diff --git a/eslint.config.mjs b/eslint.config.mjs
index 691bc03b..14717446 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -4,7 +4,7 @@ import tseslint from 'typescript-eslint'
 
 export default tseslint.config(
   {
-    ignores: ['esm/**/*', 'dist/**/*'],
+    ignores: ['esm/**/*', 'dist/**/*', '*.js', '*.mjs', 'example/*'],
   },
   {
     languageOptions: {
diff --git a/src/craiIndex.ts b/src/craiIndex.ts
index 53ce8e62..15b34da1 100644
--- a/src/craiIndex.ts
+++ b/src/craiIndex.ts
@@ -21,16 +21,17 @@ type ParsedIndex = Record<string, Slice[] | undefined>
 function addRecordToIndex(index: ParsedIndex, record: number[]) {
   const [seqId, start, span, containerStart, sliceStart, sliceBytes] = record
 
-  if (!index[seqId]) {
-    index[seqId] = []
+  const s = seqId!
+  if (!index[s]) {
+    index[s] = []
   }
 
-  index[seqId].push({
-    start,
-    span,
-    containerStart,
-    sliceStart,
-    sliceBytes,
+  index[s].push({
+    start: start!,
+    span: span!,
+    containerStart: containerStart!,
+    sliceStart: sliceStart!,
+    sliceBytes: sliceBytes!,
   })
 }
 
diff --git a/src/cramFile/codecs/byteArrayLength.ts b/src/cramFile/codecs/byteArrayLength.ts
index ce2db4b3..fb7d69ae 100644
--- a/src/cramFile/codecs/byteArrayLength.ts
+++ b/src/cramFile/codecs/byteArrayLength.ts
@@ -23,11 +23,6 @@ export default class ByteArrayStopCodec extends CramCodec<
   ) {
     super(parameters, dataType)
     this.instantiateCodec = instantiateCodec
-    if (dataType !== 'byteArray') {
-      throw new TypeError(
-        `byteArrayLength does not support data type ${dataType}`,
-      )
-    }
   }
 
   decode(
diff --git a/src/cramFile/codecs/byteArrayStop.ts b/src/cramFile/codecs/byteArrayStop.ts
index 2495e1cc..7f0a494b 100644
--- a/src/cramFile/codecs/byteArrayStop.ts
+++ b/src/cramFile/codecs/byteArrayStop.ts
@@ -10,18 +10,6 @@ export default class ByteArrayStopCodec extends CramCodec<
   'byteArray',
   ByteArrayStopCramEncoding['parameters']
 > {
-  constructor(
-    parameters: ByteArrayStopCramEncoding['parameters'],
-    dataType: 'byteArray',
-  ) {
-    super(parameters, dataType)
-    if (dataType !== 'byteArray') {
-      throw new TypeError(
-        `byteArrayStop codec does not support data type ${dataType}`,
-      )
-    }
-  }
-
   decode(
     slice: CramSlice,
     coreDataBlock: CramFileBlock,
diff --git a/src/cramFile/codecs/external.ts b/src/cramFile/codecs/external.ts
index 672e41ff..c6f144fe 100644
--- a/src/cramFile/codecs/external.ts
+++ b/src/cramFile/codecs/external.ts
@@ -64,6 +64,6 @@ export default class ExternalCodec extends CramCodec<
         'attempted to read beyond end of block. this file seems truncated.',
       )
     }
-    return contentBlock.content[cursor.bytePosition++]
+    return contentBlock.content[cursor.bytePosition++]!
   }
 }
diff --git a/src/cramFile/codecs/getBits.ts b/src/cramFile/codecs/getBits.ts
index 4edf95db..18c3f386 100644
--- a/src/cramFile/codecs/getBits.ts
+++ b/src/cramFile/codecs/getBits.ts
@@ -17,7 +17,7 @@ export function getBits(
   for (let dlen = numBits; dlen; dlen--) {
     // get the next `dlen` bits in the input, put them in val
     val <<= 1
-    val |= (data[cursor.bytePosition] >> cursor.bitPosition) & 1
+    val |= (data[cursor.bytePosition]! >> cursor.bitPosition) & 1
     cursor.bitPosition -= 1
     if (cursor.bitPosition < 0) {
       cursor.bytePosition += 1
diff --git a/src/cramFile/codecs/huffman.ts b/src/cramFile/codecs/huffman.ts
index 159ee9ae..ddd01083 100644
--- a/src/cramFile/codecs/huffman.ts
+++ b/src/cramFile/codecs/huffman.ts
@@ -48,7 +48,7 @@ export default class HuffmanIntCodec extends CramCodec<
 
