[evalTokens] Refactor the last transformation stage

lib/eval.js

@@ -58,17 +58,14 @@ function evalTokens(tokens) {
   solvePowers(tokens);
   solveOpsWithProriorityOf2(tokens);
 
-  let next,
-    operator,
-    prev;
-  for (const token of tokens) {
-    if (token.type === Token.TYPES.OPERATOR) {
-      prev = operator ? operator.perform(prev, next) : next;
-      operator = token;
-      next = undefined;
-    } else next = token.value;
+  // now we only have numbers and priority 1 operators (+-)
+  // in the form of a+b-c+d-...
+  let number = tokens[0].value;
+  for (let i = 1; i < tokens.length; i += 2) {
+    const operator = tokens[i];
+    number = operator.perform(number, tokens[i + 1].value);
   }
-  return operator ? operator.perform(prev, next) : next;
+  return number;
 }
 
 /** @param {string} expr */