How to create a function that solves basic arithmetic operations

JavaScript, algorithms, math, recursion

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 REPL
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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