diff --git a/src/components/Editor/CallSite.js b/src/components/Editor/CallSite.js
index c3dda92871..8327782e78 100644
--- a/src/components/Editor/CallSite.js
+++ b/src/components/Editor/CallSite.js
@@ -1,7 +1,7 @@
 // @flow
 import { Component } from "react";
 
-import { markText, toEditorLocation } from "../../utils/editor";
+import { markText, toEditorRange } from "../../utils/editor";
 require("./CallSite.css");
 
 type MarkerType = {
@@ -34,8 +34,8 @@ export default class CallSite extends Component {
     const { editor, callSite, breakpoint, source } = nextProps || this.props;
     const className = !breakpoint ? "call-site" : "call-site-bp";
     const sourceId = source.get("id");
-    const editorLocation = toEditorLocation(sourceId, callSite.location);
-    this.marker = markText(editor, className, editorLocation);
+    const editorRange = toEditorRange(sourceId, callSite.location);
+    this.marker = markText(editor, className, editorRange);
   }
 
   clearCallSite() {
diff --git a/src/components/Editor/Preview.js b/src/components/Editor/Preview.js
index 25ec8b7a19..dcff8a8aed 100644
--- a/src/components/Editor/Preview.js
+++ b/src/components/Editor/Preview.js
@@ -19,6 +19,8 @@ import { markText } from "../../utils/editor";
 import Rep from "../shared/Rep";
 import { MODE } from "devtools-reps";
 
+import type { EditorRange } from "../../utils/editor/types";
+
 import "./Preview.css";
 
 class Preview extends Component {
@@ -32,7 +34,7 @@ class Preview extends Component {
     value: Object,
     expression: string,
     onClose: () => void,
-    location: Object,
+    range: EditorRange,
     editor: any,
     selectSourceURL: (string, Object) => void
   };
@@ -43,10 +45,10 @@ class Preview extends Component {
       loadedObjects,
       value,
       editor,
-      location
+      range
     } = this.props;
 
-    this.marker = markText(editor, "selection", location);
+    this.marker = markText(editor, "selection", range);
 
     if (!value || !value.type == "object") {
       return;
diff --git a/src/components/Editor/index.js b/src/components/Editor/index.js
index 166d058490..85898a3abe 100644
--- a/src/components/Editor/index.js
+++ b/src/components/Editor/index.js
@@ -65,7 +65,8 @@ import {
   lineAtHeight,
   toSourceLine,
   toEditorLine,
-  toEditorLocation,
+  toEditorPosition,
+  toEditorRange,
   resetLineNumberFormat
 } from "../../utils/editor";
 
@@ -483,17 +484,10 @@ class Editor extends PureComponent {
       selectedLocation &&
       selectedFrame.location.sourceId === selectedLocation.sourceId
     ) {
-      const {
-        sourceId,
-        line: sourceLine,
-        column: sourceColumn
-      } = selectedFrame.location;
-      let { line, column } = toEditorLocation(sourceId, {
-        line: sourceLine,
-        column: sourceColumn
-      });
-      this.state.editor.codeMirror.addLineClass(line, "line", "new-debug-line");
+      const { location, sourceId } = selectedFrame;
+      const { line, column } = toEditorPosition(sourceId, location);
 
+      this.state.editor.codeMirror.addLineClass(line, "line", "new-debug-line");
       this.debugExpression = markText(this.state.editor, "debug-expression", {
         start: { line, column },
         end: { line, column: null }
@@ -615,12 +609,12 @@ class Editor extends PureComponent {
       return;
     }
 
-    const editorLocation = toEditorLocation(selectedSource.get("id"), location);
+    const editorRange = toEditorRange(selectedSource.get("id"), location);
 
     return Preview({
       value,
       editor: this.state.editor,
-      location: editorLocation,
+      range: editorRange,
       expression: expression,
       popoverPos: cursorPos,
       onClose: () => this.clearPreviewSelection()
diff --git a/src/utils/editor/index.js b/src/utils/editor/index.js
index cc610121ca..3e53a190f6 100644
--- a/src/utils/editor/index.js
+++ b/src/utils/editor/index.js
@@ -1,3 +1,5 @@
+// @flow
+
 import { isEnabled } from "devtools-config";
 import { isPretty, isJavaScript } from "../source";
 import { isOriginalId } from "devtools-source-map";
@@ -12,6 +14,9 @@ import { isWasm, lineToWasmOffset, wasmOffsetToLine } from "../wasm";
 
 import { SourceEditor, SourceEditorUtils } from "devtools-source-editor";
 
