YAML for Web Developers

A small guide to help you write and read .yml files

Translated into: Portuguese

More and more, we, web developers need to learn about different areas to become better professionals and less dependent on others for simple things.

If you start a career as FE, you might see a lot of .yml files like .travis.yml (for Travis Build), .gitlab-ci.yml (for git lab CI), etc. but, let’s be honest, what the hell?

Why would people use this kind of file? What’s the benefit of it? How does this thing work?

So, the goal of this article is to introduce you to the YAML structure and give you more confidence to understand, read, and change a file like this when you need it.

After all, we tend to feel very uncomfortable and frustrated and we need to do something and we can’t even understand what’s that.

But first, What is YAML?

According to the official website, Yaml is:

"YAML (a recursive acronym for "YAML Ain't Markup Language") is a human-friendly data serialization standard for all programming languages."

Heavily used to write configuration files, which explains A LOT, right?

People were tired to have a bunch of configs nobody could understand until someone just say:

What if we could somehow write our configuration like a “cake recipe”? I mean minimum bare text, very straight forward?

Boom, in May 2001 Yaml was created.


Surprisingly (or not really), Yaml is a superset of our well-known buddy JSON.

“Superset is A programming language that contains all the features of a given language and has been expanded or enhanced to include other features as well.” - Font

If I could give you a perspective of what it means I would say:

In a FE world, Yaml would matches for TypeScript while JSON for JavaScript

To better understand how this would be even possible let’s see this example:

  "compilerOptions": {
    "module": "system",
    "noImplicitAny": true,
    "removeComments": true,
    "preserveConstEnums": true,
    "outFile": "../../built/local/tsc.js",
    "sourceMap": false,
    "types": ["node", "lodash", "express"]
  "include": ["src/**/*"],
  "exclude": ["node_modules", "**/*.spec.ts"]

This is a tsconfig.json example. very easy to read, we can easily identify what’s what but… it has some limitations, like:

  • Can’t create variables;
  • Can’t use external variables (e.g. environment variables)
  • Override values;

In JS world, if we can create a .json a configuration file, we almost always also can create a .js (like .eslintrc or .eslint.js) which allows us to mitigate the CONS mentioned before.

But then, if you’re using other programing language, JS files isn’t an option. And it’s at this point Yaml starts to shine.

If we’d have to re-write the tsconfig.json in YAML syntax and having the exactly result, it’d be like:

  module: system
  noImplicitAny: true
  removeComments: true
  preserveConstEnums: true
  outFile: '../../built/local/tsc.js'
  sourceMap: false
    - node
    - lodash
    - express
  - src/**/*
  - node_modules
  - '**/*.spec.ts'

Note this is only an example. You cannot write your tsconfig in YAML! 😜

I hope you’re starting to get the idea from these files.

Concepts, Types, and Syntax

Now, let’s dive deep a bit in the concept of the language.


In Yaml, indentation does matter. It uses whitespace indentation to nest information. By whitespace, keep in mind tab is not allowed.

If you’re like me and uses tab for everything, install some plugin in your IDE to replace your tabs for spaces (like editorconfig). Doing that, when you hit a tab, it’ll automatically replace your tab by space and you don’t even need to use your space bar! ;)


Since indentation matters here, if there’s no space before the first declaration YAML will understand that’s the root (level 0) of your file:

  age: 20

Like we have in JSON with the first { curly brackets:

{  "person": {
    "age": 20


Like in JSON/JS, YAML also uses the key/value syntax and you can use in various ways:

key: value
key_one: value one
key one: value # This works but it's weird
'my key': somekey


To write a comment you just have to use # followed by your message.

# I'm a comment
person: # I'm also a comment
  age: 20

This is cool to document some decision or make a note. Unfortunately, we can’t do this with JSON.


There’re 2 ways to write lists:

JSON way: array of strings

Remember Yaml is a JSON’s superset? we can use its syntax:

people: ['Anne', 'John', 'Max']

Hyphen syntax

The most common (and probably recommended)

  - Anne
  - John
  - Max


There’re a few ways to declare a string in Yaml:

company: Google # Single words, no quotes
full_name: John Foo Bar Doe # Full sentence, no quotes
name: 'John' # Using single quotes
surname: "Christian Meyer" # Using double quotes

While in JSON we would have only a way to use double quotes:

  "company": "Google",
  "full_name": "John Foo Bar Doe",
  "name": "John",
  "surname": "Christian Meyer"

As a suggestion, prefer to use quotes when you want to use any special character like _, @, etc.


