diff --git a/src/aff/affix.ts b/src/aff/affix.ts
index d5f8b8a..2de5d74 100644
--- a/src/aff/affix.ts
+++ b/src/aff/affix.ts
@@ -3,9 +3,9 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { CONSTANTS as C } from "../constants"
-import { re } from "../util"
-import type { Aff, Flag, Flags } from "./index"
+import { CONSTANTS as C } from "../constants.js"
+import { re } from "../util.js"
+import type { Aff, Flag, Flags } from "./index.js"
 
 /** Base class for the {@link Prefix}/{@link Suffix} classes. Won't work by itself. */
 export abstract class Affix {
diff --git a/src/aff/casing.ts b/src/aff/casing.ts
index cf40aa3..b0b379e 100644
--- a/src/aff/casing.ts
+++ b/src/aff/casing.ts
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import { CapType } from "../constants"
+import { CapType } from "../constants.js"
 import {
   isLowercased,
   isTitlecased,
@@ -11,7 +11,7 @@ import {
   replaceRange,
   titlecase,
   uppercase
-} from "../util"
+} from "../util.js"
 
 /**
  * A class containing casing-related algorithms for a language. This is a
diff --git a/src/aff/compound-pattern.ts b/src/aff/compound-pattern.ts
index f98d240..0794085 100644
--- a/src/aff/compound-pattern.ts
+++ b/src/aff/compound-pattern.ts
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { AffixForm } from "../lookup/forms"
-import type { Flag } from "./index"
+import type { AffixForm } from "../lookup/forms.js"
+import type { Flag } from "./index.js"
 
 /**
  * A pattern for checking if a pair of {@link AffixForm}s are a valid
diff --git a/src/aff/compound-rule.ts b/src/aff/compound-rule.ts
index da0cc82..96f1a2b 100644
--- a/src/aff/compound-rule.ts
+++ b/src/aff/compound-rule.ts
@@ -3,9 +3,9 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { intersect, product, re } from "../util"
-import type { Aff, Flags, FlagSet } from "./index"
-import type { Flag } from "./types"
+import { intersect, product, re } from "../util.js"
+import type { Aff, Flags, FlagSet } from "./index.js"
+import type { Flag } from "./types.js"
 
 // TODO: make this not use a weird regex matching system
 // I couldn't exactly wrap my brain around how to avoid using regex here
diff --git a/src/aff/conv-table.ts b/src/aff/conv-table.ts
index e2a3bf2..2fcfd0e 100644
--- a/src/aff/conv-table.ts
+++ b/src/aff/conv-table.ts
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import { re } from "../util"
+import { re } from "../util.js"
 
 /**
  * A table of conversions (either input to output, or output to input) for
diff --git a/src/aff/index.ts b/src/aff/index.ts
index 26ae6c7..41b893e 100644
--- a/src/aff/index.ts
+++ b/src/aff/index.ts
@@ -3,17 +3,17 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { CONSTANTS as C } from "../constants"
-import type { Reader } from "../reader"
-import { Trie } from "../trie"
-import { escapeRegExp, re, reverse, split } from "../util"
-import { Prefix, Suffix } from "./affix"
-import { Casing, GermanCasing, TurkicCasing } from "./casing"
-import { CompoundPattern } from "./compound-pattern"
-import { CompoundRule } from "./compound-rule"
-import { ConvTable } from "./conv-table"
-import { PhonetTable } from "./phonet-table"
-import { RepPattern } from "./rep-pattern"
+import { CONSTANTS as C } from "../constants.js"
+import type { Reader } from "../reader.js"
+import { Trie } from "../trie.js"
+import { escapeRegExp, re, reverse, split } from "../util.js"
+import { Prefix, Suffix } from "./affix.js"
+import { Casing, GermanCasing, TurkicCasing } from "./casing.js"
+import { CompoundPattern } from "./compound-pattern.js"
+import { CompoundRule } from "./compound-rule.js"
+import { ConvTable } from "./conv-table.js"
+import { PhonetTable } from "./phonet-table.js"
+import { RepPattern } from "./rep-pattern.js"
 import type {
   AffData,
   CharacterMap,
diff --git a/src/aff/phonet-table.ts b/src/aff/phonet-table.ts
index 4acea31..0b690cf 100644
--- a/src/aff/phonet-table.ts
+++ b/src/aff/phonet-table.ts
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import { CONSTANTS as C } from "../constants"
-import { re, uppercase } from "../util"
+import { CONSTANTS as C } from "../constants.js"
+import { re, uppercase } from "../util.js"
 
