Add spaceBytes field to stringifyInfo() result

Author: lahmatiy

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## next
 
+- Added `spaceBytes` field to `stringifyInfo()` result, which indicates the number of bytes used for white spaces. This allows for estimating size of `JSON.stringify()` result with and without formatting (when `space` option is used) in a single pass instead of two
 - Fixed `stringifyInfo()` to correctly accept the `space` parameter from options, i.e. `stringifyInfo(data, { space: 2 })`
 
 ## 0.6.1 (2024-08-06)

index.d.ts

@@ -17,7 +17,8 @@ declare module '@discoveryjs/json-ext' {
     }
     type StringifyInfoResult = {
         bytes: number;
-        circular: Object[];
+        spaceBytes: number;
+        circular: object[];
     };
 
     export function parseChunked(input: Iterable<Chunk> | AsyncIterable<Chunk>): Promise<any>;

src/stringify-info.js

@@ -158,6 +158,7 @@ export function stringifyInfo(value, optionsOrReplacer, space) {
     const root = { '': value };
     let stop = false;
     let bytes = 0;
+    let spaceBytes = 0;
     let objects = 0;
 
     walk(root, '', value);
@@ -168,7 +169,8 @@ export function stringifyInfo(value, optionsOrReplacer, space) {
     }
 
     return {
-        bytes: isNaN(bytes) ? Infinity : bytes,
+        bytes: isNaN(bytes) ? Infinity : bytes + spaceBytes,
+        spaceBytes,
         circular: [...circular]
     };
 
@@ -232,7 +234,7 @@ export function stringifyInfo(value, optionsOrReplacer, space) {
                         let keyLen = keysLength.get(key);
 
                         if (keyLen === undefined) {
-                            keysLength.set(key, keyLen = stringLength(key) + (space > 0 ? 2 : 1)); // "key":
+                            keysLength.set(key, keyLen = stringLength(key) + 1); // "key":
                         }
 
                         // value is printed
@@ -248,9 +250,15 @@ export function stringifyInfo(value, optionsOrReplacer, space) {
                 : 1 + valueLength; // {} or [] + commas
 
             if (space > 0 && valueLength > 0) {
-                bytes +=
-                    (1 + stack.length * space) * valueLength + // for each key-value: \n{space}
-                    1 + (stack.length - 1) * space; // for }
+                spaceBytes +=
+                    // a space between ":" and a value for each object entry
+                    (Array.isArray(value) ? 0 : valueLength) +
+                    // the formula results from folding the following components:
+                    // - for each key-value or element: ident + newline
+                    //   (1 + stack.length * space) * valueLength
+                    // - ident (one space less) before "}" or "]" + newline
+                    //   (stack.length - 1) * space + 1
+                    (1 + stack.length * space) * (valueLength + 1) - space;
             }
 
             stack.pop();

src/stringify-info.test.js

@@ -19,10 +19,16 @@ function createInfoTest(value, ...args) {
 
     it(title.replace(/[\u0000-\u001f\u0100-\uffff]/g, m => '\\u' + m.charCodeAt().toString(16).padStart(4, '0')), () => {
         const native = String(wellformedStringify(value, ...args));
+        const nonFormatted = args[1] ? String(wellformedStringify(value, args[0])) : native;
         const info = stringifyInfo(value, ...args);
+        const bytes = strBytesLength(native);
+        const spaceBytes = nonFormatted !== native
+            ? bytes - strBytesLength(nonFormatted)
+            : 0;
 
         assert.deepStrictEqual(info, {
-            bytes: strBytesLength(native),
+            bytes,
+            spaceBytes,
             circular: []
         });
     });
@@ -61,6 +67,7 @@ describe('stringifyInfo()', () => {
 
         assert.deepStrictEqual(actual, {
             bytes: 18,
+            spaceBytes: 7,
             circular: []
         });
     });