", 12]
--> 25
*/
const operate = (calculation) => {
	if (calculation.length === 1) return calculation[0];
	return operate(mergeOperation(calculation, getNextOperatorIndex(calculation)))
}

/* formatCalculation converts a calculation string to an array
i.e : "13+6*2"
--> ["13", "+", "6", "*", "2"]
*/
const formatCalculation = (calculationString, arr = "") => {
	if (!calculationString) return arr.split(" ");
	const firstChar = calculationString[0];
	return formatCalculation(
		calculationString.slice(1),
		arr.concat(isOperator(firstChar) ? ` ${firstChar} ` : firstChar)
	)
}

/* takes a calculation string and returns a result */
const calculate = calculationString => {
	const calculation = formatCalculation(calculationString)
	return operate(calculation, getNextOperatorIndex(calculation));
}

console.log(calculate("13+6*2"));

I had to stop at line 20 because I 'm unable to understand starting from line 21.