A few built-in filters don't support undefined values

[pdehaan]

Related to #479
liquidjs@9.35.0

ERROR IN {{ foo | join }}: Cannot read properties of undefined (reading 'join'), line:1, col:1
ERROR IN {{ foo | map }}: Cannot read properties of undefined (reading 'split'), line:1, col:1
ERROR IN {{ foo | reverse }}: undefined is not iterable (cannot read property Symbol(Symbol.iterator)), line:1, col:1
ERROR IN {{ foo | slice }}: Cannot read properties of undefined (reading 'slice'), line:1, col:1
ERROR IN {{ foo | newline_to_br }}: Cannot read properties of undefined (reading 'replace'), line:1, col:1
ERROR IN {{ foo | strip_html }}: Cannot read properties of undefined (reading 'replace'), line:1, col:1
ERROR IN {{ foo | truncatewords }}: Cannot read properties of undefined (reading 'split'), line:1, col:1

Apologies for the bad code:

// liquid-filter-test.mjs
import {Liquid} from "liquidjs";

const APPEND_JSON_FILTER = false;

function createFilterTest(name) {
  // Filters that need a second parameter...
  const secondParam = (["append", "prepend"].includes(name) ? ": bar" : "");
  // Don't double append edge case "|json |json"...
  const jsonFilter = name !== "json" && !!APPEND_JSON_FILTER ? " | json" : "";
  return `{{ foo | ${name}${secondParam}${jsonFilter} }}`;
}

const arrayFilters = ["concat", "first", "join", "json", "last", "map", "reverse", "slice", "sort_natural", "sort", "uniq", "where"];
const dateFilters = ["date"];
const htmlUriFilters = ["escape", "escape_once", "newline_to_br", "strip_html", "url_decode", "url_encode"];
const mathFilters = ["abs", "ceil", "divided_by", "floor", "minus", "modulo", "plus", "round", "times"];
const stringFilters = ["append", "capitalize", "downcase", "lstrip", "prepend", "remove", "remove_first", "replace", "replace_first", "rstrip", "split", "strip", "strip_newlines", "truncate", "truncatewords", "upcase"];

const allFilters = [
  ...arrayFilters,
  ...dateFilters,
  ...htmlUriFilters,
  ...mathFilters,
  ...stringFilters,
].map(name => createFilterTest(name));

const engine = new Liquid();
const results = [];

for (const input of allFilters) {
  const tpl = engine.parse(input);
  try {
    const res = await engine.render(tpl, {foo: undefined, bar: undefined})
    results.push(`${input} = ${res}`);
  } catch (err) {
    console.error(`ERROR IN ${input}: ${err.message}`);
  }
}

console.log(results.sort().join("\n"));

OUTPUT

{{ foo | abs }} = NaN
{{ foo | append: bar }} = 
{{ foo | capitalize }} = 
{{ foo | ceil }} = NaN
{{ foo | concat }} = 
{{ foo | date }} = 
{{ foo | divided_by }} = NaN
{{ foo | downcase }} = 
{{ foo | escape }} = 
{{ foo | escape_once }} = undefined
{{ foo | first }} = 
{{ foo | floor }} = NaN
{{ foo | json }} = 
{{ foo | last }} = 
{{ foo | lstrip }} = 
{{ foo | minus }} = NaN
{{ foo | modulo }} = NaN
{{ foo | plus }} = NaN
{{ foo | prepend: bar }} = 
{{ foo | remove }} = 
{{ foo | remove_first }} = 
{{ foo | replace }} = 
{{ foo | replace_first }} = 
{{ foo | round }} = NaN
{{ foo | rstrip }} = 
{{ foo | sort }} = 
{{ foo | sort_natural }} = 
{{ foo | split }} = 
{{ foo | strip }} = 
{{ foo | strip_newlines }} = 
{{ foo | times }} = NaN
{{ foo | truncate }} = 
{{ foo | uniq }} = 
{{ foo | upcase }} = 
{{ foo | url_decode }} = 
{{ foo | url_encode }} = 
{{ foo | where }} =

Interestingly it looks like {{ undefined | escape_once }} returns the literal string "undefined". Otherwise nothing too suspicious looking here (all the math functions seems to return NaN, which is semi-expected).

---

If I append the | json filter to each test case, it gets more interesting:

{{ foo | abs | json }} = null
{{ foo | append: bar | json }} = ""
{{ foo | capitalize | json }} = ""
{{ foo | ceil | json }} = null
{{ foo | concat | json }} = [null,null]
{{ foo | date | json }} = 
{{ foo | divided_by | json }} = null
{{ foo | downcase | json }} = ""
{{ foo | escape | json }} = ""
{{ foo | escape_once | json }} = "undefined"
{{ foo | first | json }} = ""
{{ foo | floor | json }} = null
{{ foo | json }} = 
{{ foo | last | json }} = ""
{{ foo | lstrip | json }} = ""
{{ foo | minus | json }} = null
{{ foo | modulo | json }} = null
{{ foo | plus | json }} = null
{{ foo | prepend: bar | json }} = ""
{{ foo | remove | json }} = ""
{{ foo | remove_first | json }} = ""
{{ foo | replace | json }} = ""
{{ foo | replace_first | json }} = ""
{{ foo | round | json }} = null
{{ foo | rstrip | json }} = ""
{{ foo | sort | json }} = [null]
{{ foo | sort_natural | json }} = []
{{ foo | split | json }} = [""]
{{ foo | strip | json }} = ""
{{ foo | strip_newlines | json }} = ""
{{ foo | times | json }} = null
{{ foo | truncate | json }} = ""
{{ foo | uniq | json }} = []
{{ foo | upcase | json }} = ""
{{ foo | url_decode | json }} = ""
{{ foo | url_encode | json }} = ""
{{ foo | where | json }} = []

