microsoft/TypeScript · error · Error

Three-slash line in the middle of four-slash region at line

Error message

Three-slash line in the middle of four-slash region at line

What it means

Thrown by parseTestData when a three-slash line (starting with /// but not ////) appears after fourslash content lines (starting with ////) have begun accumulating currentFileContent. The parser treats //// as file content and /// as an ambiguous instruction that must not be interleaved with four-slash blocks.

Source

Thrown at src/harness/fourslashImpl.ts:4788

        currentFileContent = undefined;
        currentFileOptions = {};
        currentFileName = fileName;
        currentFileSymlinks = undefined;
    }

    for (let line of lines) {
        i++;
        if (line.length > 0 && line.charAt(line.length - 1) === "\r") {
            line = line.substr(0, line.length - 1);
        }

        if (line.substr(0, 4) === "////") {
            const text = line.substr(4);
            currentFileContent = currentFileContent === undefined ? text : currentFileContent + "\n" + text;
        }
        else if (line.substr(0, 3) === "///" && currentFileContent !== undefined) {
            throw new Error("Three-slash line in the middle of four-slash region at line " + i);
        }
        else if (line.substr(0, 2) === "//") {
            const possiblySymlinks = Harness.TestCaseParser.parseSymlinkFromTest(line, symlinks);
            if (possiblySymlinks) {
                symlinks = possiblySymlinks;
            }
            else {
                // Comment line, check for global/file @options and record them
                const match = optionRegex.exec(line.substr(2));
                if (match) {
                    const key = match[1].toLowerCase();
                    const value = match[2];
                    if (!ts.contains(fileMetadataNames, key)) {
                        // Check if the match is already existed in the global options
                        if (globalOptions[key] !== undefined) {
                            throw new Error(`Global option '${key}' already exists`);
                        }
                        globalOptions[key] = value;

View on GitHub (pinned to b465fdbfe1)

Solutions

Inspect the line number reported in the message and convert the /// line to //// if it is meant to be file content.
If the /// line is a real triple-slash directive, move it before the first //// block (outside the four-slash region).
Remove the stray line entirely if it is not needed.

Example fix

// before
////const x = 1;
/// <reference path="a.d.ts" />
////const y = 2;
// after — reference moved above the four-slash region
/// <reference path="a.d.ts" />
////const x = 1;
////const y = 2;

Defensive patterns

Strategy: validation

Validate before calling

// Pre-check that no /// lines appear after the first //// line in a test file
const lines = content.split("\n");
let sawFourSlash = false;
for (const line of lines) {
    if (line.startsWith("////")) sawFourSlash = true;
    else if (sawFourSlash && line.startsWith("///") && !line.startsWith("////")) {
        throw new Error(`Stray /// after //// region: ${line}`);
    }
}

Prevention

Place all triple-slash directives above the first //// content block.
When editing fourslash files, count slashes carefully — /// vs //// have distinct meanings.

When it happens

Trigger: A fourslash test file mixes //// content lines with a /// line in the same region — e.g. a stray triple-slash reference comment placed between four-slash file content lines.

Common situations: Test author adds a ///<reference> directive inside a four-slash block by mistake, or a copy-paste introduces a /// line where //// was intended. Also happens when trimming one slash from a //// comment accidentally.

Related errors

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/c239521baf8fcf5a. Report an issue: GitHub.