liquidjs/docs/source/tutorials/drops.md

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Liquid Drops

LiquidJS also provides a mechanism similar to Shopify Drops, allowing template authors to incorporate custom functionality in resolving variable values.

{% note info Drop for JavaScript %} Drop interface is implemented differently in LiquidJS compared to built-in filters and other template functionalities. Since LiquidJS runs in JavaScript, custom Drops need to be reimplemented in JavaScript anyway. There's no compatibility between JavaScript classes and Ruby classes. {% endnote %}

Basic Usage

import { Liquid, Drop } from 'liquidjs'

class SettingsDrop extends Drop {
  constructor() {
    super()
    this.foo = 'FOO'
  }
  bar() {
    return 'BAR'
  }
}

const engine = new Liquid()
const template = `foo: {{settings.foo}}, bar: {{settings.bar}}`
const context = { settings: new SettingsDrop() }
// Outputs: "foo: FOO, bar: BAR"
engine.parseAndRender(template, context).then(html => console.log(html))

Runkit link

As shown above, besides reading properties from context scopes, you can also call methods. You only need to create a custom class inherited from Drop.

{% note tip Async Methods %} LiquidJS is fully async-friendly. You can safely return a Promise in your Drop methods or define your methods in Drop as async. {% endnote %}

liquidMethodMissing

For cases when there isn't a fixed set of properties, you can leverage liquidMethodMissing to dynamically resolve the value of a variable name.

import { Liquid, Drop } from 'liquidjs'

class SettingsDrop extends Drop {
  liquidMethodMissing(key) {
    return key.toUpperCase()
  }
}

const engine = new Liquid()
// Outputs: "COO"
engine.parseAndRender("{{settings.coo}}", { settings: new SettingsDrop() })
  .then(html => console.log(html))

liquidMethodMissing supports Promise, meaning you can make async calls within it. A more useful case can be fetching the value dynamically from the database. By using Drops, you can avoid hardcoding each property into the context. For example:

import { Liquid, Drop } from 'liquidjs'

class DBDrop extends Drop {
  async liquidMethodMissing(key) {
    const record = await db.getRecordByKey(key)
    return record.value
  }
}

const engine = new Liquid()
const context = { db: new DBDrop() }
engine.parseAndRender("{{db.coo}}", context).then(html => console.log(html))

valueOf

Drops can implement a valueOf() method, the return value of which can be used to replace itself in the output. For example:

import { Liquid, Drop } from 'liquidjs'

class ColorDrop extends Drop {
  valueOf() {
    return 'red'
  }
}

const engine = new Liquid()
const context = { color: new ColorDrop() }
// Outputs: "red"
engine.parseAndRender("{{color}}", context).then(html => console.log(html))

toLiquid

toLiquid() is not a method of Drop, but it can be used to return a Drop. In cases where you have a fixed structure in the context that cannot change its values, you can implement toLiquid() to let LiquidJS use the returned value instead of itself to render the templates.

import { Liquid, Drop } from 'liquidjs'

const context = {
  person: {
    firstName: "Jun",
    lastName: "Yang",
    name: "Jun Yang",
    toLiquid: () => ({
      firstName: this.firstName,
      lastName: this.lastName,
      // use a different `name`
      name: "Yang, Jun"
    })
  }
}

const engine = new Liquid()
// Outputs: "Yang, Jun"
engine.parseAndRender("{{person.name}}", context).then(html => console.log(html))

Of course, you can also return a PersonDrop instance in the toLiquid() method and implement this functionality within PersonDrop:

import { Liquid, Drop } from 'liquidjs'

class PersonDrop extends Drop {
  constructor(person) {
    super()
    this.person = person
  }
  name() {
    return this.person.lastName + ", " + this.person.firstName
  }
}

const context = {
  person: {
    firstName: "Jun",
    lastName: "Yang",
    name: "Jun Yang",
    toLiquid: function () { return new PersonDrop(this) }
  }
}

const engine = new Liquid()
// Outputs: "Yang, Jun"
engine.parseAndRender("{{person.name}}", context).then(html => console.log(html))

{% note info toLiquid() vs. valueOf() Difference %}

• valueOf() is typically used to define how the current variable should be rendered, while toLiquid() is often used to convert an object into a Drop or another scope provided to the template.
• valueOf() is a method exclusive to Drops; whereas toLiquid() can be used on any scope object.
• valueOf() is called when the variable itself is about to be rendered, replacing itself; whereas toLiquid() is called when its properties are about to be read.

{% endnote %}

Special Drops

LiquidJS itself implements several built-in drops to facilitate template writing. This part is compatible with Shopify Liquid, as we need templates to be portable.

blank

Useful to check whether a string variable is false, null, undefined, an empty string, or a string containing only blank characters.

{% unless author == blank %}
    {{author}}
{% endif %}

empty

Useful to check if an array, string, or object is empty.

{% if authors == empty %}
    Author list is empty
{% endif %}

{% note info empty implementation %} For arrays and strings, LiquidJS checks their .length property. For objects, LiquidJS calls Object.keys() to check whether they have keys. {% endnote %}

nil

nil Drop is used to check whether a variable is not defined or defined as null or undefined, essentially equivalent to JavaScript == null check.

{% if notexist == nil %}
    null variable
{% endif %}

Other Drops

There are still several Drops for specific tags, like forloop, tablerowloop, block, which are covered by respective tag documents.