     // if this is a degenerate zero-length huffman code, special-case the
     // decoding
-    if (this.sortedCodes[0].bitLength === 0) {
+    if (this.sortedCodes[0]!.bitLength === 0) {
       this._decode = this._decodeZeroLengthCode
     }
   }
@@ -58,10 +58,10 @@ export default class HuffmanIntCodec extends CramCodec<
     let codes = new Array<{ symbol: number; bitLength: number }>(
       this.parameters.numCodes,
     )
-    for (let i = 0; i < this.parameters.numCodes; i += 1) {
+    for (let i = 0; i < this.parameters.numCodes; i++) {
       codes[i] = {
-        symbol: this.parameters.symbols[i],
-        bitLength: this.parameters.bitLengths[i],
+        symbol: this.parameters.symbols[i]!,
+        bitLength: this.parameters.bitLengths[i]!,
       }
     }
     // sort the codes by bit length and symbol value
@@ -74,7 +74,7 @@ export default class HuffmanIntCodec extends CramCodec<
       if (!this.codeBook[code.bitLength]) {
         this.codeBook[code.bitLength] = []
       }
-      this.codeBook[code.bitLength].push(code.symbol)
+      this.codeBook[code.bitLength]!.push(code.symbol)
     })
   }
 
@@ -117,7 +117,7 @@ export default class HuffmanIntCodec extends CramCodec<
 
     this.bitCodeToValue = new Array(maxBitCode + 1).fill(-1)
     for (let i = 0; i < this.sortedBitCodes.length; i += 1) {
-      this.bitCodeToValue[this.sortedCodes[i].bitCode] = i
+      this.bitCodeToValue[this.sortedCodes[i]!.bitCode] = i
     }
   }
 
@@ -136,7 +136,7 @@ export default class HuffmanIntCodec extends CramCodec<
 
   // the special case for zero-length codes
   _decodeZeroLengthCode() {
-    return this.sortedCodes[0].value
+    return this.sortedCodes[0]!.value
   }
 
   _decode(slice: CramSlice, coreDataBlock: CramFileBlock, coreCursor: Cursor) {
@@ -145,19 +145,19 @@ export default class HuffmanIntCodec extends CramCodec<
     let prevLen = 0
     let bits = 0
     for (let i = 0; i < this.sortedCodes.length; i += 1) {
-      const length = this.sortedCodes[i].bitLength
+      const length = this.sortedCodes[i]!.bitLength
       bits <<= length - prevLen
       bits |= getBits(input, coreCursor, length - prevLen)
       prevLen = length
       {
-        const index = this.bitCodeToValue[bits]
+        const index = this.bitCodeToValue[bits]!
         if (index > -1 && this.sortedBitLengthsByBitCode[index] === length) {
-          return this.sortedValuesByBitCode[index]
+          return this.sortedValuesByBitCode[index]!
         }
 
         for (
           let j = i;
-          this.sortedCodes[j + 1].bitLength === length &&
+          this.sortedCodes[j + 1]!.bitLength === length &&
           j < this.sortedCodes.length;
           j += 1
         ) {
diff --git a/src/cramFile/container/compressionScheme.ts b/src/cramFile/container/compressionScheme.ts
index a198e770..a4b0781a 100644
--- a/src/cramFile/container/compressionScheme.ts
+++ b/src/cramFile/container/compressionScheme.ts
@@ -53,30 +53,30 @@ function parseSubstitutionMatrix(byteArray: number[]) {
     matrix[i] = new Array(4)
   }
 