+import type { AstPosition, AstLocation } from "../parser/types";
+import type { EditorPosition, EditorRange } from "../editor/types";
+
 function shouldShowPrettyPrint(selectedSource) {
   if (!selectedSource) {
     return false;
@@ -78,31 +83,45 @@ function createEditor() {
   });
 }
 
-function toEditorLine(sourceId: string, lineOrOffset: number) {
+function toEditorLine(sourceId: string, lineOrOffset: number): ?number {
   return isWasm(sourceId)
     ? wasmOffsetToLine(sourceId, lineOrOffset)
     : lineOrOffset - 1;
 }
 
-function toEditorLocation(sourceId: string, location: any) {
+function toEditorPosition(
+  sourceId: string,
+  location: AstPosition
+): EditorPosition {
   return {
     line: toEditorLine(sourceId, location.line),
     column: isWasm(sourceId) ? 0 : location.column
   };
 }
 
-function toSourceLine(sourceId: string, line: number) {
+function toEditorRange(sourceId: string, location: AstLocation): EditorRange {
+  const { start, end } = location;
+  return {
+    start: toEditorPosition(sourceId, start),
+    end: toEditorPosition(sourceId, end)
+  };
+}
+
+function toSourceLine(sourceId: string, line: number): ?number {
   return isWasm(sourceId) ? lineToWasmOffset(sourceId, line) : line + 1;
 }
 
-function toSourceLocation(sourceId: string, location: any) {
+function toSourceLocation(
+  sourceId: string,
+  location: EditorPosition
+): AstPosition {
   return {
     line: toSourceLine(sourceId, location.line),
     column: isWasm(sourceId) ? undefined : location.column
   };
 }
 
-function markText(editor: any, className, location: any) {
+function markText(editor: any, className, location: EditorRange) {
   const { start, end } = location;
 
   return editor.codeMirror.markText(
@@ -127,7 +146,8 @@ module.exports = Object.assign(
     createEditor,
     isWasm,
     toEditorLine,
-    toEditorLocation,
+    toEditorPosition,
+    toEditorRange,
     toSourceLine,
     toSourceLocation,
     shouldShowPrettyPrint,
diff --git a/src/utils/editor/types.js b/src/utils/editor/types.js
new file mode 100644
index 0000000000..f0ac38685c
--- /dev/null
+++ b/src/utils/editor/types.js
@@ -0,0 +1,3 @@
+export type EditorPosition = { line: number, column: number };
+
+export type EditorRange = { end: EditorPosition, start: EditorPosition };
diff --git a/src/utils/wasm.js b/src/utils/wasm.js
index 9f05ce9ae4..775f1b220a 100644
--- a/src/utils/wasm.js
+++ b/src/utils/wasm.js
@@ -56,11 +56,11 @@ function getWasmLineNumberFormatter(sourceId: string) {
   let last0 = 7;
   return function(number: number) {
     let offset = lineToWasmOffset(sourceId, number - 1);
-    if (offset === undefined) {
+    if (offset == undefined) {
       return "";
     }
     let i = 7;
-    for (let n = offset | 0; n !== 0 && i >= 0; n >>= 4, i--) {
+    for (let n = offset; n !== 0 && i >= 0; n >>= 4, i--) {
       let nibble = n & 15;
       buffer[i] = nibble < 10 ? codeOf0 + nibble : codeOfA - 10 + nibble;
     }
@@ -84,7 +84,7 @@ function isWasm(sourceId: string) {
  * @memberof utils/wasm
  * @static
  */
-function lineToWasmOffset(sourceId: string, number: number) {
+function lineToWasmOffset(sourceId: string, number: number): ?number {
   let wasmState = wasmStates[sourceId];
   if (!wasmState) {
     return undefined;
@@ -100,7 +100,7 @@ function lineToWasmOffset(sourceId: string, number: number) {
  * @memberof utils/wasm
  * @static
  */
-function wasmOffsetToLine(sourceId: string, offset: number) {
+function wasmOffsetToLine(sourceId: string, offset: number): ?number {
   let wasmState = wasmStates[sourceId];
   if (!wasmState) {
     return undefined;