diff --git a/lib/lines.ts b/lib/lines.ts
index 22719536..38cf03ab 100644
--- a/lib/lines.ts
+++ b/lib/lines.ts
@@ -22,6 +22,10 @@ type LineInfo = {
   readonly sliceEnd: number;
 };
 
+type MutableLineInfo = {
+  -readonly [K in keyof LineInfo]: LineInfo[K];
+}
+
 export class Lines {
   public readonly length: number;
   public readonly name: string | null;
@@ -176,7 +180,7 @@ export class Lines {
     if (skipFirstLine && this.length === 1) return this;
 
     const lines = new Lines(
-      this.infos.map(function (info: any, i: any) {
+      this.infos.map(function (info, i) {
         if (info.line && (i > 0 || !skipFirstLine)) {
           info = {
             ...info,
@@ -190,7 +194,7 @@ export class Lines {
     if (this.mappings.length > 0) {
       const newMappings = lines.mappings;
       invariant(newMappings.length === 0);
-      this.mappings.forEach(function (mapping: any) {
+      this.mappings.forEach(function (mapping) {
         newMappings.push(mapping.indent(width, skipFirstLine, true));
       });
     }
@@ -204,7 +208,7 @@ export class Lines {
     }
 
     const lines = new Lines(
-      this.infos.map(function (info: any) {
+      this.infos.map(function (info) {
         if (info.line && !info.locked) {
           info = {
             ...info,
@@ -218,7 +222,7 @@ export class Lines {
     if (this.mappings.length > 0) {
       const newMappings = lines.mappings;
       invariant(newMappings.length === 0);
-      this.mappings.forEach(function (mapping: any) {
+      this.mappings.forEach(function (mapping) {
         newMappings.push(mapping.indent(by));
       });
     }
@@ -236,7 +240,7 @@ export class Lines {
     }
 
     const lines = new Lines(
-      this.infos.map(function (info: any, i: any) {
+      this.infos.map(function (info, i) {
         if (i > 0 && info.line && !info.locked) {
           info = {
             ...info,
@@ -251,7 +255,7 @@ export class Lines {
     if (this.mappings.length > 0) {
       const newMappings = lines.mappings;
       invariant(newMappings.length === 0);
-      this.mappings.forEach(function (mapping: any) {
+      this.mappings.forEach(function (mapping) {
         newMappings.push(mapping.indent(by, true));
       });
     }
@@ -265,7 +269,7 @@ export class Lines {
     }
 
     return new Lines(
-      this.infos.map((info: any, i: any) => ({
+      this.infos.map((info, i) => ({
         ...info,
         locked: i > 0,
       })),
@@ -282,7 +286,7 @@ export class Lines {
       return this.cachedTabWidth;
     }
 
-    const counts: any[] = []; // Sparse array.
+    const counts: number[] = []; // Sparse array.
     let lastIndent = 0;
 
     for (let line = 1, last = this.length; line <= last; ++line) {
@@ -527,7 +531,7 @@ export class Lines {
     } else {
       invariant(start.line < end.line);
       sliced[0] = sliceInfo(sliced[0], start.column);
-      sliced.push(sliceInfo(sliced.pop(), 0, end.column));
+      sliced.push(sliceInfo(sliced.pop()!, 0, end.column));
     }
 
     const lines = new Lines(sliced);
@@ -535,7 +539,7 @@ export class Lines {
     if (this.mappings.length > 0) {
       const newMappings = lines.mappings;
       invariant(newMappings.length === 0);
-      this.mappings.forEach(function (this: any, mapping: any) {
+      this.mappings.forEach(function (this: Lines, mapping) {
         const sliced = mapping.slice(this, start, end);
         if (sliced) {
           newMappings.push(sliced);
@@ -618,9 +622,9 @@ export class Lines {
 
   join(elements: (string | Lines)[]) {
     const separator = this;
-    const infos: any[] = [];
-    const mappings: any[] = [];
-    let prevInfo: any;
+    const infos: LineInfo[] = [];
+    const mappings: Mapping[] = [];
+    let prevInfo: MutableLineInfo | undefined;
 
     function appendLines(linesOrNull: Lines | null) {
       if (linesOrNull === null) {
@@ -648,7 +652,7 @@ export class Lines {
         prevInfo.sliceEnd = prevInfo.line.length;
 
         if (linesOrNull.mappings.length > 0) {
-          linesOrNull.mappings.forEach(function (mapping: any) {
+          linesOrNull.mappings.forEach(function (mapping) {
             mappings.push(mapping.add(prevLine, prevColumn));
           });
         }
@@ -656,7 +660,7 @@ export class Lines {
         mappings.push.apply(mappings, linesOrNull.mappings);
       }
 
-      linesOrNull.infos.forEach(function (info: any, i: any) {
+      linesOrNull.infos.forEach(function (info, i) {
         if (!prevInfo || i > 0) {
           prevInfo = { ...info };
           infos.push(prevInfo);
@@ -670,7 +674,7 @@ export class Lines {
     }
 
     elements
-      .map(function (elem: any) {
+      .map(function (elem) {
         const lines = fromString(elem);
         if (lines.isEmpty()) return null;
         return lines;
@@ -700,11 +704,11 @@ export class Lines {
   }
 }
 
-const fromStringCache: any = {};
+const fromStringCache: Record<string, Lines> = {};
 const hasOwn = fromStringCache.hasOwnProperty;
 const maxCacheKeyLen = 10;
 