-  matrix[0][(byteArray[0] >> 6) & 3] = 'C'
-  matrix[0][(byteArray[0] >> 4) & 3] = 'G'
-  matrix[0][(byteArray[0] >> 2) & 3] = 'T'
-  matrix[0][(byteArray[0] >> 0) & 3] = 'N'
-
-  matrix[1][(byteArray[1] >> 6) & 3] = 'A'
-  matrix[1][(byteArray[1] >> 4) & 3] = 'G'
-  matrix[1][(byteArray[1] >> 2) & 3] = 'T'
-  matrix[1][(byteArray[1] >> 0) & 3] = 'N'
-
-  matrix[2][(byteArray[2] >> 6) & 3] = 'A'
-  matrix[2][(byteArray[2] >> 4) & 3] = 'C'
-  matrix[2][(byteArray[2] >> 2) & 3] = 'T'
-  matrix[2][(byteArray[2] >> 0) & 3] = 'N'
-
-  matrix[3][(byteArray[3] >> 6) & 3] = 'A'
-  matrix[3][(byteArray[3] >> 4) & 3] = 'C'
-  matrix[3][(byteArray[3] >> 2) & 3] = 'G'
-  matrix[3][(byteArray[3] >> 0) & 3] = 'N'
-
-  matrix[4][(byteArray[4] >> 6) & 3] = 'A'
-  matrix[4][(byteArray[4] >> 4) & 3] = 'C'
-  matrix[4][(byteArray[4] >> 2) & 3] = 'G'
-  matrix[4][(byteArray[4] >> 0) & 3] = 'T'
+  matrix[0]![(byteArray[0]! >> 6) & 3] = 'C'
+  matrix[0]![(byteArray[0]! >> 4) & 3] = 'G'
+  matrix[0]![(byteArray[0]! >> 2) & 3] = 'T'
+  matrix[0]![(byteArray[0]! >> 0) & 3] = 'N'
+
+  matrix[1]![(byteArray[1]! >> 6) & 3] = 'A'
+  matrix[1]![(byteArray[1]! >> 4) & 3] = 'G'
+  matrix[1]![(byteArray[1]! >> 2) & 3] = 'T'
+  matrix[1]![(byteArray[1]! >> 0) & 3] = 'N'
+
+  matrix[2]![(byteArray[2]! >> 6) & 3] = 'A'
+  matrix[2]![(byteArray[2]! >> 4) & 3] = 'C'
+  matrix[2]![(byteArray[2]! >> 2) & 3] = 'T'
+  matrix[2]![(byteArray[2]! >> 0) & 3] = 'N'
+
+  matrix[3]![(byteArray[3]! >> 6) & 3] = 'A'
+  matrix[3]![(byteArray[3]! >> 4) & 3] = 'C'
+  matrix[3]![(byteArray[3]! >> 2) & 3] = 'G'
+  matrix[3]![(byteArray[3]! >> 0) & 3] = 'N'
+
+  matrix[4]![(byteArray[4]! >> 6) & 3] = 'A'
+  matrix[4]![(byteArray[4]! >> 4) & 3] = 'C'
+  matrix[4]![(byteArray[4]! >> 2) & 3] = 'G'
+  matrix[4]![(byteArray[4]! >> 0) & 3] = 'T'
 
