perf: improve stringify performance

Author: anonrig

README.md

@@ -70,15 +70,15 @@ console.log(qs.stringify({ foo: ['bar', 'baz'] }))
 ╔════════════════════════════╤═════════╤═══════════════════╤═══════════╗
 ║ Slower tests               │ Samples │            Result │ Tolerance ║
 ╟────────────────────────────┼─────────┼───────────────────┼───────────╢
-║ query-string               │   10000 │  290850.12 op/sec │  ± 1.52 % ║
-║ qs                         │   10000 │  324324.45 op/sec │  ± 2.05 % ║
-║ http-querystring-stringify │   10000 │  481327.85 op/sec │  ± 1.77 % ║
-║ URLSearchParams            │   10000 │  538867.84 op/sec │  ± 3.93 % ║
-║ querystringify             │   10000 │  774992.18 op/sec │  ± 2.51 % ║
-║ node:querystring           │   10000 │ 1827458.66 op/sec │  ± 5.41 % ║
+║ query-string               │   10000 │  284130.63 op/sec │  ± 1.62 % ║
+║ qs                         │   10000 │  334799.48 op/sec │  ± 1.93 % ║
+║ http-querystring-stringify │   10000 │  482642.49 op/sec │  ± 1.72 % ║
+║ URLSearchParams            │   10000 │  587274.65 op/sec │  ± 1.88 % ║
+║ querystringify             │   10000 │  753960.35 op/sec │  ± 2.20 % ║
+║ node:querystring           │   10000 │ 1796993.95 op/sec │  ± 5.34 % ║
 ╟────────────────────────────┼─────────┼───────────────────┼───────────╢
 ║ Fastest test               │ Samples │            Result │ Tolerance ║
 ╟────────────────────────────┼─────────┼───────────────────┼───────────╢
-║ fast-querystring           │   10000 │ 1881474.27 op/sec │  ± 4.78 % ║
+║ fast-querystring           │   10000 │ 2051022.89 op/sec │  ± 4.52 % ║
 ╚════════════════════════════╧═════════╧═══════════════════╧═══════════╝
 ```

lib/stringify.js

@@ -34,27 +34,33 @@ function stringify(input) {
 
   const keys = Object.getOwnPropertyNames(input);
   const keyLength = keys.length;
+  const separator = "&";
+  let valueLength = 0;
 
   for (let i = 0; i < keyLength; i++) {
     const key = keys[i];
     const value = input[key];
     const encodedKey = encodeString(key) + "=";
 
     if (i) {
-      result += "&";
+      result += separator;
     }
 
     if (Array.isArray(value)) {
-      const valueLength = value.length;
+      valueLength = value.length;
       for (let j = 0; j < valueLength; j++) {
         if (j) {
-          result += "&";
+          result += separator;
         }
 
-        result += encodedKey + getAsPrimitive(value[j]);
+        // Optimization: Dividing into multiple lines improves the performance.
+        // Since v8 does not need to care about the '+' character if it was one-liner.
+        result += encodedKey;
+        result += getAsPrimitive(value[j]);
       }
     } else {
-      result += encodedKey + getAsPrimitive(value);
+      result += encodedKey;
+      result += getAsPrimitive(value);
     }
   }