It looks like the NaN now converts to null (presumably NaN isn't valid JSON). Interesting cases might be:

{{ undefined | concat | json }} = [null,null]
{{ undefined | escape_once | json }} = "undefined"
{{ undefined | sort | json }} = [null]
{{ undefined | sort_natural | json }} = []
{{ undefined | split | json }} = [""]

[harttle]

I checked the behavior of shopify/liquid for these cases (except that they don't have a json filter). Here's some changes will be made on LiquidJS to align with Ruby:

• filters like join, reverse, truncatewords wont throw on nil input, LiquidJS will not throw as well
• concat, sort, sort_natural will ignore nil value input so undefined will not be added into result array. Actually, toArray will return [] for null/undefined instead of [null]/[undefined]
• split will return empty array for nil input and remove trailing empty strings

[pdehaan]

Awesome, thanks… Here's the current results for 9.35.1 (where foo is undefined):

FILTER	VALUE	+JSON FILTER	VALUE
{{ foo | abs }}	NaN	{{ foo | abs | json }}	null
{{ foo | append: bar }}		{{ foo | append: bar | json }}	""
{{ foo | capitalize }}		{{ foo | capitalize | json }}	""
{{ foo | ceil }}	NaN	{{ foo | ceil | json }}	null
{{ foo | concat }}		{{ foo | concat | json }}	[null]
{{ foo | date }}		{{ foo | date | json }}
{{ foo | divided_by }}	NaN	{{ foo | divided_by | json }}	null
{{ foo | downcase }}		{{ foo | downcase | json }}	""
{{ foo | escape }}		{{ foo | escape | json }}	""
{{ foo | escape_once }}		{{ foo | escape_once | json }}	""
{{ foo | first }}		{{ foo | first | json }}	""
{{ foo | floor }}	NaN	{{ foo | floor | json }}	null
{{ foo | join }}		{{ foo | join | json }}	""
{{ foo | json }}		{{ foo | json }}
{{ foo | last }}		{{ foo | last | json }}	""
{{ foo | lstrip }}		{{ foo | lstrip | json }}	""
{{ foo | map }}		{{ foo | map | json }}	[]
{{ foo | minus }}	NaN	{{ foo | minus | json }}	null
{{ foo | modulo }}	NaN	{{ foo | modulo | json }}	null
{{ foo | newline_to_br }}		{{ foo | newline_to_br | json }}	""
{{ foo | plus }}	NaN	{{ foo | plus | json }}	null
{{ foo | prepend: bar }}		{{ foo | prepend: bar | json }}	""
{{ foo | remove }}		{{ foo | remove | json }}	""
{{ foo | remove_first }}		{{ foo | remove_first | json }}	""
{{ foo | replace }}		{{ foo | replace | json }}	""
{{ foo | replace_first }}		{{ foo | replace_first | json }}	""
{{ foo | reverse }}		{{ foo | reverse | json }}	[]
{{ foo | round }}	NaN	{{ foo | round | json }}	null
{{ foo | rstrip }}		{{ foo | rstrip | json }}	""
{{ foo | slice }}		{{ foo | slice | json }}	[]
{{ foo | sort }}		{{ foo | sort | json }}	[]
{{ foo | sort_natural }}		{{ foo | sort_natural | json }}	[]
{{ foo | split }}		{{ foo | split | json }}	[]
{{ foo | strip }}		{{ foo | strip | json }}	""
{{ foo | strip_html }}		{{ foo | strip_html | json }}	""
{{ foo | strip_newlines }}		{{ foo | strip_newlines | json }}	""
{{ foo | times }}	NaN	{{ foo | times | json }}	null
{{ foo | truncate }}		{{ foo | truncate | json }}	""
{{ foo | truncatewords }}		{{ foo | truncatewords | json }}	""
{{ foo | uniq }}		{{ foo | uniq | json }}	[]
{{ foo | upcase }}		{{ foo | upcase | json }}	""
{{ foo | url_decode }}		{{ foo | url_decode | json }}	""
{{ foo | url_encode }}		{{ foo | url_encode | json }}	""
{{ foo | where }}		{{ foo | where | json }}	[]

Looks like the only curious entry in 9.35.1 is {{ undefined | concat }} which returns [null]:

<p>{{ foo | concat | size }} === 1</p>
<p>{{ foo | concat | json }} === [null]</p>

Liquid Playground example