   return matrix
 }
@@ -145,8 +145,10 @@ export default class CramContainerCompressionScheme {
       this.dataSeriesCodecCache[dataSeriesName]
     if (r === undefined) {
       const encodingData = this.dataSeriesEncoding[dataSeriesName]
+      // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
       if (encodingData) {
         const dataType = dataSeriesTypes[dataSeriesName]
+        // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
         if (!dataType) {
           throw new CramMalformedError(
             `data series name ${dataSeriesName} not defined in file compression header`,
diff --git a/src/cramFile/file.ts b/src/cramFile/file.ts
index 1a8d2fb1..9ba45aa7 100644
--- a/src/cramFile/file.ts
+++ b/src/cramFile/file.ts
@@ -279,7 +279,7 @@ export default class CramFile {
     },
     position: number,
     size = section.maxLength,
-    preReadBuffer = undefined,
+    preReadBuffer?: Buffer,
   ) {
     let buffer: Buffer
     if (preReadBuffer) {
diff --git a/src/cramFile/record.ts b/src/cramFile/record.ts
index 9b7f75ae..442f8856 100644
--- a/src/cramFile/record.ts
+++ b/src/cramFile/record.ts
@@ -42,7 +42,7 @@ function decodeReadSequence(cramRecord: CramRecord, refRegion: RefRegion) {
   let currentReadFeature = 0
   while (bases.length < cramRecord.readLength) {
     if (currentReadFeature < cramRecord.readFeatures.length) {
-      const feature = cramRecord.readFeatures[currentReadFeature]
+      const feature = cramRecord.readFeatures[currentReadFeature]!
       if (feature.code === 'Q' || feature.code === 'q') {
         currentReadFeature += 1
       } else if (feature.pos === bases.length + 1) {
@@ -90,10 +90,7 @@ function decodeReadSequence(cramRecord: CramRecord, refRegion: RefRegion) {
         // put down a chunk of sequence up to the next read feature
         const chunk = refRegion.seq.slice(
           regionPos,
-          regionPos +
-            cramRecord.readFeatures[currentReadFeature].pos -
-            bases.length -
-            1,
+          regionPos + feature.pos - bases.length - 1,
         )
         bases += chunk
         regionPos += chunk.length
@@ -131,10 +128,6 @@ function decodeBaseSubstitution(
   compressionScheme: CramContainerCompressionScheme,
   readFeature: ReadFeature,
 ) {
-  if (!refRegion) {
-    return
-  }
-
   // decode base substitution code using the substitution matrix
   const refCoord = readFeature.refPos - refRegion.start
   const refBase = refRegion.seq.charAt(refCoord)
@@ -145,7 +138,7 @@ function decodeBaseSubstitution(
   if (baseNumber === undefined) {
     baseNumber = 4
   }
-  const substitutionScheme = compressionScheme.substitutionMatrix[baseNumber]
+  const substitutionScheme = compressionScheme.substitutionMatrix[baseNumber]!
   const base = substitutionScheme[readFeature.data]
   if (base) {
     readFeature.sub = base
diff --git a/src/cramFile/sectionParsers.ts b/src/cramFile/sectionParsers.ts
index f9eeb1fe..d8dc3022 100644
--- a/src/cramFile/sectionParsers.ts
+++ b/src/cramFile/sectionParsers.ts
@@ -179,8 +179,8 @@ export function cramPreservationMap() {
       const ents = []
       for (let i = 0; i < mapCount; i++) {
         const key =
-          String.fromCharCode(buffer[offset]) +
-          String.fromCharCode(buffer[offset + 1])
+          String.fromCharCode(buffer[offset]!) +
+          String.fromCharCode(buffer[offset + 1]!)
         offset += 2
 
         if (
@@ -550,8 +550,8 @@ function cramDataSeriesEncodingMap() {
       const ents = []
       for (let i = 0; i < mapCount; i++) {
         const key =
-          String.fromCharCode(buffer[offset]) +
-          String.fromCharCode(buffer[offset + 1])
+          String.fromCharCode(buffer[offset]!) +
+          String.fromCharCode(buffer[offset + 1]!)
         offset += 2
 
         const { value, offset: newOffset4 } = cramEncodingSub(buffer, offset)
diff --git a/src/cramFile/slice/decodeRecord.ts b/src/cramFile/slice/decodeRecord.ts
index 2d542af4..59ff2dcd 100644
--- a/src/cramFile/slice/decodeRecord.ts
+++ b/src/cramFile/slice/decodeRecord.ts
@@ -22,7 +22,7 @@ import { DataSeriesEncodingKey } from '../codecs/dataSeriesTypes'
 function readNullTerminatedString(buffer: Uint8Array) {
   let r = ''
   for (let i = 0; i < buffer.length && buffer[i] !== 0; i++) {
-    r += String.fromCharCode(buffer[i])
+    r += String.fromCharCode(buffer[i]!)
   }
   return r
 }
@@ -32,8 +32,8 @@ function readNullTerminatedString(buffer: Uint8Array) {
  * @private
  */
 function parseTagValueArray(buffer: Buffer) {
-  const arrayType = String.fromCharCode(buffer[0])
-  const length = Int32Array.from(buffer.slice(1))[0]
+  const arrayType = String.fromCharCode(buffer[0]!)
+  const length = Int32Array.from(buffer.slice(1))[0]!
 
