Embedded JavaScript templates with stream and promise support





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EJS Promise

Embedded JavaScript templates with Generator Support.


$ npm install ejs-promise

Defferent with mde and tj 's implement of ejs

The compiled render function no longer return the rendered result directly. It instead return a Promise object. Which has a property outputStream on it. The promise object got resolved after all rendering is done, or it got rejected if there is some error during run the templete. Beside this, the outputStream on it is a ReadableStream, which will output the rendered result immediatly without wait for the render done.

In this way, ejs-promise use less time for TTFB(time to first byte) and total response time than original ejs.
image for timeline of response


  • Use Generator and Promise In the template.
  • Streamed render without wait until a whole templete is rendered
  • Control flow with <% %>
  • Escaped output with <%= %> (escape function configurable)
  • Unescaped raw output with <%- %>
  • Newline-trim mode ('newline slurping') with -%> ending tag
  • Whitespace-trim mode (slurp all whitespace) for control flow with <%_ _%>
  • Custom delimiters (e.g., use '' instead of '<% %>')
  • Includes
  • Client-side support
  • Static caching of intermediate JavaScript
  • Static caching of templates
  • Complies with the Express view system


<% if (user) { %>
  <h2><%= user.name %></h2>
<% } %>


var template = ejs.compile(str, options);
.then(function (result) {
  // => Rendered HTML string

ejs.render(str, data, options)
.then(function (result) {
  // => Rendered HTML string

ejs.renderFile(filename, data, options, function(err, resultPromise){
  .then(function (result) {
    // => Rendered HTML string

It is also possible to use ejs.render(dataAndOptions); where you pass everything in a single object. In that case, you'll end up with local variables for all the passed options. However, be aware that your code could break if we add an option with the same name as one of your data object's properties. Therefore, we do not recommend using this shortcut.


  • cache Compiled functions are cached, requires filename
  • filename The name of the file being rendered. Not required if you are using renderFile(). Used by cache to key caches, and for includes.
  • context Function execution context
  • compileDebug When false no debug instrumentation is compiled
  • client When true, compiles a function that can be rendered in the browser without needing to load the EJS Runtime (ejs.min.js).
  • delimiter Character to use with angle brackets for open/close
  • debug Output generated function body
  • strict When set to true, generated function is in strict mode
  • _with Whether or not to use with() {} constructs. If false then the locals will be stored in the locals object. Set to false in strict mode.
  • localsName Name to use for the object storing local variables when not using with Defaults to locals
  • rmWhitespace Remove all safe-to-remove whitespace, including leading and trailing whitespace. It also enables a safer version of -%> line slurping for all scriptlet tags (it does not strip new lines of tags in the middle of a line).
  • escape The escaping function used with <%= construct. It is used in rendering and is .toString()ed in the generation of client functions. (By default escapes XML).
  • streamOptions Options that will be passed to underlining PassThrough stream.

Returned promise properties (extend normal promise)

  • outputStream (stream.passThrough) stream that will pipe result out withoud render finished
  • noBuffer (method) stop buffering data, make this promise resolve empty buffer.
  • useBuffer (method) renable buffer.
  • waitFlush (method) prevent this promise from rendering more data than the stream pipe target required.
  • defered.interrupt (method) method interupt the rendering process

This project uses JSDoc. For the full public API documentation, clone the repository and run npm run doc. This will run JSDoc with the proper options and output the documentation to out/. If you want the both the public & private API docs, run npm run devdoc instead.


  • <% 'Scriptlet' tag, for control-flow, no output
  • <%_ 'Whitespace Slurping' Scriptlet tag, strips all whitespace before it
  • <%= Outputs the value into the template (escaped)
  • <%- Outputs the unescaped value into the template
  • <%# Comment tag, no execution, no output
  • <%% Outputs a literal '<%'
  • %%> Outputs a literal '%>'
  • %> Plain ending tag
  • -%> Trim-mode ('newline slurp') tag, trims following newline
  • _%> 'Whitespace Slurping' ending tag, removes all whitespace after it

For the full syntax documentation, please see docs/syntax.md.


Includes either have to be an absolute path, or, if not, are assumed as relative to the template with the include call. For example if you are including ./views/user/show.ejs from ./views/users.ejs you would use <%- include('user/show') %>.

You must specify the filename option for the template with the include call unless you are using renderFile().

