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Merged
Jarred-Sumner merged 2 commits into from
Aug 21, 2026
Merged

assert: render Myers diffs natively #39924

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049006e
assert: render Myers diffs natively instead of via a JS Diff[] array
dylan-conway Aug 21, 2026
320783e
assert: carry the diff mode in Output, add UTF-16 and colored char di…
robobun Aug 21, 2026
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30 changes: 9 additions & 21 deletions src/js/internal/assert/assertion_error.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@ const { SafeSet } = require("internal/primordials");
const { inspect } = require("internal/util/inspect");
const colors = require("internal/util/colors");
const { validateObject } = require("internal/validators");
const { myersDiff, printMyersDiff, printSimpleMyersDiff } = require("internal/assert/myers_diff") as typeof Internal;
const { printMyersDiff, printSimpleMyersDiff } = require("internal/assert/myers_diff") as typeof Internal;

const ErrorCaptureStackTrace = Error.captureStackTrace;
const ObjectAssign = Object.assign;
Expand All @@ -19,21 +19,12 @@ const StringPrototypeSlice = String.prototype.slice;
const StringPrototypeSplit = String.prototype.split;

declare namespace Internal {
const enum Operation {
Insert = 0,
Delete = 1,
Equal = 2,
}
interface Diff {
kind: Operation;
value: string;
}

function myersDiff(actual: string, expected: string, checkCommaDisparity?: boolean, lines?: boolean): string;
// todo

function printMyersDiff(...args: any[]): any;
function printSimpleMyersDiff(...args: any[]): any;
function printSimpleMyersDiff(actual: string, expected: string): string;
function printMyersDiff(
actual: string,
expected: string,
checkCommaDisparity: boolean,
): { message: string; skipped: boolean };
}

const kReadableOperator = {
Expand Down Expand Up @@ -113,9 +104,7 @@ function getColoredMyersDiff(actual, expected) {
const header = `${colors.green}actual${colors.white} ${colors.red}expected${colors.white}`;
const skipped = false;

// const diff = myersDiff(StringPrototypeSplit.$call(actual, ""), StringPrototypeSplit.$call(expected, ""));
const diff = myersDiff(actual, expected, false, false);
let message = printSimpleMyersDiff(diff);
let message = printSimpleMyersDiff(actual, expected);

if (skipped) {
message += "...";
Expand Down Expand Up @@ -224,8 +213,7 @@ function createErrDiff(actual, expected, operator, customMessage, diffType = "si
const checkCommaDisparity = actual != null && typeof actual === "object";
let myersDiffMessage;
try {
const diff = myersDiff(inspectedActual, inspectedExpected, checkCommaDisparity, true);
myersDiffMessage = printMyersDiff(diff);
myersDiffMessage = printMyersDiff(inspectedActual, inspectedExpected, checkCommaDisparity);
} catch {
myersDiffMessage = undefined;
}
Expand Down
101 changes: 16 additions & 85 deletions src/js/internal/assert/myers_diff.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -17,95 +17,26 @@ interface Diff {
}

declare namespace Internal {
export function myersDiff(
actual: string[],
expected: string[],
checkCommaDisparity?: boolean,
lines?: boolean,
): Diff[];
export function myersDiff(actual: string, expected: string, checkCommaDisparity?: boolean, lines?: boolean): Diff[];
/** Diffs by char and renders the `printSimpleMyersDiff` string. */
export function printSimpleMyersDiff(actual: string, expected: string, colors: object): string;
/** Diffs by line and renders the `printMyersDiff` message, collapsing long unchanged runs. */
export function printMyersDiff(
actual: string,
expected: string,
checkCommaDisparity: boolean,
colors: object,
): { message: string; skipped: boolean };
}

const kNopLinesToCollapse = 5;
const native = $rust("node_assert_binding.rs", "generate") as typeof Internal;

const { myersDiff } = $rust("node_assert_binding.rs", "generate") as typeof Internal;

function printSimpleMyersDiff(diff: Diff[]) {
let message = "";

for (let diffIdx = diff.length - 1; diffIdx >= 0; diffIdx--) {
let { kind, value } = diff[diffIdx];
if (typeof value === "number") {
value = String.fromCharCode(value);
}
switch (kind) {
case Operation.Insert:
message += `${colors.green}${value}${colors.white}`;
break;
case Operation.Delete:
message += `${colors.red}${value}${colors.white}`;
break;
case Operation.Equal:
message += `${colors.white}${value}${colors.white}`;
break;
default:
throw new TypeError(`Invalid diff operation kind: ${kind}`); // should be unreachable
}
}

return `\n${message}`;
function printSimpleMyersDiff(actual: string, expected: string) {
return native.printSimpleMyersDiff(actual, expected, colors);
}

function printMyersDiff(diff: Diff[], _simple = false) {
let message = "";
let skipped = false;
let nopCount = 0;

for (let diffIdx = diff.length - 1; diffIdx >= 0; diffIdx--) {
const { kind, value } = diff[diffIdx];
$assert(
typeof value !== "number",
"printMyersDiff is only called for line diffs, which never return numeric char code values.",
);
const previousType = diffIdx < diff.length - 1 ? diff[diffIdx + 1].kind : null;
const typeChanged = previousType && kind !== previousType;

if (typeChanged && previousType === Operation.Equal) {
// Avoid grouping if only one line would have been grouped otherwise
if (nopCount === kNopLinesToCollapse + 1) {
message += `${colors.white} ${diff[diffIdx + 1].value}\n`;
} else if (nopCount === kNopLinesToCollapse + 2) {
message += `${colors.white} ${diff[diffIdx + 2].value}\n`;
message += `${colors.white} ${diff[diffIdx + 1].value}\n`;
}
if (nopCount >= kNopLinesToCollapse + 3) {
message += `${colors.blue}...${colors.white}\n`;
message += `${colors.white} ${diff[diffIdx + 1].value}\n`;
skipped = true;
}
nopCount = 0;
}

switch (kind) {
case Operation.Insert:
message += `${colors.green}+${colors.white} ${value}\n`;
break;
case Operation.Delete:
message += `${colors.red}-${colors.white} ${value}\n`;
break;
case Operation.Equal:
if (nopCount < kNopLinesToCollapse) {
message += `${colors.white} ${value}\n`;
}
nopCount++;
break;
default:
throw new TypeError(`Invalid diff operation kind: ${kind}`); // should be unreachable
}
}

message = message.trimEnd();

return { message: `\n${message}`, skipped };
function printMyersDiff(actual: string, expected: string, checkCommaDisparity: boolean) {
return native.printMyersDiff(actual, expected, checkCommaDisparity, colors);
}

export default { myersDiff, printMyersDiff, printSimpleMyersDiff };
export default { myersDiff: native.myersDiff, printMyersDiff, printSimpleMyersDiff };
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