   const array: number[] = new Array(length)
   buffer = buffer.slice(5)
@@ -41,37 +41,37 @@ function parseTagValueArray(buffer: Buffer) {
   if (arrayType === 'c') {
     const arr = new Int8Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else if (arrayType === 'C') {
     const arr = new Uint8Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else if (arrayType === 's') {
     const arr = new Int16Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else if (arrayType === 'S') {
     const arr = new Uint16Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else if (arrayType === 'i') {
     const arr = new Int32Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else if (arrayType === 'I') {
     const arr = new Uint32Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else if (arrayType === 'f') {
     const arr = new Float32Array(buffer.buffer)
     for (let i = 0; i < length; i += 1) {
-      array[i] = arr[i]
+      array[i] = arr[i]!
     }
   } else {
     throw new Error(`unknown type: ${arrayType}`)
@@ -311,26 +311,17 @@ export default function decodeRecord(
 
   const tags: Record<string, any> = {}
   // TN = tag names
-  const TN = compressionScheme.getTagNames(TLindex)
+  const TN = compressionScheme.getTagNames(TLindex)!
   const ntags = TN.length
 
   for (let i = 0; i < ntags; i += 1) {
-    const tagId = TN[i]
+    const tagId = TN[i]!
     const tagName = tagId.slice(0, 2)
     const tagType = tagId.slice(2, 3)
 
-    const tagCodec = compressionScheme.getCodecForTag(tagId)
-    if (!tagCodec) {
-      throw new CramMalformedError(
-        `no codec defined for auxiliary tag ${tagId}`,
-      )
-    }
-    const tagData = tagCodec.decode(
-      slice,
-      coreDataBlock,
-      blocksByContentId,
-      cursors,
-    )
+    const tagData = compressionScheme
+      .getCodecForTag(tagId)
+      .decode(slice, coreDataBlock, blocksByContentId, cursors)
     tags[tagName] = parseTagData(tagType, tagData)
   }
 
diff --git a/src/cramFile/slice/index.ts b/src/cramFile/slice/index.ts
index f777df1a..fbe5076a 100644
--- a/src/cramFile/slice/index.ts
+++ b/src/cramFile/slice/index.ts
@@ -237,7 +237,7 @@ export default class CramSlice {
         throw new Error('block undefined')
       }
       blocks[i] = block
-      blockPosition = blocks[i]._endPosition
+      blockPosition = blocks[i]!._endPosition
     }
 
     return blocks
@@ -246,8 +246,7 @@ export default class CramSlice {
   // no memoize
   async getCoreDataBlock() {
     const blocks = await this.getBlocks()
-    // the core data block is always the first block in the slice
-    return blocks[0]
+    return blocks[0]!
   }
 
   // memoize
@@ -352,10 +351,6 @@ export default class CramSlice {
     }
 
     const sliceHeader = await this.getHeader()
-    if (sliceHeader === undefined) {
-      throw new Error('slice header undefined')
-    }
-
     const blocksByContentId = await this._getBlocksContentIdIndex()
 
