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a hack that makes jq run in the browser with emscripten.

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npm badge Mentioned in Awesome jq


This is a build of jq, the command-line JSON processor in Javascript using Emscripten along with a wrapper for making it usable as a library.

It runs in the browser.

install and use

npm install jq-web
var jq = require('jq-web')

  a: {
    big: {
      json: [
}, '.a.big.json | ["empty", .[1], "useless", .[3]] | join(" ")')

The code above returns the string "empty of useless things".

You could do the same using the promised API with jq.promised.json({...}).then(result => {}). That is useful if you're loading a .mem or .wasm file, as the library won't return the correct results until these files are asynchronously fetched by the Emscripten runtime.

What is each file

The releases page has a bunch of different files you can download, here's what they all mean:

jq.asm.jsasm.js versionruns in most placesrequires loading jq.asm.js.mem
jq.asm.min.jsminified version of the aboveidemidem
jq.asm.js.memmemory initialization file for asm.js, needed by jq.asm.js and jq.asm.min.js
jq.asm.bundle.jsasm.js version with memory initialization embeddeddoesn't require loading anythingbig and slow
jq.asm.bundle.min.jsminified version of the aboveidemthe minification has no effect in the memory initialization stuff
jq.wasm.jsWebAssembly wrappersmaller, much much fasterrequires loading jq.wasm.wasm
jq.wasm.min.jsminified WebAssembly wrappersince this is just a wrapper around jq.wasm.wasm, the minification makes almost no difference here
jq.wasm.wasmactual WebAssembly binary, loaded by jq.wasm.js and jq.wasm.min.js

When in doubt, just use jq.wasm.js, it is the best!

Webpack issues


The Emscripten runtime will try to require the fs module, and if it fails it will resort to an in-memory filesystem (almost no use of that is made of the library, but it is needed somehow). In Browserify there's a default {} that corresponds to the fs module, but in Webpack you must declare it as an empty module.

404 error when loading .wasm files

By default projects compiled with Emscripten look for .wasm files in the same directory that the .js file is run from. This causes issues when using webpack because name of the .wasm file is altered with a hash and can be placed in a different directory. To fix this problem you can use the copy-webpack-plugin to copy the jq.wasm file to the same directory that the webpack bundle is placed.


jq.json(<object>, <filter>) <object> will take a Javascript object, or scalar, whatever, and dump it to JSON, then it will return whatever your filter outputs and try to convert that into a JS object. If you're loading .mem or .wasm files asynchronously this will return {} every time you call it until the loading is finished.

jq.raw(<json-string>, <filter>) <raw-output> will take a string that will be passed as it is to jq (like if you were doing echo '<json-string>' | jq <filter> on the command line) then return a string with the raw STDOUT response. If you're loading .mem or .wasm files asynchronously this will return '{}' every time you call it until the loading is finished.

jq.onInitialized.addListener(<function>) registers a function to be called when .mem or .wasm files have finished loading and the library is ready to be used. You should register callbacks here to rerun your functions if you're using the sync API (above). If you're using the promised API (below) you don't ever need to look at this. Also, if you're using the sync API but just at a long time after the page is loaded and the user inputs something, for example, you may not need to use this at all.

jq.promised.json(<object>, <filter>) Promise<object> will do the same as jq.json() but returning a Promise to the result instead. This is safe to use anytime.

jq.promised.raw(<json-string>, <filter>) Promise<raw-output> will do the same as jq.raw() but returning a Promise to the result instead. This is safe to use anytime.


  1. Install Emscripten from source, we used 1.38.12
  2. Clone jq-web and cd into it
  3. Look over the Makefile for more Emscripten instructions
  4. make
    • This may take a while the first time if you have never ran Emscripten before


A handful of tests exist in test.js and are good place to start when verifying a build

  1. npm install or yarn
  2. node test.js
  3. ./node_modules/live-server/live-server.js --open="index.html"



Last updated on 21 Sep 2023

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