Like in any programming language, we have 2 types of number: Integer and Float:

year: 2019 # Integer
nodeVersion: 10.8 # Float

Node Anchors (variables-ish)

An anchor is a mechanism to create a group of data (an object) that can be injected or extended from other objects.

Let’s imagine you need to create a configuration for your CI. It’ll have both production and staging environments. As you can imagine, they share almost the same base settings.

In JSON world, we would have to duplicate these configs:

  "production": {
    "node_version": "13.0.0",
    "os": "ubuntu",
    "package_manager": "yarn",
    "run": ["yarn install", "NODE_ENV=${ENVIRONMENT} yarn build"],
    "env": {
      "ENVIRONMENT": "production"
  "staging": {
    "node_version": "13.0.0",
    "os": "ubuntu",
    "package_manager": "yarn",
    "run": ["yarn install", "NODE_ENV=${ENVIRONMENT} yarn build"],
    "env": {
      "ENVIRONMENT": "staging"

Copy and paste are also annoying, especially when you have to change something in all places it’s been used those Infos.

Anchors came to solve that problem. We can:

  1. First, create our anchor
# I name it as "base-config" but it can be whatever
# &base will be the "variable name" you'll use in the injection
base-config: &base  node_version: 13.0.0
  os: ubuntu
  package_manager: yarn
    - yarn install
    - NODE_ENV=${ENVIRONMENT} yarn build
  1. Then, injecting the anchor created in the level we want to have to see these values being injected:
base-config: &base
  node_version: 13.0.0
  os: ubuntu
  package_manager: yarn
    - yarn install
    - NODE_ENV=${ENVIRONMENT} yarn build

  # I'm injecting all "base" attributes and values inside production
  <<: *base  env:
    - ENVIRONMENT: production

  # I'm injecting all "base" attributes and values inside staging
  <<: *base  env:
    - ENVIRONMENT: staging

Looks simpler, right? And also easier to maintain.

If you copy this code and paste into a “Yaml to JSON converter” online tool you’ll see the same code as I mentioned early in the JSON example but within the addition of the base config:

  "base-config": {
    "node_version": "13.0.0",
    "os": "ubuntu",
    "package_manager": "yarn",
    "run": ["yarn install", "NODE_ENV=${ENVIRONMENT} yarn build"]
  "production": {    "node_version": "13.0.0",    "os": "ubuntu",    "package_manager": "yarn",    "run": ["yarn install", "NODE_ENV=${ENVIRONMENT} yarn build"],    "env": [      {        "ENVIRONMENT": "production"      }    ]  },  "staging": {    "node_version": "13.0.0",    "os": "ubuntu",    "package_manager": "yarn",    "run": ["yarn install", "NODE_ENV=${ENVIRONMENT} yarn build"],    "env": [      {        "ENVIRONMENT": "staging"      }    ]  }}

JSON syntax (yes, JSON)

As explained before a superset of a language is the base language PLUS some extra features, which means we could write a Yaml file in JSON way

  "details": {
    "company": {
      "name": "Google",
      "year": 2019,
      "active": true
    "employees": [

Doubting? Copy this code and paste it here

If you convert this YAML to JSON, you’ll have the same structure:

  "details": {
    "company": {
      "name": "Google",
      "year": 2019,
      "active": true
    "employees": ["Anne", "John", "Max"]

Shell/Bash environment

As I told at the beginning of this article, it’s very common .yml files are used as config files for many things, but especially for CI/CD environment.

For those, you’ll have to describe how the machine/docker should work, what should be installed, ran, etc.

Commonly, all those environments are Linux, which means you’ll also have access to the environment itself.

On GitLab CI, for instance, you can specify on a global level environment variables you want to have available for the whole process:

variables:  NODE_IMAGE: node:10
  - build

  image: $NODE_IMAGE  stage: build

Note that the syntax to use variables by $ isn’t from YAML but shell/bash.

What GitLab CI does is getting everything you’d defined in variables and creates shell variables.

Some other platforms also inject other values like commit ref, branch name, build time, author and also secret keys defined outside the configuration:

  NODE_IMAGE: node:10

  - build

  image: $NODE_IMAGE
  stage: build

In the example above, we’re using a $CI_COMMIT_REF_NAME external environment variable that GitLab CI platform makes available which describes The branch or tag name for which the project is built.


I hope you now understand a bit more about YAML and at least feel comfortable reading and write your files.

Keep in mind that what you’ll have access to or not, the limitations will be determined by the platform you’re using. Travis defines a different configuration than GitLab CI or CircleCI for example.

Always check the documentation from the platform you’re working on to see what’s possible or not to be done! :)