-export function countSpaces(spaces: any, tabWidth?: number) {
+export function countSpaces(spaces: string, tabWidth?: number) {
   let count = 0;
   const len = spaces.length;
 
@@ -794,7 +798,7 @@ function isOnlyWhitespace(string: string) {
   return !/\S/.test(string);
 }
 
-function sliceInfo(info: any, startCol: number, endCol?: number) {
+function sliceInfo(info: LineInfo, startCol: number, endCol?: number) {
   let sliceStart = info.sliceStart;
   let sliceEnd = info.sliceEnd;
   let indent = Math.max(info.indent, 0);
@@ -848,7 +852,7 @@ function sliceInfo(info: any, startCol: number, endCol?: number) {
   };
 }
 
-export function concat(elements: any) {
+export function concat(elements: (string | Lines)[]) {
   return emptyLines.join(elements);
 }
 
diff --git a/test/lines.ts b/test/lines.ts
index dcb0ad8e..645bb3cc 100644
--- a/test/lines.ts
+++ b/test/lines.ts
@@ -3,16 +3,20 @@ import fs from "fs";
 import path from "path";
 import { fromString, concat, countSpaces, Lines } from "../lib/lines";
 import { EOL as eol } from "os";
+import { namedTypes } from "ast-types";
 
-function check(a: any, b: any) {
-  assert.strictEqual(
-    a.toString({
-      lineTerminator: eol,
-    }),
-    b.toString({
-      lineTerminator: eol,
-    }),
-  );
+type LinesInput = string | Lines;
+
+function toString(s: LinesInput): string {
+  if (typeof s === 'string') {
+    return s;
+  }
+
+  return s.toString({ lineTerminator: eol });
+}
+
+function check(a: LinesInput, b: LinesInput) {
+  assert.strictEqual(toString(a), toString(b));
 }
 
 describe("lines", function () {
@@ -37,7 +41,7 @@ describe("lines", function () {
   });
 
   it("FromString", function () {
-    function checkIsCached(s: any) {
+    function checkIsCached(s: LinesInput) {
       assert.strictEqual(fromString(s), fromString(s));
       check(fromString(s), s);
     }
@@ -66,19 +70,19 @@ describe("lines", function () {
     check(lines.indentTail(5).indentTail(-7).indentTail(2), code);
   });
 
-  function testEachPosHelper(lines: any, code: any) {
+  function testEachPosHelper(lines: Lines, code: string) {
     check(lines, code);
 
-    const chars: any[] = [];
+    const chars: string[] = [];
     let emptyCount = 0;
 
-    function iterator(pos: any) {
+    function iterator(pos: namedTypes.Position) {
       const ch = lines.charAt(pos);
       if (ch === "") emptyCount += 1;
       chars.push(ch);
     }
 
-    lines.eachPos(iterator, null);
+    lines.eachPos(iterator, undefined);
 
     // The character at the position just past the end (as returned by
     // lastPos) should be the only empty string.
@@ -94,7 +98,7 @@ describe("lines", function () {
 
     const withoutSpaces = code.replace(/\s+/g, "");
     chars.length = emptyCount = 0;
-    lines.eachPos(iterator, null, true); // Skip spaces this time.
+    lines.eachPos(iterator, undefined, true); // Skip spaces this time.
     assert.strictEqual(emptyCount, 0);
     joined = chars.join("");
     assert.strictEqual(joined.length, withoutSpaces.length);
@@ -123,7 +127,7 @@ describe("lines", function () {
     const code = String(CharAtTest).replace(/\r\n/g, "\n");
     const lines = fromString(code);
 
-    function compare(pos: any) {
+    function compare(pos: namedTypes.Position) {
       assert.strictEqual(lines.charAt(pos), lines.bootstrapCharAt(pos));
     }
 
@@ -133,7 +137,7 @@ describe("lines", function () {
       indented = original.indentTail(4),
       reference = fromString("  ab" + eol + "      c");
 
-    function compareIndented(pos: any) {
+    function compareIndented(pos: namedTypes.Position) {
       const c = indented.charAt(pos);
       check(c, reference.charAt(pos));
       check(c, indented.bootstrapCharAt(pos));
@@ -197,7 +201,7 @@ describe("lines", function () {
   });
 
   it("Empty", function () {
-    function c(s: any) {
+    function c(s: LinesInput) {
       const lines = fromString(s);
       check(lines, s);
       assert.strictEqual(lines.isEmpty(), s.length === 0);
@@ -254,9 +258,9 @@ describe("lines", function () {
     checkAllSlices(lines);
   });
 
-  function checkAllSlices(lines: any) {
-    lines.eachPos(function (start: any) {
-      lines.eachPos(function (end: any) {
+  function checkAllSlices(lines: Lines) {
+    lines.eachPos(function (start) {
+      lines.eachPos(function (end) {
         check(lines.slice(start, end), lines.bootstrapSlice(start, end));
         check(
           lines.sliceString(start, end),
@@ -266,7 +270,7 @@ describe("lines", function () {
     });
   }
 
-  function getSourceLocation(lines: any) {
+  function getSourceLocation(lines: Lines) {
     return { start: lines.firstPos(), end: lines.lastPos() };
   }
 
@@ -274,7 +278,7 @@ describe("lines", function () {
     const code = String(GetSourceLocationTest),
       lines = fromString(code);
 
-    function verify(indent: any) {
+    function verify(indent: number) {
       const indented = lines.indentTail(indent),
         loc = getSourceLocation(indented),
         string = indented.toString(),