Move MaybeBigNumber functions into helper

packages/instant/src/components/scaling_amount_input.tsx

@@ -4,6 +4,7 @@ import * as React from 'react';
 
 import { ColorOption } from '../style/theme';
 import { MaybeBigNumber } from '../types';
+import { maybeBigNumberUtil } from '../util/maybe_big_number';
 import { util } from '../util/util';
 
 import { ScalingInput } from './scaling_input';
@@ -21,23 +22,7 @@ interface ScalingAmountInputState {
     stringValue: string;
 }
 
-const stringToMaybeBigNumber = (stringValue: string): MaybeBigNumber => {
+const { stringToMaybeBigNumber, areMaybeBigNumbersEqual } = maybeBigNumberUtil;
-    let maybeBigNumber: MaybeBigNumber;
-    try {
-        maybeBigNumber = new BigNumber(stringValue);
-    } catch {
-        maybeBigNumber = undefined;
-    }
-    return _.isNaN(maybeBigNumber) ? undefined : maybeBigNumber;
-};
-
-const areMaybeBigNumbersEqual = (val1: MaybeBigNumber, val2: MaybeBigNumber): boolean => {
-    if (!_.isUndefined(val1) && !_.isUndefined(val2)) {
-        return val1.equals(val2);
-    }
-    return _.isUndefined(val1) && _.isUndefined(val2);
-};
-
 export class ScalingAmountInput extends React.Component<ScalingAmountInputProps, ScalingAmountInputState> {
     public static defaultProps = {
         onChange: util.boundNoop,
@@ -57,6 +42,8 @@ export class ScalingAmountInput extends React.Component<ScalingAmountInputProps,
         if (!areMaybeBigNumbersEqual(parsedStateValue, currentValue)) {
             // we somehow got into the state in which the value passed in and the string value
             // in state have differed, reset state
+            // we dont expect to ever get into this state, but let's make sure
+            // we reset if we do since we're dealing with important numbers
             this.setState({
                 stringValue: _.isUndefined(currentValue) ? '' : currentValue.toString(),
             });

packages/instant/src/util/maybe_big_number.ts

@@ -0,0 +1,25 @@
+import { BigNumber } from '@0x/utils';
+import * as _ from 'lodash';
+
+import { MaybeBigNumber } from '../types';
+
+export const maybeBigNumberUtil = {
+    // converts a string to a MaybeBigNumber
+    // if string is a NaN, considered undefined
+    stringToMaybeBigNumber: (stringValue: string): MaybeBigNumber => {
+        let validBigNumber: BigNumber;
+        try {
+            validBigNumber = new BigNumber(stringValue);
+        } catch {
+            return undefined;
+        }
+
+        return validBigNumber.isNaN() ? undefined : validBigNumber;
+    },
+    areMaybeBigNumbersEqual: (val1: MaybeBigNumber, val2: MaybeBigNumber): boolean => {
+        if (!_.isUndefined(val1) && !_.isUndefined(val2)) {
+            return val1.equals(val2);
+        }
+        return _.isUndefined(val1) && _.isUndefined(val2);
+    },
+};