     // check MD5 of reference if available
@@ -424,7 +419,7 @@ export default class CramSlice {
       )
       return decoded
     }
-    let records: CramRecord[] = new Array(sliceHeader.parsedContent.numRecords)
+    const records: CramRecord[] = new Array(sliceHeader.parsedContent.numRecords)
     for (let i = 0; i < records.length; i += 1) {
       try {
         const init = decodeRecord(
@@ -451,7 +446,6 @@ export default class CramSlice {
           console.warn(
             'read attempted beyond end of buffer, file seems truncated.',
           )
-          records = records.filter(r => !!r)
           break
         } else {
           throw e
@@ -462,13 +456,13 @@ export default class CramSlice {
     // interpret `recordsToNextFragment` attributes to make standard `mate`
     // objects Resolve mate pair cross-references between records in this slice
     for (let i = 0; i < records.length; i += 1) {
-      const { mateRecordNumber } = records[i]
+      const { mateRecordNumber } = records[i]!
       if (mateRecordNumber !== undefined && mateRecordNumber >= 0) {
         associateIntraSliceMate(
           records,
           i,
-          records[i],
-          records[mateRecordNumber],
+          records[i]!,
+          records[mateRecordNumber]!,
         )
       }
     }
diff --git a/src/cramFile/util.ts b/src/cramFile/util.ts
index 5b7068e0..cd476054 100644
--- a/src/cramFile/util.ts
+++ b/src/cramFile/util.ts
@@ -20,34 +20,34 @@ export function itf8Size(v: number) {
 
 export function parseItf8(buffer: Uint8Array, initialOffset: number) {
   let offset = initialOffset
-  const countFlags = buffer[offset]
+  const countFlags = buffer[offset]!
   let result: number
   if (countFlags < 0x80) {
     result = countFlags
     offset = offset + 1
   } else if (countFlags < 0xc0) {
-    result = ((countFlags << 8) | buffer[offset + 1]) & 0x3fff
+    result = ((countFlags << 8) | buffer[offset + 1]!) & 0x3fff
     offset = offset + 2
   } else if (countFlags < 0xe0) {
     result =
-      ((countFlags << 16) | (buffer[offset + 1] << 8) | buffer[offset + 2]) &
+      ((countFlags << 16) | (buffer[offset + 1]! << 8) | buffer[offset + 2]!) &
       0x1fffff
     offset = offset + 3
   } else if (countFlags < 0xf0) {
     result =
       ((countFlags << 24) |
-        (buffer[offset + 1] << 16) |
-        (buffer[offset + 2] << 8) |
-        buffer[offset + 3]) &
+        (buffer[offset + 1]! << 16) |
+        (buffer[offset + 2]! << 8) |
+        buffer[offset + 3]!) &
       0x0fffffff
     offset = offset + 4
   } else {
     result =
       ((countFlags & 0x0f) << 28) |
-      (buffer[offset + 1] << 20) |
-      (buffer[offset + 2] << 12) |
-      (buffer[offset + 3] << 4) |
-      (buffer[offset + 4] & 0x0f)
+      (buffer[offset + 1]! << 20) |
+      (buffer[offset + 2]! << 12) |
+      (buffer[offset + 3]! << 4) |
+      (buffer[offset + 4]! & 0x0f)
     // x=((0xff & 0x0f)<<28) | (0xff<<20) | (0xff<<12) | (0xff<<4) | (0x0f & 0x0f);
     // TODO *val_p = uv < 0x80000000UL ? uv : -((int32_t) (0xffffffffUL - uv)) - 1;
     offset = offset + 5
@@ -62,56 +62,56 @@ export function parseItf8(buffer: Uint8Array, initialOffset: number) {
 