You'll likely want to use the raw output tag (<%-) with your include to avoid double-escaping the HTML output.

  <% for (var i = 0; i < users.length; i++) { var user = users[i]; %>
    <%- include('user/show', {user: user}) %>
  <% } %>

Includes are inserted at runtime, so you can use variables for the path in the include call (for example <%- include(somePath) %>). Variables in your top-level data object are available to all your includes, but local variables need to be passed down.

NOTE: Include preprocessor directives (<% include user/show %>) are still supported.

Custom delimiters

Custom delimiters can be applied on a per-template basis, or globally:

var ejs = require('ejs'),
    users = ['geddy', 'neil', 'alex'];

// Just one template
ejs.render('<?= users.join(" | "); ?>', {users: users}, {delimiter: '?'})
.then(function (result) {
  // => 'geddy | neil | alex'

// Or globally
ejs.delimiter = '$';
ejs.render('<$= users.join(" | "); $>', {users: users})
.then(function (result) {
// => 'geddy | neil | alex'


EJS ships with a basic in-process cache for caching the intermediate JavaScript functions used to render templates. It's easy to plug in LRU caching using Node's lru-cache library:

var ejs = require('ejs')
  , LRU = require('lru-cache');
ejs.cache = LRU(100); // LRU cache with 100-item limit

If you want to clear the EJS cache, call ejs.clearCache. If you're using the LRU cache and need a different limit, simple reset ejs.cache to a new instance of the LRU.


EJS does not specifically support blocks, but layouts can be implemented by including headers and footers, like so:

<%- include('header') -%>
  My page
<%- include('footer') -%>


You could use promise in templete for async control or other purpose

var ejs = require('ejs');

// Just one template
ejs.render('<?= wait() ?>', 
        wait: function (x) {
            return new Promise(function (resolve, reject) {
                setTimeout(function () {
                    resolve('wait a second!');
                }, 1000)
    }, {delimiter: '?'})
.then(function (result) {
  // => 'wait a second!'


You could stream output immediatly without wait for render to be finished.

Note: the end event will not be fired if there is a error and error handler for stream existed.

var ejs = require('ejs');

// Just one template
ejs.render('first\n<?= wait() ?>\nsecond', 
        wait: function (x) {
            return new Promise(function (resolve, reject) {
                setTimeout(function () {
                    resolve('wait a second!');
                }, 1000)
    }, {delimiter: '?'})
// output 'first' then delay a second and output 'wait a second!', 'second'

Client-side support

Go to the Latest Release, download ./ejs.js or ./ejs.min.js. Alternately, you can compile it yourself by cloning the repository and running jake build (or $(npm bin)/jake build if jake is not installed globally).

Include one of these files on your page, and ejs should be available globally.

or you cou


<div id="output"></div>
<script src="ejs.min.js"></script>
  var people = ['geddy', 'neil', 'alex'],
      promise = ejs.render('<%= people.join(", "); %>', {people: people});
  promise.then(function (html) {
    // With jQuery:
    // Vanilla JS:
    document.getElementById('output').innerHTML = html;


Most of EJS will work as expected; however, there are a few things to note:

  1. Obviously, since you do not have access to the filesystem, ejs.renderFile() won't work.

  2. For the same reason, includes do not work unless you use an IncludeCallback. Here is an example:

    var str = "Hello <%= include('file', {person: 'John'}); %>",
        fn = ejs.compile(str, {client: true});
    fn(data, null, function(path, d){ // IncludeCallback
      // path -> 'file'
      // d -> {person: 'John'}
      // Put your code here 
      // Return the contents of file as a string
    }); // returns rendering promise
  3. becuase browser won't has Stream (old browser may also lack of Promise), the {client: true} won't work unless Stream and Promise are exists

        1. you need a Stream implement to make {client: true} work 
        2. you need a Promise implement to make {client: true} work
           newer borwser may already has it, but the old won't
      <script src="stream.min.js"></script>
      <script src="promise.min.js"></script>
      <script src="ejs.min.js"></script>
        var str = "Hello",
            fn = ejs.compile(str, {client: true});
        fn(data).then(function (html) {
          // "Hello"

There are a number of implementations of EJS:


Licensed under the Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)

EJS Embedded JavaScript templates copyright 2112 mde@fleegix.org.

Modofied by EJS Embedded JavaScript templates with stream copyright 2016 mmis1000@yahoo.com.tw

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