 // TODO: use homegrown metaphone implementation (maybe)
 
diff --git a/src/aff/rep-pattern.ts b/src/aff/rep-pattern.ts
index 2499aa4..2161384 100644
--- a/src/aff/rep-pattern.ts
+++ b/src/aff/rep-pattern.ts
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import { re, replaceRange } from "../util"
+import { re, replaceRange } from "../util.js"
 
 /**
  * A replacement pattern describes a pattern and its replacement, with the
diff --git a/src/aff/types.d.ts b/src/aff/types.d.ts
index 6fd97ed..1ae6e84 100644
--- a/src/aff/types.d.ts
+++ b/src/aff/types.d.ts
@@ -2,13 +2,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { Trie } from "../trie"
-import type { Affix, Prefix, Suffix } from "./affix"
-import type { CompoundPattern } from "./compound-pattern"
-import type { CompoundRule } from "./compound-rule"
-import type { ConvTable } from "./conv-table"
-import type { PhonetTable } from "./phonet-table"
-import type { RepPattern } from "./rep-pattern"
+import type { Trie } from "../trie.js"
+import type { Affix, Prefix, Suffix } from "./affix.js"
+import type { CompoundPattern } from "./compound-pattern.js"
+import type { CompoundRule } from "./compound-rule.js"
+import type { ConvTable } from "./conv-table.js"
+import type { PhonetTable } from "./phonet-table.js"
+import type { RepPattern } from "./rep-pattern.js"
 
 /**
  * A flag is just a string label for some special property. This may mark a
diff --git a/src/dic/index.ts b/src/dic/index.ts
index a8a454b..e237b12 100644
--- a/src/dic/index.ts
+++ b/src/dic/index.ts
@@ -3,10 +3,10 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import type { Aff, Flag } from "../aff"
-import { CapType, CONSTANTS as C } from "../constants"
-import type { Reader } from "../reader"
-import { Word } from "./word"
+import type { Aff, Flag } from "../aff/index.js"
+import { CapType, CONSTANTS as C } from "../constants.js"
+import type { Reader } from "../reader.js"
+import { Word } from "./word.js"
 
 export { Word }
 
diff --git a/src/dic/word.ts b/src/dic/word.ts
index bce25d4..25ee189 100644
--- a/src/dic/word.ts
+++ b/src/dic/word.ts
@@ -3,11 +3,11 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import type { Aff, Flag, Flags, FlagSet } from "../aff"
-import type { Prefix, Suffix } from "../aff/affix"
-import { RepPattern } from "../aff/rep-pattern"
-import { CapType, CONSTANTS as C } from "../constants"
-import { split } from "../util"
+import type { Prefix, Suffix } from "../aff/affix.js"
+import type { Aff, Flag, Flags, FlagSet } from "../aff/index.js"
+import { RepPattern } from "../aff/rep-pattern.js"
+import { CapType, CONSTANTS as C } from "../constants.js"
+import { split } from "../util.js"
 
 /** A word as found in a {@link Dic} instance's index. */
 export class Word {
diff --git a/src/index.ts b/src/index.ts
index 9871190..bb9d04c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -3,16 +3,16 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { Aff, OverridableAffData } from "./aff/index"
-import { decoder } from "./constants"
-import { Dic, Word } from "./dic/index"
-import { Lookup } from "./lookup"
-import { Reader } from "./reader"
-import { Suggest } from "./suggest"
-import { concat } from "./util"
-
-export type { AffData, Flag, Flags, FlagSet, OverridableAffData } from "./aff"
-export type { LookupResult } from "./lookup"
+import { Aff, OverridableAffData } from "./aff/index.js"
+import { decoder } from "./constants.js"
+import { Dic, Word } from "./dic/index.js"
+import { Lookup } from "./lookup/index.js"
+import { Reader } from "./reader.js"
+import { Suggest } from "./suggest/index.js"
+import { concat } from "./util.js"
+
+export type { AffData, Flag, Flags, FlagSet, OverridableAffData } from "./aff/index.js"
+export type { LookupResult } from "./lookup/index.js"
 