 export function parseLtf8(buffer: Buffer, initialOffset: number) {
   let offset = initialOffset
-  const countFlags = buffer[offset]
+  const countFlags = buffer[offset]!
   let n: number | Long
   if (countFlags < 0x80) {
     n = countFlags
     offset += 1
   } else if (countFlags < 0xc0) {
-    n = ((buffer[offset] << 8) | buffer[offset + 1]) & 0x3fff
+    n = ((buffer[offset]! << 8) | buffer[offset + 1]!) & 0x3fff
     offset += 2
   } else if (countFlags < 0xe0) {
     n =
-      ((buffer[offset] << 16) |
-        (buffer[offset + 1] << 8) |
-        buffer[offset + 2]) &
+      ((buffer[offset]! << 16) |
+        (buffer[offset + 1]! << 8) |
+        buffer[offset + 2]!) &
       0x1fffff
     n = ((countFlags & 63) << 16) | buffer.readUInt16LE(offset + 1)
     offset += 3
   } else if (countFlags < 0xf0) {
     n =
-      ((buffer[offset] << 24) |
-        (buffer[offset + 1] << 16) |
-        (buffer[offset + 2] << 8) |
-        buffer[offset + 3]) &
+      ((buffer[offset]! << 24) |
+        (buffer[offset + 1]! << 16) |
+        (buffer[offset + 2]! << 8) |
+        buffer[offset + 3]!) &
       0x0fffffff
     offset += 4
   } else if (countFlags < 0xf8) {
     n =
-      ((buffer[offset] & 15) * 2 ** 32 + (buffer[offset + 1] << 24)) |
-      ((buffer[offset + 2] << 16) |
-        (buffer[offset + 3] << 8) |
-        buffer[offset + 4])
+      ((buffer[offset]! & 15) * 2 ** 32 + (buffer[offset + 1]! << 24)) |
+      ((buffer[offset + 2]! << 16) |
+        (buffer[offset + 3]! << 8) |
+        buffer[offset + 4]!)
     // TODO *val_p = uv < 0x80000000UL ? uv : -((int32_t) (0xffffffffUL - uv)) - 1;
     offset += 5
   } else if (countFlags < 0xfc) {
     n =
-      ((((buffer[offset] & 7) << 8) | buffer[offset + 1]) * 2 ** 32 +
-        (buffer[offset + 2] << 24)) |
-      ((buffer[offset + 3] << 16) |
-        (buffer[offset + 4] << 8) |
-        buffer[offset + 5])
+      ((((buffer[offset]! & 7) << 8) | buffer[offset + 1]!) * 2 ** 32 +
+        (buffer[offset + 2]! << 24)) |
+      ((buffer[offset + 3]! << 16) |
+        (buffer[offset + 4]! << 8) |
+        buffer[offset + 5]!)
     offset += 6
   } else if (countFlags < 0xfe) {
     n =
-      ((((buffer[offset] & 3) << 16) |
-        (buffer[offset + 1] << 8) |
-        buffer[offset + 2]) *
+      ((((buffer[offset]! & 3) << 16) |
+        (buffer[offset + 1]! << 8) |
+        buffer[offset + 2]!) *
         2 ** 32 +
-        (buffer[offset + 3] << 24)) |
-      ((buffer[offset + 4] << 16) |
-        (buffer[offset + 5] << 8) |
-        buffer[offset + 6])
+        (buffer[offset + 3]! << 24)) |
+      ((buffer[offset + 4]! << 16) |
+        (buffer[offset + 5]! << 8) |
+        buffer[offset + 6]!)
     offset += 7
   } else if (countFlags < 0xff) {
     n = Long.fromBytesBE(
@@ -167,6 +167,7 @@ export function tinyMemoize(_class: any, methodName: any) {
       const res = method.call(this)
       this[memoAttrName] = res
       Promise.resolve(res).catch(() => {
+        // eslint-disable-next-line @typescript-eslint/no-dynamic-delete
         delete this[memoAttrName]
       })
     }
diff --git a/src/indexedCramFile.ts b/src/indexedCramFile.ts
index 28a0608c..6245274a 100644
--- a/src/indexedCramFile.ts
+++ b/src/indexedCramFile.ts
@@ -63,9 +63,6 @@ export default class IndexedCramFile {
     }
 
     this.index = args.index
-    if (!this.index.getEntriesForRange) {
-      throw new Error('invalid arguments: not an index')
-    }
   }
 
