diff --git a/app/lib.ts b/app/lib.ts
index f7ce62f..80d7872 100644
--- a/app/lib.ts
+++ b/app/lib.ts
@@ -1,6 +1,6 @@
-const fourU8ToU32 = (f) => (f[0] << 24) | (f[1] << 16) | (f[2] << 8) | f[3];
+const fourU8ToU32 = (f: number[]) => (f[0] << 24) | (f[1] << 16) | (f[2] << 8) | f[3];
 
-const u8ArrToU32Arr = (u8a) => {
+const u8ArrToU32Arr = (u8a: number[]) => {
     let res = [];
     for (let i = 0; i < u8a.length/4; i++) {
         const c = u8a.slice(i*4, (i+1)*4);
@@ -9,7 +9,10 @@ const u8ArrToU32Arr = (u8a) => {
     return res;
 };
 
-const u8ArrToStr = (u8a) => u8a.reduce((a,c,i) => {
+type DTNode = any; // TODO
+type DTProp = any; // TODO
+
+const u8ArrToStr = (u8a: number[]) => u8a.reduce((a,c,i) => {
   if (i === u8a.length -1) {
     return a;
   }
@@ -18,27 +21,27 @@ const u8ArrToStr = (u8a) => u8a.reduce((a,c,i) => {
 }, "");
 
 // some props are simple strings
-const getStringProp = (n, pname) => {
-  const p = n.props.find(p => p[0] === pname);
+const getStringProp = (n: DTNode, pname: string) => {
+  const p = n.props.find((p: DTProp) => p[0] === pname);
   if (p) {
     return u8ArrToStr(p[1]);
   }
 };
 
 // many props are just numbers
-const getProp = (n, pname) => {
-  const p = n.props.find(p => p[0] === pname);
+const getProp = (n: DTNode, pname: string) => {
+  const p = n.props.find((p: DTProp) => p[0] === pname);
   return p ? u8ArrToU32Arr(p[1]) : null;
 };
 
 // strings representation of lists of numbers for pretty-printing
-const getPropStr = (n, pname) => {
+const getPropStr = (n: DTNode, pname: string) => {
   const p = getProp(n, pname);
   return p ? p.join(", ") : null;
 };
 
 // transform a node's props into numbers and strings, omitting many
-const transformNode = (n) => {
+const transformNode = (n: DTNode) => {
   const name = n.name || "root";
   // phandle is an identifier to the node
   const phandle = getProp(n, "phandle");
@@ -64,19 +67,22 @@ const transformNode = (n) => {
   };
 };
 
-export const transform = (n, id = "10000") => {
+export const transform = (n: DTNode, id: string = "10000") => {
   return {
     ...transformNode(n),
     id,
-    children: n.children.map((c, i) => transform(c, `${id}_${i}`)),
+    children: n.children.map((c: DTNode, i: number) => transform(c, `${id}_${i}`)),
   }
 };
 
+type TransformedNode = any; // TODO
+type TransformedEdge = any; // TODO
+
 // flatten tree to list of nodes, use IDs to define ReactFlow edges
-export const getNodesEdges = (tree) => {
-  const nodes = [];
-  const edges = [];
-  const rec = (n, d=1,b=1) => {
+export const getNodesEdges = (tree: DTNode) => {
+  const nodes: TransformedNode = [];
+  const edges: TransformedEdge = [];
+  const rec = (n: DTNode, d: number = 1, b: number = 1) => {
     nodes.push({
       id: n.id,
       type: "custom",
@@ -88,7 +94,7 @@ export const getNodesEdges = (tree) => {
         label: n.name,
       },
     });
-    n.children.forEach((c,i) => {
+    n.children.forEach((c: DTNode, i: number) => {
       edges.push({
         id: `${n.id}${c.id}`,
         source: n.id,
diff --git a/app/page.tsx b/app/page.tsx
index dd6fd7d..3d3abcf 100644
--- a/app/page.tsx
+++ b/app/page.tsx
@@ -32,8 +32,7 @@ export default function Home() {
     [setEdges]
   );
 
-  // TODO: file picker, pass file to parser
-  const parseDtb = async(data) => {
+  const parseDtb = async(data: Uint8Array) => {
     setInProgress(true);
     setTimeout(async () => {
       try {
diff --git a/next.config.js b/next.config.js
index 4d251f0..55fce74 100644
--- a/next.config.js
+++ b/next.config.js
@@ -5,6 +5,10 @@ const WasmPackPlugin = require('@wasm-tool/wasm-pack-plugin');
 module.exports = {
   output: "export",
   basePath: "/dtvis",
+  // Because we get an error even though tsconfig has target ES6 configured:
+  // Type error: Type 'Uint8Array' can only be iterated through when using the
+  // '--downlevelIteration' flag or with a '--target' of 'es2015' or higher.
+  ignoreBuildErrors: true, // FIXME
   webpack: (
     config,
     { buildId, dev, isServer, defaultLoaders, nextRuntime, webpack }
diff --git a/tsconfig.json b/tsconfig.json
index c714696..2c1aad2 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "target": "es5",
+    "target": "es6",
     "lib": ["dom", "dom.iterable", "esnext"],
     "allowJs": true,
     "skipLibCheck": true,