url: implement URLSearchParams size getter

Refs: whatwg/url#734

Author: jasnell

doc/api/url.md

@@ -940,6 +940,14 @@ console.log(params.toString());
 // Prints foo=def&abc=def&xyz=opq
 ```
 
+#### `urlSearchParams.size`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The total number of parameter entries.
+
 #### `urlSearchParams.sort()`
 
 <!-- YAML

lib/internal/url.js

@@ -271,6 +271,12 @@ class URLSearchParams {
     return `${this.constructor.name} {}`;
   }
 
+  get size() {
+    if (!isURLSearchParams(this))
+      throw new ERR_INVALID_THIS('URLSearchParams');
+    return this[searchParams].length / 2;
+  }
+
   append(name, value) {
     if (!isURLSearchParams(this))
       throw new ERR_INVALID_THIS('URLSearchParams');

test/parallel/test-whatwg-url-properties.js

@@ -65,6 +65,14 @@ const { URL, URLSearchParams, format } = require('url');
   testMethod(URLSearchParams.prototype, name, methodName);
 });
 
+{
+  const params = new URLSearchParams();
+  params.append('a', 'b');
+  params.append('a', 'c');
+  params.append('b', 'c');
+  assert.strictEqual(params.size, 3);
+}
+
 function stringifyName(name) {
   if (typeof name === 'symbol') {
     const { description } = name;