   /**
@@ -163,7 +160,7 @@ export default class IndexedCramFile {
         .sort((a, b) => a.toString().localeCompare(b.toString()))
         .filter(
           (item, pos, ary) =>
-            !pos || item.toString() !== ary[pos - 1].toString(),
+            !pos || item.toString() !== ary[pos - 1]!.toString(),
         )
 
       const mateRecordPromises = []
diff --git a/test/crai.test.ts b/test/crai.test.ts
index 7caa4615..5fc64d0e 100644
--- a/test/crai.test.ts
+++ b/test/crai.test.ts
@@ -61,7 +61,7 @@ describe('.crai reader', () => {
       () => {
         throw new Error('the getIndex call should have failed')
       },
-      err => {
+      (err: unknown) => {
         expect(`${err}`).toMatch(/invalid/)
       },
     )
@@ -145,7 +145,7 @@ describe('reading a BAI file instead', () => {
       () => {
         throw new Error('the getIndex call should have failed')
       },
-      err => {
+      (err: unknown) => {
         expect(`${err}`).toMatch(/bai/)
       },
     )
diff --git a/test/cram2sam.ts b/test/cram2sam.ts
index eb177885..6c37678a 100644
--- a/test/cram2sam.ts
+++ b/test/cram2sam.ts
@@ -109,7 +109,7 @@ function decodeSeqCigar(record: CramRecord) {
         if (oplen) {
           cigar += oplen + op
         }
-        cigar += `${1}I`
+        cigar += `1I`
         oplen = 0
       } else if (code === 'P') {
         // Padding
diff --git a/test/indexedfile.test.ts b/test/indexedfile.test.ts
index 311ac647..6f1309b9 100644
--- a/test/indexedfile.test.ts
+++ b/test/indexedfile.test.ts
@@ -56,8 +56,16 @@ describe('.crai indexed cram file', () => {
       }),
     })
 
-    const features = await cram.getRecordsForRange(0, 0, Number.POSITIVE_INFINITY)
-    const features2 = await cram.getRecordsForRange(-1, 0, Number.POSITIVE_INFINITY)
+    const features = await cram.getRecordsForRange(
+      0,
+      0,
+      Number.POSITIVE_INFINITY,
+    )
+    const features2 = await cram.getRecordsForRange(
+      -1,
+      0,
+      Number.POSITIVE_INFINITY,
+    )
     expect(features).toMatchSnapshot()
     expect(features2).toMatchSnapshot()
   })
@@ -104,7 +112,11 @@ describe('.crai indexed cram file', () => {
         index: new CraiIndex({ filehandle: testDataFile(`${filename}.crai`) }),
       })
 
-      const features = await cram.getRecordsForRange(0, 0, Number.POSITIVE_INFINITY)
+      const features = await cram.getRecordsForRange(
+        0,
+        0,
+        Number.POSITIVE_INFINITY,
+      )
       features.sort((a, b) =>
         (a.readName || '').localeCompare(b.readName || ''),
       )
@@ -118,7 +130,11 @@ describe('.crai indexed cram file', () => {
         index: new CraiIndex({ filehandle: testDataFile(`${filename}.crai`) }),
       })
 
-      const features = await cram.getRecordsForRange(1, 0, Number.POSITIVE_INFINITY)
+      const features = await cram.getRecordsForRange(
+        1,
+        0,
+        Number.POSITIVE_INFINITY,
+      )
       expect(features.length).toBeGreaterThan(-1)
       expect(features).toMatchSnapshot()
     })
@@ -239,8 +255,8 @@ test('region not downloading enough records', async () => {
   })
   const entries = await index.getEntriesForRange(0, 75100635, 75125544)
   expect(entries.length).toEqual(2)
-  expect(entries[0].start).toEqual(74378949)
-  expect(entries[1].start).toEqual(74945118)
+  expect(entries[0]!.start).toEqual(74378949)
+  expect(entries[1]!.start).toEqual(74945118)
 })
 
 test('troublesome file returns the correct sequence', async () => {
diff --git a/test/lib/fasta/index.ts b/test/lib/fasta/index.ts
index 17c1c57e..063aba02 100644
--- a/test/lib/fasta/index.ts
+++ b/test/lib/fasta/index.ts
@@ -4,7 +4,7 @@ function parseSmallFasta(text: string) {
     .filter(t => /\S/.test(t))
     .map(entryText => {
       const [defLine, ...seq] = entryText.split('\n')
-      const [id, ...des] = defLine.split(' ')
+      const [id, ...des] = defLine!.split(' ')
       const description = des.join(' ')
       const sequence = seq.join('').replace(/\s/g, '')
       return { id, description, sequence }