 export interface EspellsInitOpts {
   /** Source of a `.aff` affix file. */
diff --git a/src/lookup/affixes.ts b/src/lookup/affixes.ts
index 2e8f2cb..84220e3 100644
--- a/src/lookup/affixes.ts
+++ b/src/lookup/affixes.ts
@@ -2,13 +2,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import { CapType, CompoundPos } from "../constants"
-import type { Word } from "../dic"
-import { includes, lowercase } from "../util"
-import { decompose } from "./decompose"
-import type { AffixForm } from "./forms"
-import { LKFlags } from "./lk-flags"
-import type { LKWord } from "./lk-word"
+import { CapType, CompoundPos } from "../constants.js"
+import type { Word } from "../dic/index.js"
+import { includes, lowercase } from "../util.js"
+import { decompose } from "./decompose.js"
+import type { AffixForm } from "./forms.js"
+import { LKFlags } from "./lk-flags.js"
+import type { LKWord } from "./lk-word.js"
 
 /**
  * Yields the allowed {@link AffixForm}s for this word, as in all ways the
diff --git a/src/lookup/compounds.ts b/src/lookup/compounds.ts
index 4a44c70..529e572 100644
--- a/src/lookup/compounds.ts
+++ b/src/lookup/compounds.ts
@@ -2,16 +2,16 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { Flags } from "../aff"
-import type { CompoundRule } from "../aff/compound-rule"
-import { CapType, CompoundPos } from "../constants"
-import { Word } from "../dic"
-import { replchars } from "../permutations"
-import { any, isTriplet, isUppercased } from "../util"
-import { affixForms } from "./affixes"
-import { AffixForm } from "./forms"
-import { LKFlags } from "./lk-flags"
-import type { LKWord } from "./lk-word"
+import type { CompoundRule } from "../aff/compound-rule.js"
+import type { Flags } from "../aff/index.js"
+import { CapType, CompoundPos } from "../constants.js"
+import { Word } from "../dic/index.js"
+import { replchars } from "../permutations.js"
+import { any, isTriplet, isUppercased } from "../util.js"
+import { affixForms } from "./affixes.js"
+import { AffixForm } from "./forms.js"
+import { LKFlags } from "./lk-flags.js"
+import type { LKWord } from "./lk-word.js"
 
 /**
  * A hypothesis of how a compound word may be constructed, using an array
diff --git a/src/lookup/decompose.ts b/src/lookup/decompose.ts
index 5583a87..7e0f8d0 100644
--- a/src/lookup/decompose.ts
+++ b/src/lookup/decompose.ts
@@ -2,12 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { Aff } from "../aff"
-import { CompoundPos } from "../constants"
-import { concat, reverse } from "../util"
-import { AffixForm } from "./forms"
-import type { LKFlags } from "./lk-flags"
-import type { LKWord } from "./lk-word"
+import type { Aff } from "../aff/index.js"
+import { CompoundPos } from "../constants.js"
+import { concat, reverse } from "../util.js"
+import { AffixForm } from "./forms.js"
+import type { LKFlags } from "./lk-flags.js"
+import type { LKWord } from "./lk-word.js"
 
 export const enum AffixType {
   PREFIX,
diff --git a/src/lookup/forms.ts b/src/lookup/forms.ts
index e73036d..a37c990 100644
--- a/src/lookup/forms.ts
+++ b/src/lookup/forms.ts
@@ -2,10 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { Prefix, Suffix } from "../aff/affix"
-import type { Word } from "../dic/word"
-import { concat } from "../util"
-import { LKWord } from "./lk-word"
+import type { Prefix, Suffix } from "../aff/affix.js"
+import type { Word } from "../dic/word.js"
+import { concat } from "../util.js"
+import { LKWord } from "./lk-word.js"
 
