template-replace-stream

A high performance {{ template }} replace stream working on binary or string streams.

This module is written in pure TypeScript, consists of only 224 lines of code (including type definitions) and has no other dependencies. It is flexible and allows replacing an arbitrary wide range of template variables while being extremely fast (we reached over 20GiB/s, see Benchmarks).

Install

npm install template-replace-stream

This module contains type definitions and also an .mjs file for maximum compatibility.

Supported Node.js Versions

The following Node.js versions are tested to work with the package. Older versions are not tested but should still be able to use it.

18.x	20.x	22.x	24.x

Usage

You create a TemplateReplaceStream by passing a source of template variables and their replacement values to the constructor. This may either be a map containing key-value pairs, or a function that returns a replacement value for a given template string.

JavaScript

const { TemplateReplaceStream } = require("template-replace-stream");
const fs = require("node:fs");
const path = require("node:path");

// create a map of variables to replace. This will replace "{{replace-me}}" with "really fast"
const variables = new Map([["replace-me", "really fast"]]);

// create the streams
const readStream = fs.createReadStream(path.join(__dirname, "template.txt"));
const writeStream = fs.createWriteStream(path.join(__dirname, "example.txt"));
const templateReplaceStream = new TemplateReplaceStream(variables);

// connect the streams and put the template replace stream in the middle
readStream.pipe(templateReplaceStream).pipe(writeStream);
writeStream.on("finish", () => console.log("Finished writing example.txt"));

TypeScript

import { TemplateReplaceStream } from "template-replace-stream";
import fs from "node:fs";
import path from "node:path";

// create a map of variables to replace. This will replace "{{replace-me}}" with "really fast"
const variables = new Map([["replace-me", "really fast"]]);

// create the streams
const readStream = fs.createReadStream(path.join(__dirname, "template.txt"));
const writeStream = fs.createWriteStream(path.join(__dirname, "example.txt"));
const templateReplaceStream = new TemplateReplaceStream(variables);

// connect the streams and put the template replace stream in the middle
readStream.pipe(templateReplaceStream).pipe(writeStream);
writeStream.on("finish", () => console.log("Finished writing example.txt"));

Advanced

Readable Stream as Replacement Value Source

It's also possible to pass another Readable as replacement value source to the TemplateReplaceStream. In fact, the README you are just reading was created using this feature. This makes it possible to replace template variables with whole files without reading them into a stream before.

Advanced Example Code

import { StringSource, TemplateReplaceStream } from "template-replace-stream";
import fs from "node:fs";
import path from "node:path";
import sloc from "sloc";
import { Project, ts } from "ts-morph";

const rootDir = path.join(__dirname, "..");
const exampleFiles = ["javascript-example.js", "typescript-example.ts", "generate-readme.ts"];

const outputFilePath = path.join(rootDir, "README.md");
const sourceFilePath = path.join(rootDir, "index.ts");
const codeInfo = sloc(fs.readFileSync(sourceFilePath, "utf8"), "ts");
const loc = codeInfo.total - codeInfo.comment - codeInfo.empty;
const optionsDefinition = extractTypeDefinition("TemplateReplaceStreamOptions", sourceFilePath);

// the map of example files and their read streams and further template variables
const templateMap = new Map<string, StringSource>(
  exampleFiles.map((file) => [file, openExampleStream(file)])
);
templateMap.set("loc", loc.toString());
templateMap.set("options-definition", optionsDefinition);

// create the streams
const readmeReadStream = fs.createReadStream(path.join(rootDir, "template.md"));
const readmeWriteStream = fs.createWriteStream(outputFilePath);

// connect the streams and put the template replace stream in the middle
readmeReadStream.pipe(new TemplateReplaceStream(templateMap)).pipe(readmeWriteStream);
readmeWriteStream.on("finish", () => console.log(`Created ${outputFilePath}`));

/**
 * Opens a file stream to the given source file.
 *
 * @param file The file to read.
 */
function openExampleStream(file: string) {
  return fs.createReadStream(path.join(__dirname, file));
}

