Here is one way to implement a function that takes a string as an input.
The string contains an arithmetic operation
1+2*3/4.
There are no parens or negative numbers.
calculatorTry in REPL1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
 function solve(str) { const toAdd = str.split('+'); const toSubtract = str.split(''); const toMultiply = str.split('*'); const toDivide = str.split('/'); if (str.length === 1) { return +str; } else if (toAdd.length >= 2) { return toAdd.reduce((accum, elem) => solve('' + accum) + solve(elem)); } else if (toSubtract.length >= 2) { return toSubtract.reduce((accum, elem) => solve('' + accum)  solve(elem)); } else if (toMultiply.length >= 2) { return toMultiply.reduce((accum, elem) => solve('' + accum) * solve(elem)); } else if (toDivide.length >= 2) { return toDivide.reduce((accum, elem) => solve('' + accum) / solve(elem)); } else { return +toAdd[0]; } }

Tests can be found at REPL.it