 export interface AffixFormOpts {
   /** Outermost prefix. */
diff --git a/src/lookup/index.ts b/src/lookup/index.ts
index 4bf6d84..4f11acd 100644
--- a/src/lookup/index.ts
+++ b/src/lookup/index.ts
@@ -3,14 +3,14 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import type { Aff } from "../aff"
-import { CapType, CONSTANTS as C } from "../constants"
-import type { Dic } from "../dic"
-import { any, includes } from "../util"
-import { affixForms as affixes } from "./affixes"
-import { compoundForms as compounds } from "./compounds"
-import { breakWord } from "./decompose"
-import { LKWord } from "./lk-word"
+import type { Aff } from "../aff/index.js"
+import { CapType, CONSTANTS as C } from "../constants.js"
+import type { Dic } from "../dic/index.js"
+import { any, includes } from "../util.js"
+import { affixForms as affixes } from "./affixes.js"
+import { compoundForms as compounds } from "./compounds.js"
+import { breakWord } from "./decompose.js"
+import { LKWord } from "./lk-word.js"
 
 /** The resulting data returned from executing a lookup. */
 export interface LookupResult {
diff --git a/src/lookup/lk-flags.ts b/src/lookup/lk-flags.ts
index 92c2f27..53ad9cd 100644
--- a/src/lookup/lk-flags.ts
+++ b/src/lookup/lk-flags.ts
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { Flags } from "../aff"
+import type { Flags } from "../aff/index.js"
 
 export type LKFlagsOpts = { prefix?: Flags; suffix?: Flags; forbidden?: Flags }
 
diff --git a/src/lookup/lk-word.ts b/src/lookup/lk-word.ts
index e7011c1..43e17ec 100644
--- a/src/lookup/lk-word.ts
+++ b/src/lookup/lk-word.ts
@@ -2,9 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { Aff } from "../aff"
-import type { CapType, CompoundPos } from "../constants"
-import type { Dic } from "../dic"
+import type { Aff } from "../aff/index.js"
+import type { CapType, CompoundPos } from "../constants.js"
+import type { Dic } from "../dic/index.js"
 
 /**
  * A word (a string) wrapped with metadata. Can be iterated, and will
diff --git a/src/permutations.ts b/src/permutations.ts
index 9a294cf..2319b32 100644
--- a/src/permutations.ts
+++ b/src/permutations.ts
@@ -2,10 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { CharacterMap } from "./aff"
-import type { RepPattern } from "./aff/rep-pattern"
-import { CONSTANTS as C } from "./constants"
-import { replaceRange, uppercase } from "./util"
+import type { CharacterMap } from "./aff/index.js"
+import type { RepPattern } from "./aff/rep-pattern.js"
+import { CONSTANTS as C } from "./constants.js"
+import { replaceRange, uppercase } from "./util.js"
 
 /**
  * Uses a {@link Aff}'s {@link RepPattern} table to yield replaced
diff --git a/src/reader.ts b/src/reader.ts
index 2a00541..35f5a95 100644
--- a/src/reader.ts
+++ b/src/reader.ts
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import { decoder } from "./constants"
+import { decoder } from "./constants.js"
 
