Sven Luijten

What is a lock file, and why should you care?

Published on February 26th, 2021 2 minutes to read

As a PHP or JavaScript developer, you have probably come across composer.lock, package-lock.json, and/or yarn.lock. These files are often committed to version control, but not a lot of people realize what they do, and why they should be handled with care.

In this post, I will show you how things can go wrong without a lock file, and the way lock files protect you against those problems.

Without a lock file

Assume you are a PHP or JavaScript developer and you just joined a company. This company has the policy to never commit their lock files to version control, and has added them to .gitignore.

Meanwhile, the "blueprint" file (composer.json and package.json for PHP and JavaScript respectively) has a list of dependencies needed to run the project, for example:

    "require": {
        "super-awesome-dependency": "^1.0"

Everything works fine, and your deployment script runs install every time, happily fetching the latest 1.x.y version every time.

Now, it just so happens that the author of super-awesome-dependency accidentally introduced a terrible application-crashing bug in version 1.3.0, which is going to take a really long time to fix, because they just left for a vacation in the Bahamas.

Your deploy script will still happily install the latest 1.x version (which just so happens to be 1.3.0, the one with the bug), every time you deploy the application until you intervene and edit your "blueprint" file:

    "require": {
        "super-awesome-dependency": ">= 1.0 < 1.3"

With a lock file

Your "blueprint" file looks the same as it does above, but now, you also commit your lock file, which will look similar to this:

    "hash": "8as7d6f98a7s6dfa09w74",
    "packages": [
            "name": "super-awesome-dependency",
            "source": "",
            "sha": "9bdcaa80da69bd7e1fecfbe81319052e4da50844"
            // ...

Now, when your deploy script runs the install command, Composer/npm/yarn will read this file first, and fetch the commit that was referenced in the lock file for that package instead of trying to resolve the dependency all over again.

What if I want to update to a new version?

Updating to a newer, later version should always be something you do manually, or at least under supervision. This way you can ensure your application does not break after upgrading some faraway dependency.


For Composer, you can use composer update to update everything at once (I would not recommend this), or composer update <dependency> --with-all-dependencies. Read the documentation to learn more about how composer update works.

npm / Yarn

For npm and Yarn, use npm update <dependency> and yarn up. See their documentation (npm, Yarn) for more information.


So, the next time you join a company that has the policy to never commit the lock file, you know what to tell them. Or, better yet, send them a link to this blog post 😉