/**
 * Extracts the type definition from the given source file.
 *
 * @param typeName The name of the type to extract.
 * @param filePath The full path to the source file.
 */
function extractTypeDefinition(typeName: string, filePath: string) {
  const sourceFile = new Project().addSourceFileAtPath(filePath);
  const typeNode = sourceFile.getTypeAlias(typeName)?.compilerNode;
  if (!typeNode) throw new Error(`Type alias ${typeName} not found.`);
  const printer = ts.createPrinter({ removeComments: false });
  return printer.printNode(ts.EmitHint.Unspecified, typeNode, sourceFile.compilerNode);
}

Options

/**
 * Options for the template replace stream.
 */
export type TemplateReplaceStreamOptions = {
    /** Default: `false`. If true, the stream creates logs on debug level */
    log: boolean;
    /**
     * Default: `false`. If true, the stream throws an error when a template variable has no
     * replacement value. Takes precedence over `removeUnmatchedTemplate`.
     */
    throwOnUnmatchedTemplate: boolean;
    /**
     * Default: `100`. The maximum length of a variable name between a start and end pattern including
     * whitespaces around it. Any variable name longer than this length is ignored, i.e. the search
     * for the end pattern canceled and the stream looks for the next start pattern.
     * Note that a shorter length improves performance but may not find all variables.
     */
    maxVariableNameLength: number;
    /** Default: `'{{'`. The start pattern of a template string either as string or buffer */
    startPattern: string | Buffer;
    /** Default: `'}}'`. The end pattern of a template string either as string or buffer */
    endPattern: string | Buffer;
    /** Any options for the lower level {@link Transform} stream. Do not replace transform or flush */
    streamOptions?: TransformOptions;
};

Benchmarks

The benchmarks were run on my MacBook Pro with an Apple M1 Pro Chip. The data source were virtual files generated from- and to memory to omit any bottleneck due to the file system. The "native" data refers to reading a virtual file without doing anything else with it (native fs.Readable streams). So they are the absolute highest possible.

Replacing a single Template Variable in a large File

Like the raw file system stream, a TemplateReplaceStream becomes faster with an increasing source file size. It is more than 20x faster than the replace-stream when processing large files. The throughput of the TemplateReplaceStream was more than 20GiB/s when replacing a single variable in a 100MiB file.

Replacing a single variable in a 100MiB file takes only 6ms using a TemplateReplaceStream. Reading the whole file from the disk alone takes already more than 1ms. The stream-replace-string packages was omitted im this graph, as it took over 16s to process the 100MiB file.

Replacing 10 thousand Template Variables in a large File

You can see that the performance declines when working with more replacements. Note that one reason is the virtually generated workload (see "native" in the graph). TemplateReplaceStream still reaches 10GiB/s.

To replace ten thousand template variables in a 100MiB file, the TemplateReplaceStream takes around 10ms. Since this duration is similar for smaller file sizes, we can see that it does not perform too well in the 1MiB file. We will keep optimizing for that.

Changelog

2.2.0

• Add TemplateReplaceStream.replaceAsync() that directly returns a Promise<Buffer> with the result
• Add TemplateReplaceStream.replaceStringAsync() that directly returns a Promise<string> with the result
• Drop support for Node.js 16 and 18 (EOL) and add support for 24

2.1.2

• Support async replacement value functions (Promise<StringSource> as return value)
• Add CI to repository
• Update README

2.1.1

• Fix stream ending when replacing a template with another stream during the last chunk of data
• Update README

2.1.0

• Further improve performance by using Buffer.indexOf() to find the end of a template variable, too
• Add more benchmarks

2.0.0

• Drastically improve performance (by ~10x) by using Buffer.indexOf() instead of iterating over the buffer myself
• Rename option throwOnMissingVariable to throwOnUnmatchedTemplate
• Add benchmarks

1.0.1

• Update README

1.0.0

• Initial Release