 /** Wrapper for manipulating a document separated by lines. */
 export class Reader {
diff --git a/src/suggest/index.ts b/src/suggest/index.ts
index 9707806..37fd7c2 100644
--- a/src/suggest/index.ts
+++ b/src/suggest/index.ts
@@ -3,11 +3,11 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import type { Aff } from "../aff"
-import { CapType, CONSTANTS as C, GOOD_EDITS, SuggestionKind } from "../constants"
-import type { Dic } from "../dic"
-import type { Word } from "../dic/word"
-import type { Lookup } from "../lookup"
+import type { Aff } from "../aff/index.js"
+import { CapType, CONSTANTS as C, GOOD_EDITS, SuggestionKind } from "../constants.js"
+import type { Dic } from "../dic/index.js"
+import type { Word } from "../dic/word.js"
+import type { Lookup } from "../lookup/index.js"
 import {
   badchar,
   badcharkey,
@@ -20,11 +20,11 @@ import {
   replchars,
   swapchar,
   twowords
-} from "../permutations"
-import { intersect, limit, lowercase, uppercase } from "../util"
-import { NgramSuggestionBuilder } from "./ngram"
-import { PhonetSuggestionBuilder } from "./phonet"
-import { MultiWordSuggestion, Suggestion } from "./suggestion"
+} from "../permutations.js"
+import { intersect, limit, lowercase, uppercase } from "../util.js"
+import { NgramSuggestionBuilder } from "./ngram.js"
+import { PhonetSuggestionBuilder } from "./phonet.js"
+import { MultiWordSuggestion, Suggestion } from "./suggestion.js"
 
 /**
  * Represents a {@link Suggest} function that "handles" potential
diff --git a/src/suggest/ngram.ts b/src/suggest/ngram.ts
index 4e4de51..6a270a2 100644
--- a/src/suggest/ngram.ts
+++ b/src/suggest/ngram.ts
@@ -3,16 +3,16 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { CONSTANTS as C } from "../constants"
-import type { Word } from "../dic/word"
-import { lowercase } from "../util"
+import { CONSTANTS as C } from "../constants.js"
+import type { Word } from "../dic/word.js"
+import { lowercase } from "../util.js"
 import {
   preciseAffixScore,
   rootScore,
   roughAffixScore,
   ScoresList,
   scoreThreshold
-} from "./scores"
+} from "./scores.js"
 
 export class NgramSuggestionBuilder {
   /** The "root" scores for potential suggestions for a misspelling. */
diff --git a/src/suggest/phonet.ts b/src/suggest/phonet.ts
index c4d2019..617c6cb 100644
--- a/src/suggest/phonet.ts
+++ b/src/suggest/phonet.ts
@@ -2,11 +2,11 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { PhonetTable } from "../aff/phonet-table"
-import { CONSTANTS as C } from "../constants"
-import type { Word } from "../dic/word"
-import { lowercase, ngram } from "../util"
-import { finalScore, rootScore, ScoresList } from "./scores"
+import type { PhonetTable } from "../aff/phonet-table.js"
+import { CONSTANTS as C } from "../constants.js"
+import type { Word } from "../dic/word.js"
+import { lowercase, ngram } from "../util.js"
+import { finalScore, rootScore, ScoresList } from "./scores.js"
 
 /**
  * Builder for phonetic suggestions.
diff --git a/src/suggest/scores.ts b/src/suggest/scores.ts
index dcd5706..1755d22 100644
--- a/src/suggest/scores.ts
+++ b/src/suggest/scores.ts
@@ -3,8 +3,14 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { PriorityList } from "../plist"
-import { commonCharacters, lcslen, leftCommonSubstring, lowercase, ngram } from "../util"
+import { PriorityList } from "../plist.js"
+import {
+  commonCharacters,
+  lcslen,
+  leftCommonSubstring,
+  lowercase,
+  ngram
+} from "../util.js"
 
 /** An entry in a {@link ScoresList}. */
 export type ScoreEntry<T extends any[]> = [number, ...T]
diff --git a/src/suggest/suggestion.ts b/src/suggest/suggestion.ts
index fdf0baf..edee74a 100644
--- a/src/suggest/suggestion.ts
+++ b/src/suggest/suggestion.ts
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-import type { SuggestionKind } from "../constants"
+import type { SuggestionKind } from "../constants.js"
 
 /** The object that the {@link Suggest} class uses internally for tracking suggestions. */
 export class Suggestion {
diff --git a/src/util.ts b/src/util.ts
index 30abc82..37acd6a 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import { iterate } from "iterare"
-import { CONSTANTS as C } from "./constants"
+import { CONSTANTS as C } from "./constants.js"
 
 /**
  * Creates a `RegExp` using a string template. Supports flags.