Add withDefault

Author: pbeshai

README.md

@@ -68,6 +68,20 @@ Examples in this table assume query parameter named `qp`.
 | DelimitedArrayParam | string[] | `['a','b','c']` | `?qp=a_b_c'` |
 | DelimitedNumericArrayParam | number[] | `[1, 2, 3]` | `?qp=1_2_3'` |
 
+**Setting a default value**
+
+If you'd like to have a default value, you can wrap your param with `withDefault()`:
+
+```js
+import { withDefault, ArrayParam } from 'serialize-query-params';
+
+// by default, nulls are converted to defaults
+const NeverNullArrayParam = withDefault(ArrayParam, []);
+
+// if you don't want nulls to be included, pass false as a third arg
+const NeverUndefinedArrayParam = withDefault(ArrayParam, [], false);
+```
+
 **Example with Custom Param**
 
 You can define your own params if the ones shipped with this package don't work for you. There are included [serialization utility functions](https://github.com/pbeshai/serialize-query-params/blob/master/src/serialize.ts) to make this easier, but you can use whatever you like.

src/decodeQueryParams.ts

@@ -16,10 +16,6 @@ export function decodeQueryParams<QPCMap extends QueryParamConfigMap>(
   const paramNames = Object.keys(encodedQuery);
   for (const paramName of paramNames) {
     const encodedValue = encodedQuery[paramName];
-    if (encodedValue === undefined) {
-      decodedQuery[paramName as keyof QPCMap] = undefined;
-      continue;
-    }
 
     if (!paramConfigMap[paramName]) {
       if (process.env.NODE_ENV === 'development') {

src/encodeQueryParams.ts

@@ -16,10 +16,6 @@ export function encodeQueryParams<QPCMap extends QueryParamConfigMap>(
   const paramNames = Object.keys(query);
   for (const paramName of paramNames) {
     const decodedValue = query[paramName];
-    if (decodedValue === undefined) {
-      encodedQuery[paramName as keyof QPCMap] = undefined;
-      continue;
-    }
 
     if (!paramConfigMap[paramName]) {
       if (process.env.NODE_ENV === 'development') {

src/index.ts

@@ -1,3 +1,5 @@
+export { withDefault } from './withDefault';
+
 export {
   encodeDate,
   decodeDate,

src/types.ts

@@ -24,7 +24,7 @@ export interface QueryParamConfig<D, D2 = D> {
   encode: (value: D) => string | (string | null)[] | null | undefined;
 
   /** Convert the query param string value to its native type */
-  decode: (value: string | (string | null)[] | null) => D2;
+  decode: (value: string | (string | null)[] | null | undefined) => D2;
 }
 
 /**

src/withDefault.ts

@@ -0,0 +1,44 @@
+import { QueryParamConfig } from './types';
+
+/**
+ * Wrap a given parameter with a default value when undefined or null (optionally, default includes null)
+ * @param param QueryParamConfig - { encode, decode} to serialize a parameter
+ * @param defaultValue A default value
+ * @param includeNull
+ */
+export function withDefault<D, DefaultType extends D2, D2 = D>(
+  param: QueryParamConfig<D, D2>,
+  defaultValue: DefaultType,
+  includeNull?: false | undefined
+): QueryParamConfig<D, Exclude<D2, undefined> | DefaultType>;
+export function withDefault<D, DefaultType extends D2, D2 = D>(
+  param: QueryParamConfig<D, D2>,
+  defaultValue: DefaultType,
+  includeNull?: true
+): QueryParamConfig<D, Exclude<D2, null | undefined> | DefaultType>;
+export function withDefault<D, DefaultType extends D2, D2 = D>(
+  param: QueryParamConfig<D, D2>,
+  defaultValue: DefaultType,
+  includeNull: boolean = true
+): QueryParamConfig<D, any | DefaultType> {
+  const decodeWithDefault = (
+    ...args: Parameters<typeof param.decode>
+  ): Exclude<D2, null | undefined> | Exclude<D2, undefined> | DefaultType => {
+    const decodedValue = param.decode(...args);
+
+    if (decodedValue === undefined) {
+      return defaultValue;
+    }
+    if (includeNull) {
+      if (decodedValue === null) {
+        return defaultValue;
+      } else {
+        return decodedValue as Exclude<D2, undefined>;
+      }
+    }
+
+    return decodedValue as Exclude<D2, undefined | null>;
+  };
+  return { encode: param.encode, decode: decodeWithDefault };
+}
+export default withDefault;