replace null to undefined

lib/Symbols.js

@@ -19,7 +19,7 @@ Symbols.getSymbolsInExpression = function(expression) {
 };
 
 // Iterates through a node and returns the last term with the symbol name
-// Returns null if no terms with the symbol name are in the node.
+// Returns undefined if no terms with the symbol name are in the node.
 // e.g. 4x^2 + 2x + y + 2 with `symbolName=x` would return 2x
 Symbols.getLastSymbolTerm = function(node, symbolName) {
   // First check if the node itself is a polyomial term with symbolName
@@ -43,7 +43,7 @@ Symbols.getLastSymbolTerm = function(node, symbolName) {
     return Symbols.getLastSymbolTerm(node.content, symbolName);
   }
 
-  return null;
+  return undefined;
 };
 
 // Iterates through a node and returns the last term that does not have the
@@ -57,7 +57,7 @@ Symbols.getLastNonSymbolTerm = function(node, symbolName) {
     return new Node.PolynomialTerm(node).getCoeffNode();
   }
   else if (hasDenominatorSymbol(node, symbolName)) {
-    return null;
+    return undefined;
   }
   else if (Node.Type.isOperator(node)) {
     for (let i = node.args.length - 1; i >= 0 ; i--) {
@@ -71,7 +71,7 @@ Symbols.getLastNonSymbolTerm = function(node, symbolName) {
     }
   }
 
-  return null;
+  return undefined;
 };
 
 // Iterates through a node and returns the denominator if it has a
@@ -98,7 +98,7 @@ Symbols.getLastDenominatorWithSymbolTerm = function(node, symbolName) {
       }
     }
   }
-  return null;
+  return undefined;
 };
 
 // Returns if `node` is a term with symbol `symbolName`